Chartwork
Short Description
Position findings...
Description
Skill Development Course Chartwork and Position Fixing Instructor Manual
Chartwork and Position Fixing Instructor Manual
Contents Course Arrangements Arrangements
1
Course Introduction
4
Chart Basics
7
Measurement Basics
17
Tides
25
Position Fixing Methods
36
Finding Dive Sites
47
Voyage Planning
55
Open Forum
69
Chart Char t Exercises
71
Position Fixing Dry Practical
72
Voyage Planning Exercise
74
Practical Navigation Aoat
77
Appendix A Tailoring the course
81
Appendix B Estimating distance
84
Appendix C Tide differences
85
Appendix D Chart for offsetting tide against course
86
Appendix E Magnetic variation
87
Appendix F Model voyage plan
90
Appendix G Further reading
91
Issue 1.2, February 2010 Copyright © British Sub-Aqua Club 2010 Published in the United Kingdom British Sub-Aqua Club, Telford’s Quay, South Pier Road, Ellesmere Port, Cheshire CH65 4FL Telephone: 0151-350 6200
Fax: 0151-350 6215
www.bsac.com
All rights reserved. This Instructor Manual may not, in whole or part, be copied, photocopied, reproduced or translated, or converted into any electronic or readable form without prior written consent of British Sub Aqua Club
V1.2 Copyright Copyrigh t © BSAC 2010
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Chartwork and Position Fixing Instructor Manual
BSAC is grateful to the members who contributed to the development of this course, i ts supporting materials, and the previous versions it has been developed from. In particular: David George Tim Moxey Mark Murphy Richard Scarsbrook
Where illustrations have been sourced from the internet or from photographers who have requested attribution, the acknowledgement and the licensing arrangement where applicable is shown on the Notes page of the relevant Powerpoint slide The chart extracts used in this manual and the accompanying visual aids are NOT TO BE USED FOR NAVIGATION . © Crown Copyright and/or database rights. Reproduced by permission of the Controller of Her Majesty’s Stationery Ofce and the UK Hydrographic Ofc e ( www.ukho.gov.uk ). The extract from Chart 2693 also appears by permission of Trinity House.
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Course Aim To instruct members in the basic techniques of marine navigation and dive site location to a level which will help them as active divers in planning and managing dives undertaken by a typical BSAC branch.
Course Arrangements
Session
Introduction
Course Overview This is a practical course which has the students using charts and chartwork instruments right from the start. It culminates in a day at sea, navigating to two dive sites and locating them by echo sounder, directed by a voyage plan built up gradually over the rst day. The rst part of the course teaches the basics of charts, measurement and tides, interspersed with exercises. This is followed by position xing techniques and a session on the specic problems involved in locating dive sites. The last part of day one looks at how all the techniques are brought together to create a voyage plan. The focus of the course is developing practical navigation skills, not boat handling or dive management, which are covered thoroughly on other BSAC courses. The course is aimed primarily at daylight navigation, and makes extensive use of GPS and echo sounders. On the practical day students receive coaching in the mental approach that good navigators employ in addition to mastery of specic techniques. Because the course uses actual data from ofcial nautical reference materials to plan and execute navigational exercises at sea, it requires a small amount of tailoring of the visual aids and class exercises to make them specic to the local area where the practical navigation aoat is to be run. Full instructions for tailoring are included in Appendix A of this manual.
Chart Basics
Chart Exercises
Measurement Basics
Tides
Contents Day 1 Assemble, introductions, administration Chart availability, major brands paper and digital, Admiralty, updates available from website; scale; general layout; colours - land, intertidal, submerged; chart symbols - depths (spot, drying, isobath), wrecks, rocks etc, nature of seabed, buoys and lights, buoyage, landmarks; lat/long - what it is, how to write it down, how to determine lat/ long of a point on chart; Exercises Terminology; bearings, variation, compass rose, measuring at sea; drawing on charts, measuring angles, reciprocals, measuring distance; speed distance & time, calculations and tables; measuring with GPS; water v ground track Exercises Tidal terminology; cause of tides; springs and neaps; tide tables; tidal range; calculating depth of water; rule of twelfths; tidal diamonds, tidal stream atlas, local effects, calculating slack water; chart symbols - tides and currents; implications for launch/recovery, diving, navigation
Mins
15
60
30
60
60
Exercises
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Position Fixing Methods
Position Fixing Dry Practical
Finding Dive Sites
Voyage Planning
Voyage Planning Exercise
Simple x; position lines – compass bearing, transits, uses of position lines clearing lines, leading lines, x; position x; visual x accuracy; GPS; GPS limitations, geodetic datum; other methods - depth sounding, radar. Take transits. Exchange with other groups and use to relocate hidden objects. Outdoors. Accuracy v precision; problems with positions, types of error/inaccuracy; dive site information needed for searches, echo sounder basics, approaching the dive site; search patterns, pros and cons Waypoints, waypoint navigation, using GPS; choosing waypoints; techniques - aiming off, attack points, buoy hopping, straight to GPS; preparing the voyage plan, planning process, documentation; allowing for tide; practical navigation; charter boats; navigation Zen; model voyage plan Work in groups to build up complete voyage plan to be used on Day 2
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Entry Level BSAC member.
45
30
Open Forum
Duration 2 days. 2
Qualication card conrming course attendance, supplied by BSAC HQ on payment of fees and conrmation of attendance on the course. There are no examinations or assessments.
Course Registration Branches wishing to run a Skill Development Course must order the packs two weeks in advance from the BSAC Shop and pay the relevant course pack/ registration fees for all students on the course. This can be done online via the BSAC website, by telephone, or by post. 45
Instructor Requirements ●
75
Day 2 6h45' Course debrieng; issue documentation; disperse
Previous experience is not essential, but it will enhance students' experience of the course if they have experience of diving from boats, and if they are able to drive prociently the type of boats used on the course.
Qualications Awarded
7h30' Practical Navigation Aoat
Classroom lessons may be split over a number of (evening class) sessions if required. This approach allows time for additional exercises to be set, which some students may nd helpful. It would also be possible to split the practical day into two sessions. A Voyage planning exercise should immediately precede every practical session aoat.
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Qualications Each instructor should be a BSAC Open Water Instructor or above, with practical experience of navigating dive boats. All instructors should be familiar with the entire contents of the course, and should be capable of performing all elements of the practical exercises competently and condently at the chosen location. Knowledge of the content of previous versions of CPF alone is not sufcient. If necessary, instructors should rehearse the practical skills with other instructors at the location chosen for the practical day before teaching/supervising other students. Each boat should be manned by a person suitably qualied to take charge of it (see Equipment , below) and with a thorough knowledge of how to operate the electronic navigation aids. Normally this should be the instructor. V1.2 Copyright © BSAC 2010
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Student:Instructor ratio The maximum recommended ratio is four students to each instructor for all parts of the course. One lecturer may address any number of students in the theory lessons, but additional instructors should be on hand to assist with any exercises. For the at sea exercises it is preferable to have two instructors in the boat because the physical layout of most RIBs makes it difcult to instruct more than two students at a time whilst the boat is making way. While one student is carrying out an exercise, with a second student observing if appropriate, the second instructor can coach the others.
Equipment Up-to-date charts of the area to be used during the practical lessons, plus dive guides, almanacs, tide tables, reference books as appropriate, and chartwork instruments. Students should work from full colour versions of charts and similar navigational publications. Internet access for planning sessions if possible. Specic requirements are shown under Additional visual aids at the start of the notes for each lesson. Boats tted with a GPS, echo sounder and steering compass for practical lessons - one boat for each group of four students and instructor. Any type of dive boat may be used, including large charter boats driven by a professional skipper. However, to ensure that students get sufcient attention and hands-on experience, there should not be more than four students per boat unless the duration of the course is extended accordingly.
Course Arrangements
they were dive sites. Inland sites will not normally make suitable venues for this course because they lack tides. Refer to Appendix A of this manual for full instructions on tailoring the course to local conditions.
Administration HQ will supply the appropriate course packs and a Course Report form, on receipt of an order. After the course the organiser should return the completed form without delay to BSAC HQ, who will issue qualication cards to students and record their course attendance on the membership database. For regionally run courses: on completion of the course, instructors should forward outstanding fees, accounts and expense claims etc, to the course organiser (some Regions may operate different procedures).
Costs BSAC course fees cover the cost of course packs and certication cards. Fees must be paid to HQ at the time of ordering, in order to register and obtain student packs. For branch run courses, instructor expenses, launching fees, boat fuel, boat hire, and any other costs are the responsibility of the students and their organisers. For regionally run courses, students must pay the appropriate regional course fee to the Skill Development Course Organiser at time of booking.
Club and private boats used on this course should be operated in accordance with BSAC Safe Diving and any other applicable BSAC or BSAC-recommended guidelines. Check the BSAC website for the latest information. Charter boats should be operating under the appropriate MCA Codes of Practice.
Venue Facilities Adequate classroom facilities with a suitable table to lay out charts for each pair of students. Spacious outdoor area for position xing exercises. Coastal site with boat access to an area suitable for the practical activities. Preferably the area should contain known dive sites, but if not it should contain underwater features which can be navigated to and located by echo sounder as if V1.2 Copyright © BSAC 2010
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INTRODUCTION Lesson Objectives This lesson sets the scene for the course overall. It briey outlines the course content, domestic/logistics and timetable.
Achievement Targets At the end of this lesson students should: ●
understand the objectives and structure of the course
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understand what is required of them during the course
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understand the domestic and logistical arrangements for the course
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understand the course timetable
Additional visual aids On courses where the instructors and students do not know one another, it may be useful to insert an extra VA introducing each instructor. Paper copies of blank voyage plan forms CPF09 VPslate v1.2.doc (or later version)
Introduction Chartwork and Position Fiing Introductions Introduce the instructors, and get the students to introduce themselves. Introductions can be minimised or omitted on courses where the participants already know one another. Ask the students to describe any previous experience of navigation and boat handling.
Course aims The aims of the course are to instruct members in the basic techniques of marine navigation and dive site location to a level which will help them as active divers in planning and managing dives undertaken by a typical BSAC branches from RIBs. This means that you can read a chart to identify dangers and things of interest to divers, work out slack water and the approximate depth, get to the dive site on time, and then nd the exact spot on the seabed.
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basic techniques of marine navigation
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suitable level for dive planning & management
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typical BSAC branch RIB dives
If planning a walk in open country or even a day long car tour, it is most likely that you would consult maps to see what and where the attractions are, and listen to the weather forecast. Diving requires similar preparation, to nd out how to get to and locate the site, and to determine what is likely to be found on the seabed. Divers also have to contend with a third factor - tides. This course teaches you how to read and employ charts and tide tables to safely navigate small boats, and as aids to dive planning. Point out that this course focuses on the techniques of chartwork, position xing, and 4
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navigation. It is not a boat handling course, or a dive management course. To take full charge of diving from small boats you need those skills as well.
Domestic arrangements Fire exits, catering facilities, house rules, etc
Locations Cover the different venues that will be used during the course.
Timings Outline the timetable for the course, including breaks, lunch, and nish time. Either issue a printed timetable, or add it to this VA. Guideline durations for each session are provided in the Course Arrangements section of these notes.
Course Outline Day one ●
chart basics
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measurement basics
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tides
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position xing methods
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nding dive sites
● voyage planning
Day two ●
navigation at sea - all day on the boats
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course debrief
Voage plan Explain how the course is centred around the production and use of a voyage plan, the key output from chartwork and position xing. The voyage plan will be built up progressively during the course. Hand out blank copies of the voyage plan forms to each student.
Understand charts Select dive sites Plot courses Write it down – VP ●
build up in stages
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nish off in last session
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use ‘tomorrow’
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Navigate to dive site Locate the dive site For us as divers, this is the extra step without which all the others are pointless [and the bit other navigation courses do not tell you about ].
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CHART BASICS Lesson Objectives This lesson describes the types of chart that students are likely to encounter, how to understand the information that is shown on them, and how to plot and record positions using a chart.
Achievement Targets At the end of this lesson students should: ●
be aware of the types of chart useful to divers
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be able to extract information from a chart with the aid of 5011
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be capable of plotting a position on a chart
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be capable of reading a position from a chart
Additional visual aids Local charts, examples of different paper chart formats, 5011.
CPF09_exercises handout.doc (see Appendix A for tailoring instructions)
Chart Basics Outline Objectives ●
introduction to nautical charts
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learn to extract information relevant to divers
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plot and record positions
Availability
Scale Chart laout Smbols and abbreviations Latitude & longitude Eercises
Availability Charts can be purchased from Admiralty Chart Agents and leading yacht chandlers. Similar charts are published in other countries so charts should be available for almost all sea areas of the world.
Primaril designed for mariners Charts are maps of the sea and its adjacent coast line, designed for marine navigation. V1.2 Copyright © BSAC 2010
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Charts concentrate on the seabed, coastal topography, and particular features such as potential hazards to navigation. What are hazards for large ships are often the sites that are of interest to divers using smaller boats. For example, large ships need to avoid wrecks and reefs at certain depths, but these may provide a good dive site.
Paper or digital Paper charts come in a variety of formats. The most common is a large (typically about 1 metre by 2/3rds meter) sheet rolled, or at and folded in half. Char ts for leisure use may come folded like a map, or be split into a number of smaller, separate sheets (typically about A3 size). Some charts are available printed on waterproof and/or plastic paper, so that they can be used in rugged environments such as a RIB. Show examples of various types of chart. Point out that paper char ts are still valuable even in the digital age. There are two different types of digital chart. A raster chart is basically a digital photograph of a paper chart. An electronic chart system which uses raster charts can overlay the boat's course and position on the chart, but will not be able to recognise any of the information printed on it. They are relatively cheap, but need a reasonably large screen (laptop and upwards) to be used effectively. Most electronic navigation systems use vector charts. A vector chart consists of a database which holds all the elements of the chart such as spot depths, wrecks, lights, boundaries and so on as separate data items. This means that in addition to overlaying boat position information, the software can process all the information on the chart. A major advantage for small boat users, who use GPS plotter units with small screens, is improved display scaleability. Data items that would otherwise clutter the screen can be omitted or reduced in size when zooming in so that the screen remains readable. Depending on the manufacturer, digital charts may be supplied as downloads, on CD/ DVD or on memory cards (which themselves come in a number of different types eg, SD, MMC, etc). The le formats used by different digital chart vendors vary, and each manufacturer may sell a range of charts containing different levels of information. All this means that not all digital charts can be used in all electronic navigation devices. It is important to get up to date advice before buying.
Main brands [BSAC instruction in chartwork assumes use of British Admiralty Charts. Overseas, instructors should tailor the following list to use any other charts which are in common use in their part of the world. This information is correct in January 2010.]
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Admiralty Published by the UK Hydrographic Ofce (UKHO) for the Royal Navy, they are amongst the best and most detailed in the world and are available to anyone who goes to sea. They include seabed information of interest to divers. Admiralty Charts cover all coastal waters of the UK and many other coastlines of the world. UKHO products include paper charts in standard, leisure, and Tough Chart formats (waterproof, but only available for some areas egs, S Coast, N Wales, Clyde), as well as raster (ARCS) and vector (AVCS) digital versions.
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Imray, Stanford Both produce paper charts, principally for yachtsmen. Small boat versions available.
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C-Map Digital charts for a wide variety of GPS units and electronic navigation systems.
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Digital charts for a wide variety of GPS units including Eagle, Furuno, Lowrance, and Humminbird.
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BlueChart Digital charts for Garmin systems.
Chart agents, chandlers, websites Most chandlers carry stocks of paper charts covering their local area, and digital charts which are compatible with the brands of equipment they sell. Charts can also be purchased online.
Admiralt charts The full worldwide range of Admiralty charts can be purchased from Admiralty Chart Agents.
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catalogues The full list of Admiralty charts is published on the UKHO website. Paper catalogues showing various subsets of the full range of charts and publications are also available. NP109, the NW Europe Catalogue (pictured on VA) shows all the UK charts. Other manufacturers also provide catalogues of the charts they produce.
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Notices to Mariners on web Over time the information shown on charts becomes out of date as buoys are moved, new wrecks occur, channels are dredged or silt up, and so on. Changes to charts are published on the UKHO website as they occur. The updates are collectively known as Notices to Mariners and can be viewed free of charge, so that charts can be kept up to date. Most vector chart suppliers provide an update service for their products.
Scale Show examples of large and small scale charts for your local area.
Various scales (1:2,500 - 1:20,000,000) ●
small scale - large area - little detail Small scale charts cover large areas in little detail. For example, a chart of the entire North Sea would be a small scale chart. Small scale big number eg 1:20000000.
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large scale - small area - more detail – best for divers Large scale charts cover small areas in great detail. For identifying dive sites and navigating in their vicinity, use large scale char ts where available. Large scale small number eg 1:2500.
Chart laout Use this VA to introduce charts in general and to go over the main topics and points found on them. Chart layout includes various pieces of information which apply to the whole of the chart. The various topics do not occur in a xed position, but are usually printed on land mass parts of the chart so they do not obscure soundings and seabed information. Where space is available they are printed together, but the nature of the area covered by the chart sometimes means that this is not possible. Point out the various features on an actual chart. V1.2 Copyright © BSAC 2010
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Title and number The chart title gives a full description of the area covered, the date of survey, denes units and lists cautions, etc. Admiralty Charts are printed in metric units, so depths of water and heights of land are quoted in metres. The chart number is printed at the top left and bottom right corners of the chart. On standard charts, the title and number are also printed on the back.
Scale The scale is printed near the title.
Positions/datum Charts are drawn to a particular geodetic datum, to which all positions are referred. Older charts may also contain a block entitled “Satellite-derived Positions”. The signicance of these items is covered later in the course. The relevant information is printed in the title block.
Cautions A caution is a hazard which is not described by any particular chart symbol. Usually the hazard applies generally across a substantial area of the chart. Examples include marine farms, military operations, areas which become exceptionally dangerous in bad weather, and depths/channels which change frequently.
Source data There is normally a small chartlet which gives details of the hydrographic surveys which provided the data used to populate the various areas of the chart. This is useful for assessing the reliability of chart data when looking for new dive sites. This topic is discussed more fully in the lesson Finding Dive Sites.
Corrections A record of all the Notices to Mariners corrections that had been applied at the time of sale is shown at the bottom left of the char t.
Tidal levels This is a table showing the mean heights of the tide at various points on the chart.
Tidal streams The Tidal Streams table is used in conjunction with tidal diamonds. It is usually found close to the chart title. Its function is explained in the Tides lesson.
Compass rose Point it out. Its use is covered in Measurement basics.
Smbols and abbreviations Road or Ordnance Survey maps give a coded representation of how the land would look, with its features of towns and roads drawn to a scale. Charts are similar but concentrate on the seabed, coastal topography, particular features, such as potential hazards to navigation.
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Over 1000 different smbols and abbreviations Charts contain a huge amount of information. They can be used to pick interesting dive sites; to see what they are, how deep they are, what the seabed is like, and so on. But the main purpose of charts is to assist mariners in the safe navigation of the seas. This requires an understanding of everything which appears on the chart, not just those things needed to nd dive sites. It is unrealistic to expect mariners to remember every symbol, but you do need to know the main ones, and to know how to nd out about ones you are unfamiliar with.
Chart 5011 The UKHO publish a book entitled Symbols and Abbreviations used on Admiralty Charts. It is normally known as Chart 5011 or simply 5011 for short. Anyone navigating a vessel at sea should have access to a copy and should make themself familiar with its contents. Show a copy.
For this course The following VAs show the main categories of symbols that diver navigators need to be familiar with. They include common examples of symbols within each category, but not the entire set.
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colours
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depths Depths at CD are covered in this lesson but actual depths (ie, including tides) are covered in the Tides lesson.
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buoys
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wrecks
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rocks
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landmarks
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nature of seabed
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tides and currents The symbols relating to tides and currents are covered in the Tides lesson.
For each of the following VAs 7-14, get the students to nd examples of the relevant symbols on the local char ts, show them the relevant pages in 5011, and if practicable get them to use 5011 to nd the meaning of additional symbols in the same category that appear on the chart but not on the VA. [ Include any additional symbols that are important locally (such as trafc separation schemes) either here, or in the Voyage Planning Exercise].
Colours Dr land is shown as a buff colour Areas that cover and uncover with the tide are green Sea is mainl white, with shallow areas either shades of solid blue or edged in blue Point out blue and blue-edged areas on your local charts. V1.2 Copyright © BSAC 2010
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Depths Explain that the depths shown on a chart are at chart datum, which is approximately the lowest astronomical tide, which means that the actual depth will almost always be deeper than shown. Calculating the actual depth taking into account the rise and fall of the tide is covered in detail later, in the Tides lesson.
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146 sounding 14.6m
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15
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----10---- depth contour (isobath) 10m
drying height 1.5m
Isobaths full a similar function to height contours on a land map, except that whereas the latter has contours spaced at regular intervals (eg every 10m on a 1:25000 OS map), isobaths are spaced more closely near the surface than in deeper water, and the actual spacing varies from chart to chart.
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merging contours = wall Mention, and if possible demonstrate on a local chart, how isobaths coming close together or merging can indicate an underwater cliff which might make a good wall dive. The VA shows the excellent vertical wall at Calve Island in the Sound of Mull.
Buoys Colour and shape, light characteristics Point out that this course covers daylight navigation and so lights are not covered. However many buoys are lit, including those students may be using in daylight, which is why the examples on the VA include lit buoys, recognisable by the purple blob. Students do not need to know what the light characteristics mean, but can look them up in 5011 later if they are interested.
Meaning ●
lateral - marks edge of channel Lateral marks (port, starboard, preferred channel) are generally used to mark well dened channels.
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cardinal - marks area to avoid Cardinal marks indicate safe water to the named side of the mark.
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others Other marks are used to show a variety of things such as isolated dangers, safe water in mid-channel and at landfall, and sewage outfalls.
Buoage - lateral [Note that the VAs deal with IALA area A (red to port) which includes the UK and most of the world except the Americas. In IALA area B lateral buoyage is the other way around.].
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red = port green = starboard
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going in direction of rising tide
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going in direction of falling tide
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direction of buoyage symbol used where direction not obvious This symbol is used where the direction of buoyage is not obvious eg, in a narrow channel open to the sea at both ends such as the Solent, and the Menai Strait.
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Cardinal [Although the course deals with daylight navigation only the light characteristics of cardinal marks are shown because they are always the same, and thus easy to identify even without a chart. Lateral mark characteristics, apart from their colour, vary from buoy to buoy.]
Wrecks Explain how the presence of a wreck symbol on the chart does not necessarily mean there is actually a diveable wreck there. Point out the difference in certainty between symbols that indicate specic details such as swept by wire or exact soundings, and those that do not. Draw attention to symbols such as PA and PD in 5011.
Rocks Rock symbols often indicate the presence of reefs and pinnacles which may yield good dive sites.
Landmarks Landmarks are important for visual position xing. The rst six examples are man made objects. The last two illustrate natural features, which can also be useful for position xing, and in remote areas may be all that is available.
Nature of seabed For navigators, the nature of the seabed is of interest mainly to assess the suitability of a potential anchorage. For divers it can suggest what types of marine life might be encountered, and give clues as to the likely visibility.
Abbreviations for material ●
R = rock
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G = gravel
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S = sand
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Sh = shells
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Sn = shingle People often get Sh and Sn confused.
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M = mud, etc
Qualifing terms ●
bk = broken
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f = ne
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m = medium, etc
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Used in combination Material abbreviations are separated by a full stop eg M.S as shown on the VA. Qualifying terms may be included, as in the following example.
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fS.M.Sh? Ask what this combination would signify.
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ne sand.mud.shells
Grid reference sstem Chart makers face the problem of representing the seas of the world on a at piece of paper. As the world is a sphere (for purists it is an oblate spheroid, slightly attened at the poles), transferring its shape to a at surface will always means distortion. However, chart makers have allowed for this by drawing char ts as 'projections' using mathematical formulae. As maps use a grid reference to be able to locate a position, so do charts and these grid lines are referred to as the parallels of latitude and meridians of longitude.
Parallels of latitude Meridians of longitude
Latitude Imagine the inside of the world scooped out except for the equator plane and a rod joining the two poles. This rod passes through the centre of the equator plane and, therefore, the centre of the Earth .
Equator to Pole angle = 90° The angle measured at the centre of the earth to one of other of the poles will be no more than 90° to the equatorial plane.
Parallels of latitude Now imagine a pencil, held at a 50° angle from the equatorial plane at the centre of the earth. Rotating the pencil around this centre point, not changing the angle, would mark a circle on the earth's surface parallel to the equator circle and always at 50°; this is a parallel of latitude.
Equator divides earth into north & south The Equator divides the earth into two, the northern and southern hemispheres.
Longitude To complete the earth's grid system, the meridians of longitude need to be put in place.
Looking down on equator plan from North Pole Imagine standing on the North Pole and looking down through a transparent earth onto the equatorial plane. Radiating lines, like spokes of a wheel can be drawn from the centre to the edge of the equator plane. Circles drawn around the world, through the north and south poles and touching the 14
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equator, divide the world vertically, rather like segments of an orange.
Greenwich Meridian The base line of 0° is an internationally agreed meridian - the Greenwich meridian, which passes through the UK and this divides the world into two. One hemisphere is west of Greenwich and the other hemisphere to the east. Longitude measurements are from 0° - 180° west, and 0° - 180° east.
Latitude & longitude Describe the notations used for expressing positions.
Lat & long angles measured in ●
degrees° minutes.decimal’ (hemisphere) egs, 55° 31.55', 07° 06.8', 005° 59'. It is usual to include leading zeros.
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hemisphere = N, S, E, W It is important, when using the Earth's grid of latitude and longitude - the abbreviation is lat and long - to give complete information to dene a position. Just saying 50° and 30° could mean north of the equator and either east or west; or it could mean south of the equator and either east or west. The latitude position should always be sufxed with either N or S and the longitude position always sufxed with either W or E. Then the correct position is given.
A position ●
latitude = 51° 43.250´ N
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longitude = 002° 59.950´ W
1 degree = 1° = 60 minutes = 60´ Each minute can be subdivided with decimals to an precision required ●
50°30´N 50°30.2´N 50°30.21´N 50°30.207´N 50°30.20669´N However there is no point expressing a position to a greater precision than the accuracy with which it was determined warrants. Three places of decimals in minutes of latitude correspond to a distance of about 2 metres.
Mapping programs sometimes use decimal degrees ●
50°30´N = 50.(30/60) = 50.5° This notation expresses each coordinate with a single signed number (south and west are negative, east and north are positive), which is more efcient for computation in software. It is easy to convert between the different notations, as in the above example: divide minutes or seconds by 60 to get decimal degrees or minutes respectively. Multiply the decimal part of degrees or minutes by 60 to get minutes or seconds.
Degrees/minutes/seconds becoming obsolete ●
51° 43.250´ = 51° 43´ 15˝ In the past, positions were often expressed using seconds instead of decimals of minutes. One minute contains 60 seconds (1'=60"). So 50°30.2’ = 50°30’ 12". This notation may still be encountered in old guidebooks and articles. You will not need to use it on this course.
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Latitude & longitude The latitude and longitude of any point on a chart can be read using scales along its borders. Demonstrate the procedure by stepping through the examples on the VA.
Summary Availability
Scale Chart laout Smbols and abbreviations Latitude & longitude
Eercises Examples of questions and answers for the Chart Exercises practical can be found after the black slide VA23 at the end of this lesson. These exercises are based on Admiralty Charts 2172 Harbours and Anchorages on the South Coast of England and 2611 Poole Harbour and Approaches . See Appendix A Tailoring the course.
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MEASUREMENT BASICS Lesson Objectives This lesson covers the basic techniques and equipment for measuring angles, distance and speed on charts and at sea.
Achievement Targets At the end of this lesson students should: ●
understand how to use a compass rose
●
be able to plot & measure course directions on a chart
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appreciate the difference between true and magnetic bearings
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be able to measure distances from a chart
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know how to convert speed, time and distance
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appreciate the difference between travel over the ground and travel through the water
Additional visual aids Local charts Parallel rule, Portland plotter and dividers Hand bearing compass, several types if possible.
Measurement Basics Outline Direction ●
bearings
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magnetic variation
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compass rose
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measuring at sea
Chartwork instruments ●
drawing on charts
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bearings
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distance
Speed time and distance ●
relationships
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measuring at sea
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Direction There are a few basic terms which students need to be familiar with. Star t to get across the concept that a boat moves through or over the water, which itself may be moving relative to the ground.
Nautical direction terms ●
heading - direction in which the boat is pointing Heading is the direction through the water (ignoring effects of wind, and assuming the boat is moving).
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course - direction in which the boat is (or should be) steered Course is normally a direction over the ground.
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track - direction in which the boat is moving If the track equals course, the boat is on course. The track and the heading may be different if the water is moving relative to the ground.
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bearing - direction of an object Bearing is the term used to describe the direction of one object from another, illustrated in the VA by the relative positions of the boat and the yellow blob (representing an island). Bearing is also used as a generic term for all direction measurements, including heading, course and track.
Direction epressed as an angle Direction is an angular measurement relative to true north. A bearing describes the direction of an object from you in relation to north, measured in degrees true or degrees magnetic.
Bearings Bearings in degrees For navigation, bearings are measured in degrees relative to north, which is dened as 0°. Bearings increase in a clockwise direction, and reset to 0° after a full circle. A bearing in degrees may be expressed to any level of precision required, although a whole number of degrees is more than adequate for small boat navigation.
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relative to North = 0°
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clockwise
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360° = 0°
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precise
Cardinal points Cardinal points (N, S, E, W) and their subdivisions (NE, SSW, etc) are often used to express approximate directions. When bearings expressed as cardinal points are intended to be taken as precise, the prex 'due' is normally used.
●
often used for approximate direction
●
‘due’ N, S etc if precise
Understand both Students need to be comfortable using both notations, and should be able to convert between them easily. 18
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Magnetic North Charts aligned to North Pole North on a chart refers to geographic (true) north: the top of the globe where all the meridians of longitude converge.
Compass points to Magnetic Pole A compass needle points not to geographic north, but to the magnetic north pole, which is hundreds of miles away from geographic north and also moves about slowly. [Technically a compass does not actually point to the magnetic north pole; it aligns with the local magnetic lines of force, which emanate from the magnetic north pole. ]
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angle between true and magnetic is ‘variation’
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different in different places
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changes slowly with time
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currently about 3-5°W in UK
Variation not signicant for us 'Us' means UK divers. Variation is less than the accuracy with which a small boat can be steered using a magnetic compass, so it can be disregarded for small boat navigation. Any errors are swept up by the methods described later in the course. However in other parts of the world variation can be substantial eg, at the south tip of New Zealand it approaches 25°E. See Course Manual appendices for a char t of global magnetic variation and notes on how to work out and use variation. [ Appendix E in this manual].
Distinguish between ●
chart measurements - °T
●
compass measurements - °M
Even though variation is small, it is good practice when recording bearings to state whether they are relative to true or magnetic North, in case the readings are later used for something where precision is important.
Measuring bearings at sea Fied compass ●
shows boat’s heading To take a bearing of an object you would need to point the boat at it.
●
smallest graduation 5° on the model shown, which is a common one. This is commensurate with the accuracy to be expected from these compasses.
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deviation ○
error from nearby ferrous metal, magnets, electrics The loudspeaker magnets in radios and mobile phones can have a large effect on a compass. On large vessels, the compass can be isolated from most magnetic inuences, and a deviation card can be carried, which shows the corrections to be applied to bearings on different headings.
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Bearings corrected for variation and deviation are shown as °C, whereas bearings corrected for variation alone are shown as °M.
○
not usually considered on small boats Although a xed compass should always be mounted as far away as possible from extraneous magnetic inuences, in the cramped and chaotic environment of a small dive boat it is impractical to ensure a constant magnetic environment. For this reason deviation is normally ignored, and any compass errors are compensated by other methods when steering.
Hand bearing compass A hand bearing compass is impractical for steering, but is a convenient and relatively accurate tool for measuring the bearing of objects.
●
used to measure bearing of objects
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smallest graduation 1°
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can distance from sources of magnetism – less deviation
Demonstrate a selection of hand bearing compasses.
Record as °M
Drawing on a chart Charts are expensive and relatively fragile, so it is important to treat them carefully.
Paper charts ●
soft pencil (2B) Anything harder than 2B is likely to leave an imprint on the paper and be difcult to erase. Erasing removes material from the paper, so the less rubbing that is required the better.
●
draw gently
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plastic eraser for corrections
Laminated charts Charts taken to sea in an open boat must be weatherproof. Plastic and/or waterproof charts are available, or you can make copies from paper or electronic originals and laminate them. Copyright restrictions may apply.
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indelible marker pen
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clean with solvent
Eercise ●
draw a line from slipway/harbour to rst waypoint on chart Specify the rst waypoint (after the launch point) (see Appendix A Tailoring the course)
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Measuring bearings on charts Demonstrate the use of each instrument in turn, then get the students to have a go.
Parallel rules ●
line up on along direction of travel
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walk rules to centre of nearest compass rose
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read bearing from true scale
Portland plotter ●
line up along direction of travel
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twist circular scale to line up grid lines with grids on chart, north pointer to North
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read the bearing from plotter
Record as °T Eercise ●
measure the bearing of the line just drawn, enter on voyage plan
Reciprocal bearings Explain the concept of a reciprocal bearing by working through the example on the VA.
Bearing ●
direction of island from boat
Reciprocal bearing ●
direction of boat from island
●
add 180° (or subtract 180° if answer > 360°) ○
90°→270°
○
350°→170°
● your bearing from object This can be important in some circumstances: for example the preferred method of stating your position at sea in a distress call (if an accurate lat/long is not available) is bearing and distance from a landmark.
● your way home For an out and back trip, planning your return passage is simply a matter of working out the reciprocal bearing of each leg of your outward voyage.
Add reciprocal to voage plan Get the students to do this exercise.
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Measuring distance on charts Distances at sea are measured in Nautical Miles. The nautical mile is based on an angular measurement of ‘one minute of arc’. Imagine two straight lines with an angle of one minute between them projected outwards from the centre of the earth. At the surface of the earth the lines would be one nautical mile apart. The equivalent distances in metres and statute miles are 1852 and 1.15 respectively. The border on the sides (not top or bottom) of a chart includes a latitude scale. It is always subdivided into units of 1 minute of arc, and on larger scale char ts it is further subdivided into tenths of a minute. Each subdivision then represents one tenth of a nautical mile so on large scale charts, it is possible to work accurately to one twentieth of a nautical mile or better. Dividers or drawing compass are used to compare a distance between two points shown or plotted on the chart with the adjacent latitude scale, thus determining the actual distance between them.
1 Nautical Mile (nm or Nm or NM or M) = [Several different abbreviations are used for nautical miles. There is no single 'correct' abbreviation. The International Hydrographic Organization uses M while the International Civil Aviation Organization prefers NM. The abbreviations nm, though conicting with the SI symbol for the nanometre, and Nm are also widely used. 'nm' is used throughout this course. ]
●
1´ latitude
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1852m
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1.15 statute miles
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1/10nm = 1 cable (ca) Short distances are often described in cables in nautical publications.
Measuring length of line Demonstrate the procedure.
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span using dividers
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or set dividers to one nm & walk length of line
Convert to distance ●
read off distance in nm from latitude scale
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latitude scale widens towards poles so use scale at same latitude as line The latitude scale widens because of the way the Mercator projection works. On large scale charts the effect is negligible.
Eercise: measure line on chart and add to voage plan
Speed, time and distance Speed Speed at sea is measured in nautical miles per hour. 1 nautical mile per hour = 1 knot.
●
1 knot (kn) = 1nm / hour = 1.15mph
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Explain the relationship between distance, speed and time, and run through the options available for calculating one parameter when the other two are known. Numerate students will nd this topic trivial, but other students may take some time.
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Distance (nm) = Speed (kn) x Time (hr)
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Speed = Distance / Time
●
Time = Distance / Speed
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○
for Time (min) use Speed (kn) /60
○
or Time (min) = Time (hr) x 60
or use triangle, if easier ○
●
cover required item, read the others
or use look-up tables Look-up tables can be found in most nautical almanacs. Tables have the advantage of being easy to use at sea, whatever your level of numeracy.
○
nautical almanac
○
easy to use at sea
Eamples and eercise Run through the examples. If necessary, set additional ones and coach the students who struggle.
Eamples ●
how long to cover 0.5nm at 15kn?
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how far after 30min at 12kn?
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how long for 7nm at 21kn?
Eercise ●
complete details for 1 st leg on voyage plan Add the estimated speed for the leg from the slipway to waypoint 1 and calculate the elapsed time.
Measuring distance & speed at sea For divers, GPS has superseded traditional methods of speed and distance measurement.
GPS ●
distance travelled A GPS unit obtains position xes by using satellite signals, and calculates the distance travelled between xes.
●
calculates speed A GPS unit does not measure speed directly, but it can calculate it knowing the distance travelled in a given time. Typically, the speed displayed on a GPS is averaged over a few seconds.
Ground track v water track This is an important concept to get across, alluded to earlier on VA3 when talking about V1.2 Copyright © BSAC 2010
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course, heading, and track. Click through the diagram, explaining how the distance in 1 hour through water is different from the distance in 1 hour over ground when a tide is moving across the boat's heading.
Summary Direction ●
bearings
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magnetic variation
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compass rose
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measuring at sea
Chartwork instruments ●
drawing
●
bearings
●
distance
Speed time and distance
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●
relationships
●
measuring at sea
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TIDES Lesson Objectives This lesson covers basic information about the nature and causes of tides and explains how to obtain tidal information from tidal stream atlases, charts and tide tables. It explains the hazards that tidal streams create for diver navigators.
Achievement Targets At the end of this lesson students should: ●
be able to extract tidal information from tide tables
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be able to calculate the depth of water at any place on a chart at any time
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be capable of using tidal ow information from charts and tidal stream atlases to estimate the rate and direction of tidal currents
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know how to determine the time and duration of slack water
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know how to choose days when the tides are good for diving
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understand the importance of knowing the depth of water at launch and dive sites
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appreciate the implications of tidal currents for diver navigators
Additional visual aids local charts tidal stream atlas
Tides Outline Tidal terms Depth and tides Tide tables ●
times and heights of HW/LW
●
springs and neaps
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depth of water for launch, recovery, diving and navigation
Tidal diamonds & atlases ●
slack time and duration
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rates for navigation
Tidal hazards
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Tidal terms The blue diagram on the VA illustrates the way the height of the tide changes with time. Explain the following terms:
High water (normally abbreviated as HW) Low water (normally abbreviated as LW) Tidal range Ebb Flood Tidal cycle - the time between consecutive high waters is approximately 12.5 hours. The time between HW and LW, and between LW and HW is usually half a tidal cycle, but this may vary due to local effects. [In some parts of the world, chiey around the Indian and Pacic Ocean coasts, the tidal regime is more complex, and varies from the above pattern during some or all of each lunar month. Instructors in such areas should adjust the material in this lesson accordingly. ]
State - is a generic term used to describe the position in the tidal cycle: HW, LW, ebbing, ooding, half-tide, etc.
Depth and tides Knowing the maximum depth on a site is important for planning when considering the level of divers, diving gas and decompression requirements. Navigators need to know that there is sufcient depth of water for safe passage, and they need to know if and when the boat will dry out when moored or at anchor. The tides are caused by the moon and, to a lesser extent, the sun. The moon's gravitational pull on the ocean creates a bulge of high water with a balancing bulge on the opposite side of the Earth. [ Students may ask why there is a balancing bulge. The maths is difcult, but qualitatively it is because as the ocean nearest to the moon bulges towards it, the gravitational pull of the bulge itself on the ocean at the opposite side of the Earth is reduced because it is further away. ] The sun's gravity also has an effect on the oceans, but although the sun is a more massive body than the moon, it is much farther away and so has a weaker effect. As the sea surface rises and falls with the pull of the moon, it is the Ear th's crust spinning with its skin of seawater that gives an observer standing on one piece of land the impression that the tide is rising and falling. On a perfect sphere, the water would remain almost stationary. But, as the world is not a perfect sphere and with the shape of the land and seabed affecting the way the water distributes itself, the rise and fall of the tides is accompanied by horizontal water movement in many areas of the world.
Spring tides ●
greatest tidal range If the moon and sun are in line, which occurs at the new and full moon, both their gravitational pulls combine causing a big rise and fall of the sea level. These are known as spring tides.
○
Lower LW
○
Higher HW A large tidal range can be expected on spring tides, giving a high HW level and a low LW level.
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Neap tides ●
Least tidal range If the moon and sun are at right angles to each other, the sun's small gravitational pull conicts with that of the moon and reduces its 'pull' effect on the sea. The resulting rise and fall of the sea is known as neap tides.
○
Higher LW
○
Lower HW
A smaller tidal range can be expected on neap tides, giving a lower HW level and higher LW level when compared to spring tides.
Abbreviations ●
MHWS MLWS MHWN MLWN Published information about tides often needs to distinguish between springs and neaps, usually referring to mean levels. The following abbreviations are used: Mean High Water Springs, Mean Low Water Springs, Mean High Water Neaps, Mean Low Water Neaps. Check that the students can put them in the correct order of height. Point out that since these are means; actual tides may be higher or lower.
Tide tables Tide tables are used to nd out tidal heights and times. You can nd them in almanacs, where they are likely to cover the entire UK coast for a calendar year. You can also buy cheap booklets from chandlers and marinas, covering the local area for a year. Tide tables covering the next few days are published in some newspapers, and tidal information may also be found on the internet although some of it is of unknown accuracy. Some electronic navigation systems also display tidal information.
Tides are predictable ●
calculated for standard ports Tide tables are computed and published for selected locations around the country, known as standard ports.
●
differences for secondary ports Almanacs and local tide tables usually contain a list of corrections which can be used to nd tide times at other 'secondary' ports adjacent to each standard port. For example, HW at Menai Bridge occurs 28 minutes before HW at Liverpool, a standard port. The correction or 'tidal difference' is -0028. [Some almanacs also publish corrections for tidal heights at secondary ports, but these are outside the scope of the course ]. Use the tables for the nearest available port, unless directed otherwise [eg tidal diamonds, below ]. Similarly, using the Time Differences table in the Course Manual [and in Appendix C of this manual], you can nd HW times at any standard port in the British Isles from HW Dover, which is normally quoted in national newspapers. For example, the tidal difference of Whitby on Dover is +0500, ie 5 hours after Dover. Tide times calculated in this way can vary from the actual predictions for the port in question by a few minutes.
●
usually Universal Time (UT), add 1hr for British Summer Time (BST) Tide tables are usually published in UT (formerly known as GMT). However, this is not always the case, so it is important to check.
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Tide tables give the time and height on every day in the year, of HW and LW heights are given in metres.
Spring & neap tides Looking at the tide tables will show when spring and neap tides occur. Some tide tables include small moon symbols against the date to indicate spring tides. Otherwise, springs and neaps can be recognised by looking at the heights of the tides and applying the classications from the previous VA:
●
Springs ○
Higher HW
○
Lower LW
The highest high water and lowest low water heights indicate spring tides
●
Neaps ○
Lower HW
○
Higher LW
The lowest high water and highest low water heights indicate neap tides
Tide table eercises Go through the exercises on the VA with the students.
Eercises ●
we need a neap tide to dive a wreck. Which of the three weekends would you recommend and why? 29th – smallest tide
●
if we had to dive at LW on 21st July on which tide would you plan the dive? 11:00 – dark at 23:19 [Unless a night dive is required..........]
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what will be the states of the tide at 14:00 and at 21:00 on 22nd July? 14:00 ood 21:00 ebb
●
what will be the tidal range on the morning of 22nd July? 5.6m
●
are the times shown BST or UT? The information given does not specify: you would need to check, and add an hour if the table is in UT.
Calculating depth This VA covers how to calculate depth at HW and LW.
Charted depths Remind the students of the material covered in Chart basics showing how depths are indicated on charts.
●
depth marked Depths are marked on a chart as depth 'soundings' in metres.
●
lowest predictable level The soundings show the depth at chart datum, the lowest astronomical tide. [Tide levels can vary from astronomical predictions due to weather conditions. In
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extreme weather tides may be a metre or so higher or lower than predicted ].
●
does not include the water above chart datum - tides However, the depth at chart datum does not include the water that will usually be above it due to the effects of tides and different tidal ranges. To nd out the expected depth for a dive on a particular day, requires the charted depth and tidal variation to be combined.
For example, the charted depth of the seabed is 24m. Looking at the tide tables for the day, they show 0800 LW as 1.5m, 1405 HW as 5.5m. The difference between LW and HW, the tidal range is 4m. From the tide table information at LW there will be 1.5m of water over chart datum. So, at low water, the actual depth will be 25.5m. At HW, there will be 5.5m of water over chart datum. So, the actual depth will be 29.5m. This shows the importance of planning. For example, If the dive was planned just on the chart datum, it would seem ideal as a site for a new Sport Diver's rst dive to build their depth experience with a planned maximum depth of 24m to the seabed. But diving the seabed at LW would break the dive plan by 1.5m and, at HW, by 5.5m A second example of why depth planning is important is that of Nitrox divers. They will be carrying a pre-mixed gas with a MOD selected to be compatible with the planned depth. Getting out to a site and nding, from the echo sounder check, that the site is deeper because the effect of tides has not been included, could make a dive manager extremely unpopular. Depth planning is also of obvious importance to navigators: they can check that there will be sufcient water to pass safely over submerged hazards; and for harbours and slipways which are only useable at certain states of the tide, they can check that it will be possible to get in and out at the required times.
Rule of Twelfths So far we have shown how to calculate the depth at HW and LW. This VA shows how to calculate the depth at intermediate times using the Rule of Twelfths, an easily applied rule of thumb which gives an approximation to the rise and fall of the tide without needing extra documentation and is therefore easy to use in any situation including small open boats. Go through the example on the VA. Tide tables indicates that LW is at 0630 0.5m, HW is at 1245 5.3m. The tidal range is therefore 4.8m. To use the Rule of Twelfths, divide the tidal range, 4.8m, by 12. One twelfth of the tidal range = 0.4m.
The planned dive shows 24m at chart datum (CD) and the dive time is planned for 1030. Low water is 0.5m above chart datum = 24.5m. High water is 5.3m above chart datum = 29.3m.
The Rule of Twelfths is applied as follows: In the rst hour from LW the tide rises by one 12th: 24.5m + 0.4m = 24.9m In the second hour, the tide rises by two more 12ths: 24.9m + 0.8m = 25.7m In the third hour, the tide rises by three more 12ths: 25.7m + 1.2m = 26.9m In the fourth hour, the tide rises by three more 12ths: 26.9m + 1.2m = 28.1m V1.2 Copyright © BSAC 2010
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In the fth hour, the tide rises by two more 12ths: 28.1m + 0.8m = 28.9m In the sixth hour, the tide rises by one more 12th: 28.9m + 0.4m = 29.3m HW Planning the dive at 10.30 shows the anticipated depth is 28.1m. Put another way, at 10.30, in the fourth hour there is a total of 9/12 of the tidal range to add to LW 24.5m+9 x 0.4m (3.6m) = 28.1m. The same procedure can be used to determine the depth between HW and LW. [In certain areas, such as around the Solent, and wherever accuracy is critical, it may be better to use tidal curves. Tidal curves are not taught on this course, but details may be found in Lesson AT1 of the BSAC Advanced Diver course, and in nautical almanacs. There are now several proprietary software packages (eg Belleld Tidal Plotter, available on the BSAC website), and some websites (eg Admiralty EasyTide ), which generate tide tables and calculate tidal heights at any state of the tide. They are likely to make the manual use of tidal curves a thing of the past. ]
Depth at an time Using a chart and a tide table we can calculate the depth at HW and at LW. By using the rise of tide worked out with the Rule of Twelfths we can calculate the depth at any time. Step through the VA, explaining each point.
●
CD + LW Tide tables give the height of the tide above CD at LW.
●
CD + LW + rise of tide Adding the rise of the tide from the Rule of Twelfths gives the height of the tide at any time.
●
CD + HW - fall of tide We could also calculate the height of the tide at any time by nding the height of the tide at HW from tide tables then subtracting the fall of the tide worked out with the Rule of Twelfths. Either method gives the same result. It requires slightly less arithmetic to apply the Rule of Twelfths if you use whichever of HW or LW is nearest to the time for which you wish to know the depth.
●
height of tide The height of the tide above CD is calculated using information from tide tables. To nd out the actual depth we need to refer to the chart and nd the depth symbol nearest to the point required.
Actual depth = height of tide above CD...... ●
plus charted depth Depths are below CD, so simply add the charted depth to the height of tide above CD to get the actual depth.
●
minus drying height Drying heights are above CD, so subtract the drying height from the height of tide above CD to get the actual depth.
●
drying height > height of tide = above water If the drying height is greater than the height of the tide it means that the point is above water at the time in question. Subtract the height of the tide from the drying height to nd out by how much.
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Depth calculation eercises Direct the students to do the rst four exercises using the tide table extract on the VA. Students should assume that all times are BST, for the purposes of the exercise (see Appendix A Tailoring the course). Go through the answers. Set the nal question, which involves updating the voyage plan with local information, from the relevant tide tables. If appropriate, demonstrate how using a secondary port can give improved accuracy. Students must specify BST or GMT on the voyage plan.
Tidal ow In tidal waters, charts show the speed and direction of currents that can be expected in the general area at hourly intervals throughout the rise and fall of the tide. Tidal ow information is of interest for several reasons: many dive sites can only be dived when the tidal ow has stopped; dive managers need to know the strength and direction of the current if a drift dive is to be planned safely; the strength and direction of the current may affect the navigation of the boat; and some areas may become dangerous at certain states of the tide. Calculated slack water times are notoriously imprecise, and actual slack may occur before or after the calculated time. Divers are a dvised to get to sites early.
Time of least water movement ●
slack water Tidal currents usually ow in one direction for a few hours and then reverse and ow in the opposite direction. The point when the rate of tidal ow is minimal is known as slack water. It usually occurs close to the time of HW or LW, but local geographical features can bring about slack water as much as halfway between HW and LW. Slack water is not a momentary point in time, but generally represents a period during which there is negligible water ow. For diving, ‘slack’ generally refers to the period when the current is half a knot or less. [There is no precise denition, and the tolerable amount of current may also depend on the capability of the divers and the nature of the dive]. The length of this period is known as the duration of slack.
●
longer slack at neaps The bigger the tidal range, the more water there is to move about in the same time. Hence tidal streams are stronger on springs and weaker on neaps, and slack water is longer on neaps and shorter (or non-existent) on springs.
Tidal diamonds The diamond shape with the letter A on the chartlet is known as a tidal diamond (proper title "position of tabulated tidal data with designation"). It is a pointer to a table elsewhere on the chart which lists the tidal ow at the position of each tidal diamond. The nearest tidal diamond to your point of interest is usually the best, but this may not necessarily always be the case, especially around the entrances to harbours / river estuaries.
Table on chart The tidal stream information is given in 4 columns.
●
hours before and after HW The rst column gives the hours before and af ter HW (this is HW for a reference
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port, not necessarily the nearest standard port, which may be some distance away, so high water times may be considerably different), so the time of HW needs to be conrmed using tide tables for the day of the dive.
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direction of ow The next column indicates the direction of ow of the tidal stream as a bearing relative to true north. The direction given is that which the tide is owing to.
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speed on spring tides The next column indicates the speed (aka rate) of the ow in knots on springs.
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speed on neap tides The last column indicates the rate in knots on neaps.
The greatest tidal ranges occur on springs so the rates are faster than on neaps, when the least tidal ranges occur. The rates shown refer to mean spring and mean neap tides. Using the table on the VA, show how to determine slack by locating the periods when the current is least, and the periods either side when the current is 0.5kn or less. Use the Sp or Np columns as appropriate. Point out how the ow changes direction during this period. On springs, the least water movement is 5 hours before HW at 0.2kn, or with a bit of current, 0.5kn, 2 hours after HW. On neaps tides there is a slack 5 hours before HW and quite a large slack window from HW to 3 hours after. Away from mean spring and neap tides, rates will be faster or slower from those shown in the table. They can be interpolated or extrapolated. For example, on a day half way between springs and neaps, take a rate half way between the gures given. Demonstrate this idea at HW, where the spring rate is 1.3 and neap rate is 0.6, making half way 0.95: say 1 knot.
Tidal stream atlas Charts are not the only source of tidal stream information. Tidal stream atlases are often a useful source, not least because they can convey much more information in a single glance than detailed examination of the tidal streams tables on a chart, especially in conned waters.
Set of diagrams showing currents in area ●
Admiralty publications, and in almanacs A set of UKHO publications covers the whole UK coastline. Small scale chartlets cover large areas where there are few local variations, and inset or special chartlets cover smaller areas with strong and localised tidal streams. The example of Portland Bill on the VA is one such local area.
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12 or 13 diagrams cover tidal cycle
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time relative to HW Standard Port
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arrows indicate strength & direction
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numbers show speed in 1/10kn
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2.5kn neaps, 5.0kn springs
Show the students the tidal stream atlas for the local area.
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Tidal stream eercises Direct the students to do the exercises and then go through the answers with them (see Appendix A Tailoring the course).
Tides and currents smbols Flood tide stream Ebb tide stream The above two symbols may be found on some charts, showing the direction of tidal ows, and optionally the mean spring rate, at specic points on the chart.
Overfalls Overfalls, tide rips, and races occur where strong tides cause surface disturbances, with the possibility of large and/or breaking waves which are potentially hazardous to boats.
Eddies Eddies occur where strong tides cause horizontal turbulence, forming vortexes which have the potential to throw a boat violently off course.
Implications of tidal ows Focus the students' attention on the implications of tidal ows for them as navigators, thus putting the calculation methods taught earlier into context. There are some things that they need to be able to get right quantitatively, and other areas where they need to be able to identify potential hazards. Navigators should discuss the diving issues with the dive manager.
Slack water diving As navigators of a dive boat, students need to be able to get to slack water dive sites in plenty of time for the dive to take place safely. This means that they must know how to work out when slack water occurs and how long it will last. Then they can work backwards from the time of slack to produce a voyage plan that will get them there on time. The remainder of the course will show them how to do that.
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speed of tidal ow < 0.5kn
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not always at HW or LW Apart from getting the timing right, it is important to know which way the current will be running before and after slack. This helps with determining whether you are before or after (no tidal predictions are 100% accurate), and helps the dive manager to arrange appropriate cover for the divers.
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tidal diamonds and atlases fairly reliable offshore
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localised effects around islands, headlands and channels Additional tidal stream information can be found in pilot books, sailing directions and dive guidebooks (which generally give slack water times only). Close inshore tidal streams can be extremely localised, and eddies and counter-currents can occur. Currents are usually strongest around headlands, in deep water, and in narrow channels. These localised effects are not always well documented so accurate local knowledge is the gold standard for information.
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Drift and current-free diving Not all dives require slack water. For drift dives, the issues include: selecting a time when the current speed and trend (picking up or slowing down) will be appropriate for the skill level of the divers involved identifying the direction that the current will be owing in, and checking that it will not take the divers into hazardous areas such as turbulent water in which they cannot control their buoyancy, or into excessively deep water. Current -free areas, perhaps required as easy sites for beginner divers, or as backup sites that are free from slack water constraints, can sometimes be identied from charts. Knowing the direction of the tidal streams it may be possible to spot areas that will be out of the tide. The same technique can be used as a sanity check on dive sites recommended by others if you are not sure how reliable the information is. It is also important to be aware of nearby areas where there are currents which could cause divers to be lost at the surface or taken into places the boat cannot reach, so that appropriate precautions can be taken.
Navigation ●
effect of tide on speed and direction Strong tides will affect a boat's speed and may deect it from its course unless action is taken. How to deal with this is covered later in the course, but the rst step is to do your chartwork and work out what tidal streams will need to be dealt with.
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hazards – turbulent water When tides ow rapidly over an uneven seabed or through narrow channels, turbulence can develop. This can take the form of overfalls, tide races, eddies and whirlpools. They can contain confused water and short steep seas, dangerous to navigation because of the risk of capsize or swamping. Such hazards are normally identied on charts so that they can be avoided. They may only become dangerous at certain states of the tide, and are usually safe around slack water. It is important to be able to work out at what times they must be avoided, and to plan the voyage accordingly. Seek local information where necessary.
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wind over tide When a strong tide runs in the opposite direction to the wind it has the effect of steepening the waves, sometimes to the extent of causing them to break. This effect is particularly severe where the tide is running over uneven ground and overfalls are present. The stronger the wind and the faster the tide, the greater the effect. These conditions can be dangerous for boats. In contrast, if the tide is running with the wind the size and steepness of the waves is reduced. It is important to know the direction and strength of the tide at all times when planning a passage and whilst at sea, so that hazardous areas can be avoided.
Enter details of tidal hazards on the voage plan Direct the students to look over the local charts, and tidal stream atlas if appropriate, and note any tidal hazards on their voyage plans. If the charts cover a much larger area than will be visited during the practical, tell them which parts of the chart to concentrate on and which to ignore.
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Summary Tidal terms Depth and tides Tide tables ●
times and heights of HW/LW
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springs and neaps
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depth of water for launch, recovery, diving and navigation
Tidal diamonds & atlases ●
slack time and duration
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rates for navigation
Tidal hazards
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POSITION FIxING METHODS Lesson Objectives This lesson introduces the concepts of position lines and xes, and describes their uses. It covers a variety of methods including visual xes, bearings, GPS, radar, and echo sounders. The strengths and weaknesses of the different dif ferent methods methods are described.
Achievement Targets At the end of this lesson students should: ●
understand what a position line is
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know the difference between a transit and a bearing
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know how to record a set of transits
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understand what a x is
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be able to read position lines from a chart
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know how to read position lines at sea
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appreciate the limitations of visual xes
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appreciate the abstract nature of a GPS x
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appreciate the advantages of using GPS
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understand the limitations of GPS
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know the importance of geodetic datum
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be aware of alternative position xing methods
Position Fiing Methods Outline Simple Position lines ●
compass bearing
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transits
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uses of position lines
Position Visual accurac GPS Other methods
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Simple Simple The photograph is an example of a simple x. Get the students to suggest others.
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close to you
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xed & clearly identiable
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shown on chart
Ensure object is correct one This is the main precaution necessary when using simple xes. A useful check is for the navigator to ask another member of the crew to describe the object. For example the navigator might ask "what name is written on the buoy?", and avoid telling them what to look for. This gets over the problem that we often see what we are expecting to see, not what is actually there.
Detour for simple often better than b other means Navigation by following following a series of simple xes is sometimes called pilotage.
Is GPS position a simple ? The VA shows a typical chart plotter screen, where the ashing pink blob shows the current position on a chart. Ask the students the question.
No, a lat/long is not clearly identiable The answer is no, because a lat/long is not a clearly identiable object in the real world, and the electronic chart on which it is displayed is not part of the real world either. A GPS equivalent of a simple x would require the pink blob to be ashing on the water itself!
Position lines Lines that can be identied at sea
and on on
a chart
Tpes ●
compass bearing [3 on-click examples]
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transits [3 on-click examples]
[There are also position circles (eg, the distance off an object lies on a circle) but they are outside the scope of the course ].
Transits Two identiable objects in line with each other Transits can be obtained from a variety of different sources.
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taken from a chart to use at sea
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recorded at sea for later use
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obtained from books or divers with local knowledge
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Their main characteristics are:
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can be very accurate ±0.5°
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not visible at distance, at night, in fog or in mist
Choosing transits “Fast” Use examples in and around the classroom to illustrate the following points. Make sure that each student actually sees a transit in operation.
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objects wide apart
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near mark close to observer Make sure that there is some horizontal distance between the objects: for the greatest accuracy, the distance between you (the observer) and the nearest object should be less than the distance between the two objects.
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easy to see if lined up
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sensitive to sideways movement
If necessary, emphasise the value of fast transits by demonstrating a slow one.
Suitable landmarks ●
easily identied Landmarks need to be chosen so that they can be distinguished from their surroundings. For example, if the landmark is one of a group of houses or static caravans, you may need to record distinctive detail such as position (egs '8th row from the left', 'immediately right of the road'). They also need to be chosen so that the parts of the landmarks which are to form the transit are distinct and have clear outlines which are easy to line up. Vertical edges which are sharply dened are good such as edges of buildings, pylons, poles and clefts in rocks, as are sharp points such as spires. Landscape features such as summits and dips in the skyline must be sharply dened: if it is unclear exactly where the top of a peak or the bottom of a dip lies, the transit will not be accurate. Avoid considerable height differences between marks.
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long lasting How long is long depends on the purpose to which the transit will be put. For one-off use such as on this course, even a parked car might be a sufciently long lasting object. In general, natural landscape features such as headlands and islands are the longest lasting. All man made objects are subject to change over time: they may be repainted or altered, extended or demolished. New structures may be erected which obscure existing ones. Even natural features are subject to change through landslide, erosion, and so on.
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xed to the spot Some permanent objects may move around over time. Select ones that do not, such as the following.
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○
headlands
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conspicuous buildings
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easily identiable rocks etc
avoid vegetation, buoys and tide dependent marks Vegetation may undergo dramatic changes in appearance as the seasons change. Buoys may be attached to the seabed by a chain which allows them to
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move around with the wind and tide (however navigational marks which stand on the ground make good transit marks). Conspicuous rocks may be covered at some states of the tide, and vertical edges may not be vertical throughout the tidal range.
Recording transits Reiterate the principal requirements of a good transit.
Conspicuous Permanent Precise Well-recorded If transits are not recorded properly, they will be unusable. This is especially important when transits are recorded by one person but used by another. A properly recorded transit (the example shown is for the Lucy wreck in Pembrokeshire, from an old BSAC Wreck Register ) should have the following components.
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sketch A sketch for each transit, showing how it looks when on site (only one transit is sketched on the VA). A useful technique is to indicate exactly what is to be lined up with what by a vertical dotted line, unless it is obvious. [click].
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bearing The bearing of each transit should be shown so that the user knows roughly what direction to look in. The transit line on each sketch could be annotated with its bearing, [click] or all the transits can be shown together. Bearings need not be precise since they is not being used for position xing. In the example, the bearings are implied by the position of the lines drawn on the north-up chartlet.
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context The objects used to form the transits should be shown in relation to their surroundings. The navigator needs to be able to relate what can be seen looking 360° around the boat to the position of the transit objects. In the example the authors have provided an overall chartlet and sketches for each individual transit. Where identication of a transit object is not obvious, such as when a building is one of several in a group, the sketches and/or chartlet should show sufcient of the surrounding context to make identication unambiguous.
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narrative Each transit should be described in words, including precise identication details of each object and an accurate description of the way in which the objects are aligned.
Uses of position lines Clearing line A clearing line is a useful navigational tool used to keep a boat in safe water when passing hazards such as dangerous rocks. On one side of the line is safe water, on the other lies danger. The example shows a clearing line preprinted on the char t (see if students can spot the second one). Navigators could also dene their own. Clearing lines are also useful for giving a simple visual reference as to when it is safe to make a turn. V1.2 Copyright © BSAC 2010
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Step through the example. The nal chartlet shows what the navigator might see as the boat is driven along the clearing line. Referring back to the idea of transits, ask the students how the view would change if the boat was steered towards danger, and how it would change as it was steered away from danger. [Further instruction on the idea of opening and closing marks is given in Voyage Planning ].
Leading line Leading lines are used to guide a boat along a safe course through water with hazards either side. Unlike a clearing line, the boat must stay on the line rather then just keep to the safe side of it. The example uses a bearing of an object to dene the position line. In other situations, such as leading lines for entry to a harbour, a transit on specially placed leading marks and/or lights may be used. Ask the students why a bearing is necessary in the example. Why could you not just steer towards the beacon?
Fi ●
an intersection of 2 or more position lines
The intersection of two position lines denes a point. This is known as a x. Fixes are very useful because they enable navigators to locate their position on the chart and on the ground, and for divers because xes can be obtained to locate interesting features like wrecks and reefs.
Fi Run through the points on the VA.
Intersection of 2 or more position lines denes a point ●
bearings
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transits
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a mixture of transits and bearings
The only useful type of visual x for locating dive sites uses transits only. Bearings are not accurate enough for xing the position of anything as small as a wreck or a pinnacle.
Eercise: 'Mster Point' [Mystery Point is a position that students must navigate to on the practical day using transits they have taken off the chart. See Appendix A Tailoring the course. ] Instruct the students to nd transits from the chart which will enable them to nd this position. They must record the transits in a format that they will be able to use at sea.
Visual accurac Errors ●
all position lines have errors
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error effect increases with distance The best transits have an error of about ±0.5°. Compass bearings taken from a
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small boat achieve an accuracy of about ±5°. Point out on the table on the VA what this means in terms of the potential position error at various distances from the objects used to form the position line. The error distance shown corresponds to the distance between the red arrows in the error diagram at the top right of the VA: it is the total not a ± distance. Eg for an accuracy of ±5° the error is the distance subtended by +5 and -5 = 10° at the range shown.
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area of uncertainty When two position lines cross, their respective errors combine to form the shaded area of uncertainty shown in the diagram.
Improving accurac ●
position lines at right angles This minimises the area of uncertainty.
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use transits if possible
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extra position lines show area of uncertainty
The shape formed where three position lines intersect (they are unlikely to meet at a point) is called a 'cocked-hat'.
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boat movement during x A common mistake that navigators make when taking xes at sea is to forget that the boat is moving all the time under the inuence of the wind, the tide and its own momentum, even when the engine is in neutral. You must take xes quickly, and if necessary keep returning to the star ting point to ensure that each position line relates to the same position.
What accurac needed? Clearly, a dive site such as a small wreck well away from land needs an accurate position x or else the entire dive trip may be wasted. But it is worth remembering that not all xes need be precise.
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clearing lines Provided the marks for a clearing line are easy to use, and it is set to keep the boat well away from danger, it need not be particularly accurate.
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intermediate points In waypoint navigation (to be covered in detail later) a voyage is built up from a number of legs, which means that a number of intermediate points are dened at which the boat is steered from one leg on to the next. If these intermediate points are in clear water well away from dangers there is no need for a high degree of precision in determining when they have been reached.
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ease of navigation Sometimes it is helpful to identify a series of landmarks that should be passed during a voyage, to give you a simple check that you are on course. For example over a 5 mile passage you might expect to pass a lighthouse, followed by some tall masts on a headland, followed by an east cardinal buoy a mile or so out to sea. None of these plays any part in shaping your course, except to give you reassurance that you are heading the right way, so there would be no point in xing their positions exactly.
One of the skills of a good navigator is to be able to apply precision when it is needed, but another is to not waste time and effort applying it when it is not needed.
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GPS GPS is a superb system now used almost universally for navigation at sea, though as we shall see there are reasons for not relying on it totally.
Global Positioning Sstem Briey describe how the system works. Many students may have GPS navigation devices of their own in cars, on mobile phones, or in portable units for walking and cycling, etc. GPS consists of about 30 satellites in highly stable orbits around the Earth at an altitude of about 20000km. It was developed and installed by the US military, and is still operated by the US government. Each satellite sends out a signal giving its identity, precise details of its orbit and the exact time.
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receiver determines position using signal from 4+ satellites The receiver contains navigation software which needs a signal from four or more satellites to work out its own position. The software uses the information from the four signals to determine 4 unknowns - three spatial dimensions, and time. Normally a GPS receiver will be able to see 6 or more satellites at any one time. The more satellites, the more accurate the position. [Instructors requiring further information about how GPS works will nd all they need, and more, on the internet. The tutorial on the Trimble website is a good start. ]
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augmentation systems EGNOS/WAAS There are various sources of error inherent in GPS (outside the scope of this course). To overcome these, and in particular to make GPS more useful for aviators, satellite augmentation systems have been developed. In the USA, and generically, this is called WAAS (Wide Area Augmentation System), and in Europe it is called EGNOS (European Geostationary Navigation Overlay Service). They consist of ground-based stations whose position is known very accurately, which receive normal GPS signals and calculate the difference between their GPS position and their actual position. They then transmit corrections to special satellites, which re-broadcast the corrections to normal GPS receivers. All modern GPS receivers are WAAS-enabled. [There are 3 EGNOS satellites, in geostationary orbit over the equator. They are low in the sky in the UK, which can cause reception problems when close to high ground. Differential GPS is a similar augmentation system which broadcasts corrections by radio rather than via satellite. For most users, including divers, differential GPS has been superseded by WAAS.]
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GPS set displays lat/long
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calculates distance/bearing to any other lat/long Point out that GPS will give a distance/bearing regardless of the fact that this may be straight through a headland or across land.
24/7 all-weather availabilit Accurate and eas to use Accurac ●
±15m and ±3-5m with EGNOS Specifying the accuracy of GPS is actually rather complicated: it depends on how it is dened and on how it is measured. For example, any quoted gure will only apply for a certain percentage (eg, 95%) of the time. But what does that mean? Those interested should consult the extensive technical documentation that is
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available on the internet. Everyone else, whilst remembering that manufacturers tend to use the most favourable measures to describe their products, should understand that used properly, GPS is more than adequate to locate any dive site for which an accurate position is known.
●
= good transits at 1nm and 400m [Note: in terms of the table on VA10, ±15m corresponds to an error of 30m and ±3-5 to 6-10m.]
Also easy to misuse! It is easy to make mistakes, and not always easy to spot them. GPS is not idiot-proof.
GPS limitations Although GPS is a fantastic system, which no dive boat should be without, there are some important things users must understand in order to use it safely and effectively.
Lat/long is just a number..... If we describe positions using visual xes, it is easy for us to picture in our mind where things are, and how they relate to other features in the area: 'the wreck is about 50m W of that red buoy just outside the breakwater', 'follow the red and green channel markers for about two miles then you will see a big white sign marking the entrance to the marina', 'we're 2 miles due south of Jackson's Point', 'we're in the George and Dragon, upstairs by the window'. In contrast 'my position is 53°24.237′N 002°19.419′W' means nothing unless it is plotted on a chart. Lat/long is just a number, ideal for computation in GPS units and chart plotters, but unfortunately lat/long on its own means little to most humans. This has two implications. Firstly, it means that it is par ticularly difcult to spot mistakes in a lat/long, which means that we have to take precautions to guard against errors. Secondly, because it is impracticable to plot positions on a chart in a small boat (unless you use an electronic chart system which does it for you), nearly all the navigation for a small boat has to be prepared before going to sea.
Geodetic datum Even for a GPS unit, a lat/long is not quite enough to display a position precisely. It is necessary to state the geodetic datum to which the position applies.
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Earth not an exact sphere The Earth is not a perfect sphere but is slightly attened at the poles [illustrated by the squashed orange]. Its shape is not constant either; it changes slowly due to the movement of tectonic plates and other geological processes. To cater for this, positions are referred to a mathematical model of the Earth's surface, called a geodetic datum, in order to calculate a specic latitude and longitude for a particular point.
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different chart makers use different models There are many different geodetic datums in existence, some produced in the past when the Earth's shape was not known as precisely as it is today, and others produced to emphasise regional features at the expense of global ones: all geodetic datums contain some compromises. Every chart is drawn to a particular geodetic datum, which will be identied somewhere on the title block. Point out the datum on one or more charts. All new charts are drawn to a datum called WGS84 [actually UKHO charts are being drawn to ETRS89, which is compatible with WGS84. ] However there are still many charts in use drawn to different datums. By default, GPS units display positions referenced to WGS84, but most units can
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be set to use any of a large range of alternatives. For the last 20 years or so, charts drawn to older datums have included instructions for converting positions between WGS84 and the chart datum, in a block entitled Satellite Derived Positions. Point out a block on a chart. Avoid the need to make conversions by using the latest charts wherever possible
●
signicant differences [The distorted coloured triangles on the orange on the VA illustrate how points move depending on the exact shape chosen to represent the Earth’s surface }. The errors introduced by using the wrong datum are signicant. The diagram shows how the apparent position of what is actually the same point on the Earth's surface varies using three commonly encountered datums. WGS84 is the modern standard; OSGB36 was the previous standard for Admiralty charts (UKHO is converting all its older charts but had not nished the process in 2009); and ED50 is the previous standard used for many European charts.
Limitations [Mention some in-car GPS navigator stories if you have them egs, cars driving into a river which was not shown on the map; people entering the wrong postcode and ending up miles from their intended destination, having failed to notice where they were going .]
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must use correct datum Using the wrong datum could easily put you over 100m from where you think you are, which could lead the boat into danger, and could also result in you failing to nd your dive site.
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garbage in garbage out GPS units rarely validate the positions entered into them. If you make a keying error, or enter a position that is otherwise incorrect, the GPS will happily accept it and process it.
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large errors if satellites obscured GPS accuracy declines if the unit can only see the minimum number of satellites. Out at sea in an open boat this is never a problem, but close up against high ground or harbour walls it can be. GPS units will not work properly if the aerial is indoors: this could be a problem if you are trying to use a por table unit inside a large boat.
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relies on accurate chart Provided the above points are addressed, a GPS unit will provide an accurate lat/ long referenced to the chosen datum. For many purposes, the lat/long will then be displayed on a char t, either manually or electronically. How well the resulting picture mirrors the boat's actual position relative to its surroundings depends on how accurate the chart is. Although modern charts are based on recent and accurate surveys for areas that are heavily frequented by shipping, some of the areas that divers operate in, close in amongst rocks for example, are areas that most chart users avoid and they may not have been surveyed for many years. Charts can have signicant errors in such places, and an electronic display may show you in safe water when you are not. In these areas it is prudent to navigate using the echo sounder and the mark one eyeball.
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can become inoperable By denition GPS navigation relies on having the equipment available and working. GPS units can and do become inoperable for many reasons. Some risks can be mitigated by carrying a spare unit, perhaps a cheap handheld device, but others are system wide. Prudent navigators should always be able to navigate their boats to safety using traditional methods. A good dive boat navigator will
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also be able to nd a selection of dive sites this way as well, though it is likely that the loss of GPS and/or the echo sounder will rule out most offshore sites.
○
electrical faults Loss of power supply, aerial problems, wiring faults, internal faults, corrosion
○
damage, theft Water ingress, hit by dive equipment or falling bodies, clumsy handling, vandalism, theft
○
military operations GPS is ultimately a military system. Sometimes military exercises may involve jamming of signals locally, or reduction of accuracy for civilian users.
Other methods GPS and visual xes are the prime methods of position xing, but the following are useful additions.
Depth soundings ●
checks other x if deeper or shallower water nearby If the expected position is in an area close to other areas that are signicantly different in depth, seeing the expected depth on the sounder provides conrmation that you are where you should be. If the depth is similar for miles around, it proves nothing.
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get-me-home in fog In poor visibility, with no GPS and no visual xes, it may be possible to navigate by following a depth contour along the edge of a steep slope.
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crude position line where depth changes sharply The previous example involved using the position line marked by the edge of a steep slope as a kind of leading line. This technique involves crossing such a position line. When the depth changes abruptly, you know you have crossed it and, assuming it was what you had expected, your (rough) position is conrmed.
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apply tidal height and transducer offset if depth variation is small If applying the above methods where the change in depth between 'deep' and 'shallow' is about the same as the tidal range or smaller, apply the tidal range (and on larger boats transducer offset if appropriate) so that you know the expected depths more precisely.
Radar Radar is a superb tool for larger boats, particularly in poor visibility when it compensates for the restrictions on visual navigation. It is sometimes tted to small open boats, where its benets may be rather limited.
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range and bearing of land It is possible to obtain a position x by measuring the range and bearing of a conspicuous radar target such as a headland. The bearing gives a position line, and where the distance off intersects it gives the x.
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height of aerial problem for small boats Radar works on line of sight. The lower the aerial the shorter the distance the radar can see, particularly in bad weather when waves and heavy rain may
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cause clutter on the screen. The impracticality of mounting the heavy aerial unit sufciently high rules it out as a cost effective option for most small boats.
Summary Simple Position lines ●
compass bearing
●
transits
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uses of position lines
Position Visual accurac GPS Other methods
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FINDING DIVE SITES Lesson Objectives This lesson looks at the specic problems of locating dive sites, and describes how they can be overcome.
Achievement Targets At the end of this lesson students should: ●
appreciate the various ways in which supplied positions may be wrong
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appreciate the type of information about a dive site that is required to help nd it
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understand the implications of sonar beam width on surface searches
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know how to conduct a surface search
Finding Dive Sites Outline Most of the skills taught on this course are the same as those used by all mariners to navigate from place to place across the surface of the sea. If the dive site is a continuation of a feature which shows above the surface, or if it is marked by a buoy, these skills are all you need. But for dive sites that are not marked, divers must also be able to locate submerged objects precisely, from the surface. This lesson deals with the skills and knowledge required.
Accurac v precision Problems with positions Dive site information Site location Echo sounders Search patterns
Accurac v precision In planning your dive trip, you will have obtained a position for the dive site, either from a chart, from a previous visit, or from a third par ty via guidebooks, websites, articles, etc. You can plan a voyage to arrive at that surface position, but if the dive site is unmarked you must then ensure that you are actually over the wreck, reef, or whatever it is that you are expecting to dive. How easy this is depends on the quality of the position, in terms of how closely it corresponds to the position of the dive site on the seabed. The blue square on the VA represents an area of seabed. The following positions each have a different precision: the gure in parentheses indicates the distance corresponding to a change of one in the last decimal place.
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50 30.1234N (±18cm)
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001 10.4321W
This position describes a tiny area, and the precision is greater than normal GPS units are capable of (3 decimal places is the normal maximum) so one might expect that no searching would be necessary.
Less precise ●
50 30.10N (±18m)
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001 10.12W
This level of precision is probably as good as you will get reading from a chart.
Imprecise ●
50 30.1N (±180m)
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001 10.1W
Accurate The actual location of the wreck shows why accuracy is more important than precision. Not only does the (spuriously) precise position not put you over the wreck, but it also fails to warn you in advance that a surface search may be required. In contrast the imprecise position tells you that a search should be planned for.
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accuracy essential Without accuracy you are lost. The dive site could be anywhere. However, multiple (believed to be inaccurate) positions may be better than nothing for either illustrating the potential size of a search area, or for bolstering one's condence in a single (believed to be accurate) position.
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precision needed depends on nature of dive site Small sites such as a wreck or a pinnacle need a precise position, but for say a scallop dive or a dive on a large reef it may be sufcient simply to drop divers in the right general area.
Problems with positions Some positions are spot on If you have a position known to be accurate and sufciently precise, nding the dive site should be easy. Whether the marks are GPS or visual: just navigate to the spot, and drop the shot.
Others are not.......... Unfortunately even the best marks can be corrupted if data entry errors occur during input to the GPS, and unveried positions are subject to errors from several different sources.
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notation – eg 58° 53´ .50″N Positions written in degree/minute/second notation may be treated as if the seconds were decimals of a minute. In the example, 50" might be entered as 0.50' which is wrong: half a minute is only 30". 50" is actually 0.83'.
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mistakes – eg 58° 35.40´S Basic keying errors are common: numbers transposed eg 53 becomes 35, wrong numbers eg 4 instead of 5, and wrong N/S or E/W sufx. GPS units used on small
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boats do not usually have a full keyboard for data entry and often use some kind of scrolling system with the arrow and enter keys instead. It is all too easy to skip over a digit so that it remains at its default value.
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hand-me-downs Many of the positions found in guidebooks, articles and websites have a long history. They may have been compiled into lists which were then combined with other lists, and then copied into spreadsheets where they were sorted and analysed and extracted, then put onto websites from which they were copied and merged with yet more lists, and so on. In the process they may have accumulated a whole series of errors, as like was inadvertently combined with unlike, as individual positions were 'tweaked', and so on.
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obsolete systems – Decca, SA Some positions may have been taken using older, less accurate navigation systems which are now obsolete. In particular the radio-based Decca navigation system which was widely used until the late 1990s generated lat/longs that could be hundreds of metres away from the equivalent GPS position, and GPS positions recorded by civilian users before May 2000 were subject to a deliberate random error called selective availability (SA) which could be up to ±100m.
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geodetic datum The signicance of geodetic datum was mentioned in the Position Fixing lesson. With positions obtained from third parties, the geodetic datum may simply be unknown. Until fairly recently many people were unaware of its signicance.
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deliberate It is not unknown for positions given in certain guidebooks and on certain websites to contain deliberate errors.
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age of chart source data There is normally a small chartlet which gives details of the hydrographic surveys which provided the data used to populate the various areas of the chart. It contains a table which lists the date and scale of each survey and assigns it a letter. There is also a miniature version of the entire chart, divided up into areas. Each area contains the letter corresponding to the survey from which that part of the chart was drawn. The example on the VA shows that some of the surveys were conducted over a century ago, and several of them pre-date modern satellite position xing methods. It is still common for the less frequented areas shown on charts to be based on very old surveys. Old surveys are unlikely to be accurate. Be aware of this when taking positions of dive sites from charts.
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unspecied The above list is not exhaustive, and one should never rule out the possibility of unknown unknowns affecting the quality of dive site positions.
SMS Köln in
Scapa Flow
Run through the example to illustrate the inaccuracies which may occur in positions used to locate dive sites on a real example (although locating the Köln is unlikely to be a diving issue for most people since all the major Scapa wrecks are buoyed, and their exact positions are well known to the commercial skippers from whose boats the vast majority of dives are made). The positions quoted in this example were all current in 2009.
Do our homework!
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Dive site information The more information the navigator has about the target to be searched for, the better the chances of nding it. The information required is different from that needed by a dive manager, although there is some overlap. Dive managers need to know about points of interest and hazards, so that they can brief divers appropriately. Navigators need to know how to distinguish the dive site from its surroundings.
Sea bed depth and atness Depth determines the sonar beam width (covered in more detail later), and hence the width of search lanes. If there are extremely shallow parts there may be a risk of the boat hitting them. If the target rises abruptly from a at seabed it will be easier to detect than if it is amidst undulating ground.
Height of the site above the sea bed If the site stands well clear of everything around it, it should be easy to spot, but if it is very close to the seabed it will not be so easy. This information can help in selecting the best settings on the echo sounder.
Site prole – where are the tall parts? Knowing the prole of the site will make it easier to nd the highest parts of the site once some part of it has been located. It is normal practice to place the shot on the shallowest part of a dive site.
Orientation – direction of the long side Knowing the orientation of the site if it is long and thin (a wreck is the obvious example) is important when planning the best search pattern. It also helps navigation around the site once it has been located.
Pilotage Guidebooks (and other divers) sometimes provide annotated photographs or diagrams of dive sites which are close up against land. Armed with this information, assuming it is correct, you may be able to navigate right on top of the dive site without the need for transits or GPS.
Know what ou are looking for
Echo sounders The echo sounder is the most commonly used detection device. Briey explain how it works, and hence what it would show. Give examples of what it could be used to search for.
Principles of sonar ●
transducer emits sound pulses
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time delay of reected pulse indicates depth
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results displayed on scrolling screen
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strength of echoes often colour coded The strength of an echo is inuenced by the type of material, so for example soft materials like mud and weed give weaker echoes than hard ones like rock and metal.
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Using echo sounders A target which is large compared to its surroundings, such as an underwater pinnacle or a wreck standing well clear of a at bottom, is normally easy to identify using an echo sounder. However locating targets which are less distinct, and identifying details on even the largest targets, is more difcult.
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beam angle affects footprint and denition The beam angle depends on the transducer. Most manufacturers offer narrow (10-12°) and wide (20+°) transducers, and some units may have both. A wide beam has the bigger footprint at any given depth, which means that the area of seabed searched is greater but the signals returned from the target are weaker because the transducer's power is dispersed over a larger area. On the other hand, a narrow beam concentrates the power, giving better denition of detail. Echo sounder footprints are quite small. A narrow beam transducer in 15m of water may have a footprint of as little of 3m. Note that this is comparable to the accuracy of GPS. Even transducers described as wide beam may have a footprint of less than 15m in 40m of water. Point out the implications for getting 100% coverage with a search.
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screens need interpreting Put simply, the trace displayed from a single ping is a vertical line which is a composite of all the reections within the transducer's footprint, so the prole of the seabed drawn on the screen as the boat moves along is not an exact representation of the seabed directly underneath the boat but an average of everything within the path swept out along the seabed by the footprint. The screen scrolls at a constant rate so the relationship of the trace to the actual seabed prole is also affected by boat speed. For example, a sloping seabed will appear steeper the faster the boat is going. Interpreting the echoes shown on the screen can be quite an art.
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read the manual Although most echo sounders have an auto setting which means they will work straight out of the box, they normally have a variety of controls which adjust the way that signals are processed, in addition to functions which activate alarms and adjust the display settings. To achieve optimum results in different conditions, learn how to use the unit to its full potential. Start by reading the manual, or getting someone to demonstrate each function in detail.
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practise, practise, practise! Using an echo sounder for surface searching is far from an exact science. It demands detailed knowledge of the equipment used and experience in interpreting sonar screens. This can only be gained through practice.
Site location This VA describes the procedure for approaching the position of the dive site. If using a shotline you may decide to prepare it before locating the dive site and shot the site on the rst pass, or you may decide to nd the site rst and shot it later. Begin by following the route towards the site until ready to turn on to the nal approach.
Using transits If using transits to x the dive site, use one of them as a leading line.
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keep nearest transit to tide in line Wherever possible, choose a transit leading into the tide (or wind if that is having
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the greatest effect on the boat). This is the easiest way to hold the boat on a steady course, and to stop it when required.
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slow down as 2nd transit comes up As the dive site approaches you must slow down so that you have time to monitor the echo sounder properly, you do not overshoot the site, and if necessary the crew can deploy the shot. 3-4 knots is fast enough.
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monitor echo sounder
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when target appears ○
mark if necessary Mark a site with a shotline, or place an electronic marker on the GPS screen. All marine GPS units have a man overboard function (MOB) which could be used, but most allow instant placement of a waypoint at the current position which is better, since it reserves the MOB for emergencies.
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search for best bit If nothing is found, begin a search pattern.
Using GPS The procedure when using GPS is very similar. Head into the wind/tide where possible. The GPS unit will indicate the required course.
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follow GPS/compass course towards waypoint The GPS will always give a reliable course to steer to the waypoint, but do not rely on the GPS to give the boat's current heading. Use the steering compass to follow course that the GPS asks for. At slow speeds, and during turns, the heading displayed by a GPS always lags behind the actual heading. This can lead to the helmsman overcorrecting, making it difcult to settle on a steady course.
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slow down when close
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use sounder and mark target as above
If you have good pilotage information for locating a dive site close to land [example on Finding VA5] use GPS to navigate to a waypoint near the area and then motor slowly over to the marks. Conrm using GPS coordinates, if available. Wrecks close up against the shore are often broken up and scattered over uneven rocky ground, making them almost impossible to identify positively on the sounder.
Searches Search patterns Various search patterns are possible. Draw parallels with those used for underwater searches, eg circular around an initially deployed shotline or electronic marker; or ‘lane’ pattern with or without deployed shotline(s) to help. Each search pattern has its advantages and disadvantages, but the nal choice of search pattern also depends on factors including the size of the expected search area, the weather and sea conditions, the nature of the target, and the technology and resources available.
Spiral search Circling around a mark in ever increasing circles, or spiral, needs a good judge of distance, so it is only suitable for searching a relatively small area. As the original compass heading is approached at the end of each circuit, the distance from the mark 52
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is increased to widen the search area. The constantly changing heading makes it very difcult for the helmsman to correct for any tide, which means that spiral searches do not work well in a current.
Square spiral search An increasing square search pattern can be made using compass headings and timed 'legs'. This pattern has similar applications to the spiral search, but the use of straight 'legs' means that it can be extended over a larger area, further from the datum, without becoming haphazard.
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spiral where early result expected Both types of spiral search work outward from a point, which makes them useful for a quick search when the mark is known to be on or close to the target, so that an early result is expected eg, looking for the highest point of a site after it has been located.
Rectangular search A rectangular search pattern can be made, using compass or GPS headings and timed or distance-measured 'legs'.
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rectangular for large areas This pattern is suitable for methodical searches of larger areas, as might be required when only a rough position of a wreck is available. It is important to ensure 100% coverage, and to have a means of recording which areas have been searched.
Search datum ●
electronic marker An electronic marker on the GPS screen makes it easy to monitor the coverage of the search. The 'snail trail' on the screen shows the areas searched and those not, provided that the screen has sufcient size and resolution to record the boat’s track in a readable manner. However when using this method it is important to guard against getting xated by the screen and so failing to look where you are going.
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shot A shotline can be used alone for searches of a sufciently small area that distance from the buoy can be judged sufciently accurately by eye. Sometimes it is useful to place a shot to aid orientation even when GPS is being used.
Searching for new sites can be time consuming and it needs to be methodical. On nding the site, another datum marker/shot should be deployed and, if wanting to locate the site again in the future, transits and/or GPS position noted.
Consider ●
sonar footprint v lane width The distance between ‘lanes’ is determined by the width of the coverage of the seabed provided by the equipment. As discussed earlier, the footprint of an echo sounder can be quite small, which means that search lanes are narrow and accurate helming is necessary to ensure 100% coverage.
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wind/tide/sea state The weather and tidal conditions inuence the choice of search pattern because they determine how easy it is to hold the boat on any particular course. It is normally best to steer directly into the tide or to run with it, because a cross
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tide deects a boat from its heading especially when it is travelling slowly, as it is during a search. However wind, and more particularly waves, can make a particular course uncomfortable. Also, signicant amounts of rolling can affect the operation of the echo sounder. These considerations mean that the rectangular search pattern is usually preferable in all but the most benign conditions.
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shape of target When searching for long narrow targets, such as shipwrecks, it makes sense to search at right angles to their length rather than along it (assuming the orientation is known) because it increases the number of passes which will go over the wreck, and hence increases the chance of detecting it.
Summary The problems of identifying dive sites can largely be eliminated by sticking to well known sites or by using a hired skipper with a charter boat. But that also eliminates the achievement of doing your own site location, and the pleasure and adventure to be gained from exploring new sites.
Accurac v precision Problems with positions Dive site information Site location Echo sounders Search patterns
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VOyAGE PLANNING Lesson Objectives Voyage planning is thinking about what you are going to do before you do it. Before putting to sea, every skipper is required by maritime law to have considered the passage to be undertaken in the light of the expected conditions and the capabilities of the vessel and its crew, and to have made appropriate contingency plans. This duty applies to the person in charge of a RIB or a hardboat just as much as to the captain of a supertanker. On larger vessels the ne detail of navigating the route may be worked out during the voyage. But this is impractical in fast or open boats, and so all aspects of navigation have to be prepared in advance. This lesson teaches how to do it.
Achievement Targets At the end of this lesson students should: ●
understand the concept of waypoint planning
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know how to choose waypoints
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be able to select appropriate practical navigational methods to make it easier to locate the target
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appreciate the importance of eliminating input errors when using GPS
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be able to list the topics covered in a voyage plan
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appreciate the legal duties placed on ship' masters by SOLA S V
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know how to deal with the effect of tide on a boat
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appreciate the mental approach required for successful navigation
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appreciate how voyage planning methods can be applied when charter boat diving
Additional visual aids A copy of the Model voyage plan for each student
Voage Planning Outline Wapoint navigation Choosing wapoints Voage planning Allowing for tide Practical navigation Charter boats Navigation Zen
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Wapoints Click through the VA and describe what a waypoint is.
Wapoint ●
a navigationally signicant point along a boat’s route
Eamples ●
destination/dive site
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home port
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intermediate points
Established b Waypoints are selected from a chart, but they need to be identied at sea using one or more of the various methods of position xing:
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simple x
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compass bearing and/or transits
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position from GPS
Choosing wapoints Most of the time simply following a series of positions on the GPS will work, but a good navigator will try to choose the waypoints so that progress can be conrmed by other means. This makes the voyage plan more robust if there are mistakes or if equipment fails. It gives added condence to the person driving the boat and by providing an element of cross checking, helps identify errors in good time. Even if no visual xes are available, it is good practice to use the calculated elapsed time for each leg to check that each waypoint is reached in roughly the expected time, allowing for any variations in boat speed.
Look for visual es The exact position of intermediate waypoints is usually somewhat arbitrary, in which case provided they are surrounded by safe water a precise visual x is not necessary. For inshore work the crossing of a position line at roughly the right distance from land is adequate.
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transits – opening, closing The classic transit of objects in line is ideal, but where exact positioning is not needed opening and closing are useful concepts. Open means that a view has opened up. For example when running along a chain of islands the channels between them will not be visible until you reach each one. The channel opens as you reach it and have an unobstructed view through it. Closing means the opposite. For example you may be able to see two headlands ahead of you, one more distant than the other. If you turn inshore before the rst headland the distant one will eventually be obscured. It is then said to have closed. If necessary use fur ther practical examples around the classroom to get these ideas across.
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bearings of nearby objects Suitable transits may not be available, so bearings can be used instead. It is difcult to steer and take a bearing at the same time so it is helpful i f someone
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other than the helmsman can take it. It is important to understand how the bearing should change as you travel past the waypoint. Set the students an example around the classroom and let them work it out.
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depth changes
Courses with visual aim point Courses with visual aim points make it easy for the helmsman to hold the course and look where the boat is going at the same time. Using GPS alone to keep on course tempts you to look at the instruments rather than the surroundings.
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leading lines
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clearing lines
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bearing on objects/features Where the bearing denes the course, the steering compass is used, so the helmsman can check the bearing and keep a lookout at the same time. Ask the students why you need a bearing. Why not just aim at the object?
Techniques ●
aiming off This technique is useful where a dive site lies alongside a linear feature like a cliff. If you were to aim directly at the dive site, errors could throw you off to either side of it so when you arrived you would be unsure which side it was on. By aiming for the linear feature well to one side, on arrival you turn in the appropriate direction and follow it down to the dive site.
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attack points An attack point can be used when you are trying to nd a very small feature. If you attempted to navigate to it from a long way away you could easily miss it. By aiming for a nearby much larger feature which can be found easily, the attack point, you will reach a known point from which the small feature can be found more easily.
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buoy hopping Buoy hopping consists of navigating by following from one simple x to another. The classic example from which the technique gets its name is following a buoyed channel. However the same principle can be applied when following any type of simple x. This method of navigation is also known as pilotage.
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straight to GPS position Often with an accurate GPS position surrounded by clear water there is no need for any special techniques. If so, head directly towards the dive site. Navigational techniques are a means to an end. If they are not needed do not use them.
Wapoint navigation Run through the VA describing each step in creating a passage plan. The illustration is based on the form used throughout the course, but the method would apply just the same using an electronic chart system, or even the back of an envelope. Using an electronic system details such as bearings, distances and lat/longs will be captured automatically, saving time and eliminating the possibility of transcription errors.
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Plot course on chart
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Dene and list waypoints There are two columns on the form. Waypoint details contains the meat: lat/long,
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nature of waypoint etc. WPT is simply an identier for the waypoint. When waypoints are entered into a GPS unit they must have an identier code attached to them so that the unit can distinguish one waypoint from another. Usually the GPS names them by default eg 0001, 0002, etc. Many club GPS units are full of waypoints with meaningless names, often with multiple entries of the same position. Assigning unambiguous codes to your waypoints may make reusing them easier.
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Measure bearings The bearing is the bearing of the leg from the previous waypoint to the current one. Writing down the reciprocal means that you can easily use the waypoints in reverse order for the homeward trip, if necessary.
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Measure distances The distance is the length of the leg from the previous waypoint to the current one.
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Estimate speed Obviously, this depends on the boat and the conditions. Make your best guess taking into account all the factors. The method allows a different speed for each leg so that you can take account of, for example, travelling in areas of speed restriction such as harbours, travelling in sheltered water, and exposed legs where rough water is expected.
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Estimate time
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Notes as required Record details of xes, aiming points, hazards, etc which apply to the leg.
Result ●
Overall distance & time This is crucial information for working out actual departure and arrival times, and for estimating fuel consumption. If there is uncertainty about what boat speed can be achieved, work out the overall time at a variety of possible speeds to see how much contingency would be required in different circumstances.
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Step-by-step passage plan You have created a concise step by step plan which should be sufcient to navigate the boat to and from the dive site without needing to do any working out at sea.
Wapoint navigation with GPS Series of wapoints = route Most units will accept a series of waypoints as a route, so that when one waypoint is reached the unit automatically starts to navigate towards the next. Alternatively the navigator can switch between waypoints manually. The GPS unit will calculate and display a variety of useful parameters based on the boat's progress towards the next waypoint. Depending on the particular model it may be necessary to select one or more dif ferent screens in order to see all the information.
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bearing & distance to next waypoint
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averaged speed, course, eta
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continuously updated If the boat deviates from its intended course the GPS will automatically change the required bearing and update the distance to go and eta accordingly.
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straight lines only! A GPS unit provides all of the above information for any waypoint entered, wherever it is. The bearing and distance will be in a straight line from the current position, regardless of any obstacles that may be in the way. It is very important to be aware of this at all times. The planned course which was entered into the GPS will have been free of obstructions, but if the boat deviates from it the new headings provided by the GPS could lead into danger.
Garbage in garbage out ●
GPS and entered lat/long must be same datum – normally WGS84
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apply correction where different
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check for input errors If a lat/long has been entered incorrectly, one or both of bearing and distance of a leg will be wrong. Most GPS units have a planning mode which enables you to check each leg against what was expected from the chart as you enter the data. With high precision waypoints, such as a wreck, the lat/long itself should be checked because even small errors are unacceptable.
Alwas plot our route on a chart rst A GPS unit has no local information. This can only be obtained from a chart. Plotting your route on a chart helps to visualise it and eliminate mistakes.
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paper Paper charts are excellent. You can write on them, and you can see large areas and full detail simultaneously.
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electronic may simplify or eliminate measurements and data entry Electronic charts are excellent productivity tools. For planning, they are best when viewed on a large screen. PC-based planning tools based on Admiralty raster charts can be purchased very cheaply [Belleld c£70, UKHO c£50, in January 2010 - check latest products and availability before giving out details].
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take waterproof copy with you The navigator should always have a copy of the relevant charts available at sea. Any electronic system can fail, so there must be a paper backup. In an open boat it must be waterproof or carried in a waterproof case. Remind students of the weatherproof versions of char ts mentioned in Chart basics. Electronic chart users may be able to print a chart (copyright permitting) and laminate it.
Preparing the voage plan It is a legal requirement for the master of a vessel to make a voyage plan before going to sea.
Route alone is not enough For a fast open boat it is essential to have worked out the route details required to navigate to the dive site in advance. However, a detailed route plan is not a legal requirement, whereas other things are. The proposed voyage must be assessed against the prevailing conditions to make sure that it can be conducted safely.
SOLAS V regulations The legal requirements for voyage planning are set out in Annex ve of the Safety of Life at Sea convention SOLAS V. This course is about navigation techniques, not the
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responsibilities of skippers, coxswains and dive managers, so some aspects of SOLAS V are greyed out in the following list, and are just given a mention. Under BSAC guidelines, all boats used for diving should be under the supervision of someone who has completed a BSAC Boathandling course, as a minimum, and has therefore been trained in these topics.
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weather The master should obtain an up to date weather forecast and use it to determine if conditions are suitable for the planned voyage. Swell forecasts are also useful in areas subject to swell from distant storms. Force 4 is the maximum wind strength for diving in open water (ie, places exposed to the full force of the wind and waves).
○
exposure of site/route On this course, students should think about the vulnerability of their chosen route to the weather: wind, waves, and swell, and record any particularly exposed sections, and places subject to wind over tide, on the voyage plan.
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shelter from wind/waves Waves and swell can make life uncomfortable for passengers, even when they pose no hazard to navigation, so where practicable, choose a route which is sheltered from the weather. Shelter from the waves can be found behind any ground on which the waves break, even rocks just below the surface. High ground gives shelter from wind and waves. Point out that the wind will tend to blow strongly through narrow channels with high ground either side,
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tides
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limitations of the vessel Students on this course should check that the length of the voyage is within the boat's range, by estimating the fuel requirements.
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crew
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navigational dangers
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contingency plan Students should identify safe havens which can be reached in case of unexpected bad weather or other emergencies, and record sufcient detail to navigate to them. Emphasise that this is for emergencies not alternative dive sites.
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leave information ashore
If using RIBs ●
1 sheet, waterproof, legible The voyage plan needs to be concise so that it is easy to use in an open boat.
Unnecessar paperwork? The law does not actually require any aspects of voyage planning to be written down; it simply requires that the required tasks should be carried out. This course assumes that students are planning voyages to destinations that are new to them in unfamiliar waters, which means that to ensure safe navigation detailed plans should be taken to sea. [ At this stage, students are better with too much detail than not enough, but with experience, they should learn to judge what constitutes 'enough' for them ]. However, divers will not see many charter boat skippers operating from detailed voyage 60
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plans: if you genuinely 'know these waters like the back of your hand', have made the voyage in question many times before, and are condent that you can still navigate safely if contingencies arise, there is no seamanship value in plotting out and recording a voyage plan every time. Those fearful of litigation and insurance claims may wish to keep comprehensive written records of all their boating and diving activities as possible evidence. That is a personal decision, outside the scope of this course.
Planning process Summarise the whole planning process, emphasising where steps are dependent on their predecessors. The detail of what happens in each step is covered later in the lesson.
Pick sites After picking the dive sites you can get on with the next four tasks in parallel.
Determine slack Determine tides Get local info ●
Check launch Using the tide times, work out the times when launching and recovery will be possible.
Plan route Once you have worked out the route on the chart you can get on with these next three tasks in parallel.
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Contingency
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Check fuel
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Set up GPS
Set timings ●
Waterproof backup
Check weather Note that this task stands alone, with no predecessors: you can't plan the weather!
Go diving Pre-planning Although planning is shown as a linear process, it makes sense to do some rough estimates to establish the feasibility of a proposed dive trip before planning it in detail. It may be necessary to consider several proposals before coming up with one that will work. For example, sometimes the starting point may be a timeslot rather than a particular site, so you would begin by looking at the tides and seeing what would be possible.
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Voage plan Explain what goes in each section of this part of the voyage plan. Details of the route are recorded on the form covered earlier (or its equivalent).
Dive site This section should contain the information necessary to locate the dive site, such as transits, depths, orientation, dimensions etc. Point out that dive managers will require extra and different information in order to plan the diving and brief the divers, which is not necessary for navigation and need only be recorded here if convenient.
Tides ●
port, HW/LW, heights, slack, streams, direction All these topics have been covered earlier in the course. It is important to take the relevant information to sea.
Local information Local information may be scattered across many different sources. Some harbour authorities and local councils publish booklets of rules and regulations relating to their area. There are often launching fees to pay, and slipway owners may want to see evidence of insurance and boating qualications. In some areas boats have to be registered with the local authorities. Details may be found in almanacs, and on the internet. Remind students that local rules and regulations can change so it is important to use up to date and reliable sources of information.
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launching/recovery Some slipways are only useable at certain states of the tide. Slipways may also become unusable in certain weather conditions. Record such limitations on the voyage plan.
● VHF channels, Coastguard In busy shipping areas VHF channel usage may be strictly controlled, and it is important to know which channels are used for which purpose. This information can be found from almanacs. Coastguard operations are controlled from units called an MRCC (Maritime Rescue Co-ordination Centre). Each MRCC covers a very large area so it is not obvious which one to call unless you check. In some areas there are more local coastguard units called MRSCs. Contact information can be found in almanacs and on the MCA website.
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hazards, rules Natural hazards, such as areas of overfalls, exposure to waves and swell, and strong tidal streams, should be identied from the char ts and navigational publications used during route planning and should be recorded on the voyage plan. Other hazards and rules such as high speed ferries, exclusion zones, military exercise areas, etc should also be recorded.
Navigation The rst two items below are recorded separately on the detailed plan described earlier.
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The boat's range is how far it can travel on the amount of fuel it can carr y. The range is determined by the boat’s design, its loading, the sea conditions, and the manner and speed in which it is driven. Use the boat's actual fuel consumption if known and if not use the formula at the bottom of the voyage plan. Apply the rule of thirds (one third to get there, one third back, one third for contingency) and do not forget that the boat will also be burning fuel whilst patrolling the dive site. [If operating well offshore range also depends on whether the boat is designed and equipped for the purpose, and on whether the crew are suitably qualied and experienced. However that is outside the scope of this course. ]
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contingency Record any contingency arrangements, such as nearby safe havens or landing places that could be used in emergency. Provide enough detail that they could be navigated to easily. Point out that for most dives, nothing elaborate is necessary: if distances are fairly short, return to base is adequate contingency; and provided there are no off-lying dangers, heading for the coast and then turning left or right for home may be all the navigation that is required.
Weather This is space [for the person in charge (skipper/coxswain/dive manager)] to record the weather forecast for the day. How to do this is covered on the BSAC Dive Leader course , and also on the Boat Handling and Dive Planning and Management SDCs.
Allowing for tide None of the planning covered so far has allowed for the effect that the tide may have on a boat's course. Usually, the helmsman can compensate for the tide by the way the boat is steered, but this is not always the case so navigators must understand how the tide affects a boat.
Tide will push boat off course ●
biggest effect ○
tide at 90° The deection is greatest when the tide is at right angles to the direction of travel. The smaller the angle the less the effect. A tide on the bow or dead astern will cause no deection, but it will af fect the boat's speed. With large boats, which usually travel relatively slowly, the direction of the tide can make a substantial dif ference to journey times.
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fast tide The faster the tide, the bigger the effect.
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slow boat The ratio between boat speed and tide speed is signicant. If the boat is travelling much faster (at least 5 or 6 times) than the tide, the effect will be fairly small. Even so, the distance off course will add up over a long distance. If the boat and tide are going at similar speeds the tide has a large effect on the boat's motion over the ground. In some circumstances a boat may even be driven backwards. [One implication is that some large boats may be unable to provide good safety cover for divers in very tidal areas.]
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45° possible when manoeuvring At low speeds boats may have to ferry glide to manoeuvre: they will appear to approach their target almost sideways.
Correct b ●
with GPS use XTE or course line The easiest way to cope with tides is to use the cross track error (XTE) display on a GPS. The unit will calculate how much the boat has been deected from its course from the origin to the next waypoint, and give the required course corrections. Alternatively using a chart plotter the helmsman can adjust the heading manually to stay on the displayed course line. Effectively, following the GPS enables you to more or less ignore the tide while travelling at high speed to and from the dive site.
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manually adjust course Using a visual x such as a leading line, the helmsman will be able to steer so as to stay on course and keep a lookout at the same time. It is helpful if the navigator can plan the route so that suitable visual marks can be used. If the boat has no GPS and no visual xes are available, such as when heading straight out to sea, it may be necessary to make a manual course adjustment. This means adjusting the bearing taken from the char t to give a new course which allows for the tide. This method is not often required in diving situations.
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adjustment chart in notes The chart in Appendix D shows the course correction that would be required for various combinations of boat/tide speed ratio and tide angle of to boat course.
Practical navigation Use the following miscellaneous items to reinforce points made earlier, and to offer solutions to common problems.
Don’t get set on to dangers The tide can push the boat sideways into danger. In a situation like the one depicted, the mark will always be directly ahead of the boat, fooling the helmsman into thinking the boat is travelling in a straight line. Look for additional visual references, or use the GPS, and/or plan ahead.
Get positive ID on marks It is said that most groundings occur in broad daylight and good visibility to vessels whose crew thought they knew where they were. People see what they expect to see, so mistakes are not detected until it is too late. Do not be casual or lazy: take the trouble to get positive conrmation on all marks.
Use back bearings When travelling out to sea there is usually a shortage of visual marks to aim for. Back bearings are often a useful substitute, although less convenient for the helmsman.
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Look behind as ou leave a wapoint to identif the view for the wa back It is not only specic marks, but also having a general picture of what the land looks like that makes navigation easier and more reliable. By looking behind as you leave a waypoint you can see what to look for on your return.
Keep track of time Use the elapsed times for each leg to check that you are on course. There are so many factors that can affect boat speed - sea state, loading, passenger comfort, slowing to adjust things that have come loose, etc - that it is not sensible to expect timings to be accurate to the second. But if the plan was for the next waypoint to appear in 10 minutes and there is no sign of it after 15, it may be a sign that something has gone wrong. It is also worth keeping track of the eta at the dive site. For example, if you have a long journey to a slack water dive and it looks like you will not be there in time, it may make sense to abort and go to a nearer site rather than have a wasted journey. [The recommendation to use time as the principal method of High Speed Navigation which was in earlier versions of CPF has been superseded.]
Charter boats This VA looks at how the techniques learned on this course may still be useful when diving from a charter boat with a professional skipper.
Voage planning applies to all Although this course is primarily aimed at users of small open dive boats the basic techniques, and the requirements of SOLAS V, apply to all types of vessel regardless of size or function.
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charter boat = skipper On a charter boat, the skipper is responsible for navigation of the boat and for locating the dive sites. In terms of navigation, safety and responsibility, there is no need for anyone in the dive party to do any voyage planning at all.
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distance and time = cost A charter boat is a commercial operation whose principal variable costs are the fuel used and the wages (or perhaps on a liveaboard, working hours) of the skipper and crew. Both of these items are closely related to the distance and duration of the dive trip.
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large boats more affected by tide Liveaboards and some day boats are displacement craft, which means they are quite slow with top speeds typically in the 7-10kn range. Tidal streams of 2kn or more are common around the UK, which is quite signicant for a slow boat. For example, tides at the southern end of the Sound of Mull reach 3kn on springs. A boat travelling at 7kn through the water might take an hour and a half to cover this section against the tide but only 45 minutes going with it. Large boats are also less manoeuvrable, particularly in strong tides and in amongst rocks, making them unsuitable for some types of dive site unless they carry a tender capable of carrying divers to and from the dive site.
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overall plan for multi-day trips Unless it is intended to return to the same port each night, multi-day trips (normally on a liveaboard) have extra planning considerations to be taken into account. The skipper will want to minimise the overall distance travelled by
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avoiding zigzagging between dive sites, and the plan will have to be exible enough to cope with bad weather. On trips lasting several days it is likely that at some point the boat will be more than a day's sail from the home port, so the skipper will not want to get into a situation where bad weather stops the boat getting back in time for the next charter. For this reason it is a good idea to head for the furthest point of a trip early on, allowing more time for the return.
Do rough voage plans Some divers are happy to let the skipper do everything, including selecting the dive sites. There is nothing wrong with that, but if you wish to have more control over where and how you dive it is well worth doing some rough planning of your own.
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understand what’s possible If you have some particular sites that you wish to dive, do a rough voyage plan, checking that what you want to do is feasible in terms of tides, distances and timings. Think about the effects of the weather on the site: for example, an onshore wind may make it difcult to pick up divers who surface close to the rocks. For multi-day trips plan to visit the dive sites and the overnight stops in a sensible order without excessive zigzagging and going back on yourself, which would unnecessarily lengthen the journey and extend the skipper and crew's working day.
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shape expectations Organisers of dive trips can use voyage planning to help shape the expectations of the other divers on the trip. Many divers have little awareness of their surroundings when they are on a boat: some may have no idea of the geography connecting the various sites they have dived or the ports they have visited. Doing rough voyage plans will help to include requested sites where possible, and to explain why diving other sites is not possible.
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improve negotiations If for any reason you wish to change what the skipper thinks should happen in terms of the dive plans, you will be in a stronger position to negotiate if you have done your homework. Skippers rely on repeat business and if what you request is well thought out and does not require extra work for no extra income, most will be happy to agree.
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mutual respect? Obviously there is a great more to interpersonal relations than attending a CPF course, but understanding voyage planning will help you understand a dive trip from the skipper's point of view, and demonstrating that you know what you are talking about may make the skipper more ready to see your point of view.
Dive site research if eplorator diving By denition, exploratory diving involves locating and diving sites that are unknown even to the skipper. Adventurous divers who wish to do this potentially very rewarding type of diving will need to make full use of the techniques covered in the lesson Finding Dive Sites. In particular they will need to do as much research as possible to nd out about the target and its surroundings, to maximise the chances of nding it. This type of diving needs to be planned by or in conjunction with a dive manager with the appropriate skills and experience.
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Navigation Zen To be a good navigator requires more than just the ability to apply a set of procedures. It requires the right state of mind as well. Having the right mental approach helps the navigator cope with all the problems and uncertainties that can arise. There may be mistakes or equipment may fail; things may not be where they should be or they may not look like you expected them to look; you can never be sure what might be lurking just below the surface of the sea; the weather may be bad with poor visibility and rough seas. Unless you only ever go on voyages you have made many times before, in benign conditions and with good luck, you will never be completely sure of your exact position at all times. In these circumstances condence is important. Knowing how to apply the navigation methods taught on this course should give you condence, but applying the following mental techniques listed below will ensure that your condence remains well founded.
Navigation is not just a set of procedures ●
stay alert
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use a wide variety of techniques Cross check the results. If they are not consistent nd out why. There may be a problem.
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develop a mental picture of your route
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compare what you can see with your mental picture
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keep track of where you are
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look where you are going Getting xated by looking at a screen is a common problem for the helmsman, but they can be distracted by other things as well. Point out that this point is not about keeping a good lookout, though that is essential, but about checking the boat's progress on its voyage.
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look around you It is important to gather as much information from the surroundings as possible, which means looking in every direction not just along the boat's heading.
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KISS Keep It Simple Stupid
The students will get the chance to practise these techniques on the practical day.
Eample Take the students through the model voyage plan. They should each have a paper colour copy of the handout CPF09 VPexample. Refer to the instructor's version CPF09 VPexample_annotated which contains further notes on the entries. [The model voyage plan in VA14-15 may be replaced by an example based on a local dive site familiar to the students, at the instructor's discretion. ]
RIB dive in Loch Nevis on W coast of Scotland ●
launch/recovery at Mallaig
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dive Smyth Rock
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● voyage plan ●
marked up chart
Eample Eercise Introduce the exercise, which will be the next session on the course.
Work in groups with an instructor Voage planning onl, not dive planning ●
charts of the area
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dive guides, almanac
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local information
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chart 5011
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local tidal information
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current weather forecast
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waterproof ‘slate’ for recording voyage plan
Da 2 ●
carry out your plan
Summary Considered all aspects of voage planning relevant to operating area for practical navigation eercises ●
identied relevant information from a chart
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obtained tidal information & understood its impact on the practical navigation exercises
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dened a route
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documented the information needed to navigate the route
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considered contingency plans
Planning eercise If required display VA19 (requires tailoring) during the Voyage planning exercise which follows this lesson.
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OPEN FORUM Lesson Objectives This brief lesson ends the course by setting char twork and position xing in the context of dive organisation and management as a whole. It should summarise the key points which the students should take away with them, hand out any remaining course documentation, and give students a nal opportunity to ask questions. The logistics for the course are such that it would not normally resume at the dry venue after the practical aoat, so the contents of this session should be delivered informally 'in the car park' as the nal debrief at the end of the course.
Achievement Targets At the end of this lesson students should: ●
understand the role that chartwork and position xing plays in dive management and planning, and in boathandling
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have had an opportunity to ask any remaining questions
Additional Visual Aids There are no VAs for this session.
Contents Course debrieng Highlight the key messages from the course and comment upon the students' performances where appropriate.
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chart basics
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measurement basics
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tides
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position xing methods
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nding dive sites
● voyage planning ●
navigation at sea - all day on the boats
Conclusion The techniques learned can be applied to the navigation of any type of vessel. The course has used paper charts, but the principles apply equally well when using electronic navigation systems. Always remember that it is unwise to rely on electronic systems alone. On this course you have learned all the techniques that you will ever need to nd 99% of all dive sites. What you must do now is go out and gain experience by using them. Practise practise practise.
Other BSAC courses Remind students that taking responsibility for dive management and for being in V1.2 Copyright © BSAC 2010
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charge of a boat require additional skills to those taught on the course. They can learn these skills on the Dive Leader course, the Boat handling and Dive Planning and Management SDCs, and at a higher level by going for the Advanced Diver and First Class Diver qualications.
Course documentation Hand out any remaining course documentation.
Open forum Take questions and comments from the students.
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CHART ExERCISES Lesson Objectives These exercises provide further practice in basic chartwork.
Achievement Targets At the end of this lesson students should: ●
be able to record the lat/long of a point on a char t
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be able to locate a point on a chart from a given lat/long
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be able to identify any symbol at any point on a chart, using 5011 where necessary
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have recorded the starting point for the practical day aoat on their voyage plan
Equipment Needed Copies of Admiralty Charts 2611 and 2172 or local charts if using tailored exercises. Copies of 5011.
Lesson Contents 1. Eercises ●
Supply each student with a printed copy of CPF09_exercises handout.doc to work from. It covers all the exercises for the theory part of the course, but requires tailoring (see Appendix A).
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When the students have nished, go through the answers with the whole group.
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Once the answers have been checked, get the students to enter the starting point for the practical day aoat on their voyage plans.
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Provide individual coaching as necessary.
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POSITION FIxING DRy PRACTICAL Lesson Objectives This lesson gives students practical experience in nding, documenting and using transits, and in using a hand bearing compass. It is also intended to provide a break from classroom lessons.
Achievement Targets At the end of this lesson students should: ●
be able to nd a transit and record it well enough for a competent third party to use it successfully
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have experienced using a hand bearing compass
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have successfully used transits to locate a hidden object
Equipment Needed Hand bearing compass Weatherproof writing materials Outdoor clothing.
Lesson Contents In this exercise students are put into small groups (ideally pairs). They go to an outdoors area and hide a small, inconspicuous object on the ground. They then take transits and bearings to x the location of the hidden object. They document the transits as taught in the lesson Position Fixing Methods. Finally the groups exchange transits and use them to locate the hidden objects. Optionally, instructors could also place objects in pre-planned locations for which they have documented transits. Each group uses the transits to locate an object. It may add an element of challenge and/or fun if each group provides another with a moderately valuable item such as a banknote or key for them to hide. The hidden objects could also be something like a rafe ticket, which wins a 'prize' such as a packet of biscuits or a cake to be shared during a break.
1. Brieng ●
Allocate the students to groups.
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Brief the groups
2. Hide ●
Groups exchange objects to hide (optional).
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Groups disperse over the area and hide objects.
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Take transits to x position of object.
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Take bearing of each transit.
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Record transits.
3. Seek ●
Groups exchange transits.
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Use supplied transits to locate and recover objects.
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(optional) Each group locates an object pre-placed by the instructors using supplied transits.
4. Review ●
Instructors review results.
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Instructors check recorded transits against criteria • Conspicuous Permanent. • To make the exercise work it may be necessary to set a fairly low standard for 'permanent' eg use of parked cars and vegetation. • Precise • Well-recorded
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•
sketch
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bearing
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context
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narrative
Debrief. Go over the key points with the students.
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VOyAGE PLANNING ExERCISE Lesson Objectives This exercise gives students practical experience of preparing a voyage plan. Part or all of the voyage plan will be used at sea during the Practical navigation aoat exercise.
Achievement Targets At the end of this lesson students should have: ●
completed the voyage plan built up during the preceding lessons, making use of the following: • charts of the area • dive guides, almanac • local information • chart 5011 • local tidal information
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documented a route using waypoint planning techniques
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researched known dive sites and documented the information needed to locate them
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put their voyage plan into a weatherproof format suitable for taking to sea in a small open boat
Equipment Needed Each group requires: charts of the area dive guides, almanac local information chart 5011 local tidal information weather forecast waterproof ‘slate’ for recording voyage plan plus suitable writing implement and eraser chartwork instruments if available, laptop with internet access
Lesson Contents The voyage should visit the following points:
Launch site (provided during Chart basics) Waypoint 1 (provided during Measurement basics. This waypoint should be a simple x.) Exercise area 1 (location to be provided by the instructors) Mystery Point (provided during Position xing methods) Lunch spot (anchorage or landing. Location to be provided by instructors. Students to work out position)
Dive site 1 (position to be found by students from research given site name) Dive site 2 (position to be found by students from research given site name) Recovery (normally the launch site) 74
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Students work in groups with an instructor to complete all aspects of the voyage plan. They should add whatever intermediate waypoints they think necessary, and provide suitable visual xes where appropriate.
1. Voage planning Build up the voyage plan starting from the partly completed documents from earlier lessons. The instructor should provide guidance and coaching where necessary, including explaining any locally important chart symbols (such as trafc separation schemes) that were not covered in Chart Basics. Follow the planning process shown in Voyage planning VA8. ●
Pick sites Sites are specied by the instructors. Use the materials supplied (and internet if available) to nd the necessary details for site location and diving. Note that dive site references may provide details on slack water times.
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Determine slack Work out the time and duration of slack at each dive site, and note the tide direction before and after. Also note how the tide will be running around the area of the exercise throughout the day.
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Determine tides This should have been completed during Tides.
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Get local information Gather information about local hazards, byelaws, regulations, etc
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Check launching Find out about the availability of the slipway, and any rules, fees or other associated bureaucracy. Check the tides to make sure that launching and recovery will be possible at the required times.
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Plan route Do the detailed waypoint planning, using the supplied waypoints and adding others as necessary. Work out visual marks and bearings to assist in navigating the voyage.
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Set up GPS Consider the naming of waypoints for GPS entry. The actual entry may be deferred until later. The instructors should agree amongst themselves what waypoints will actually be entered, since it may be impractical for every group's plans to be entered.
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Check fuel Work out how much fuel will be required for the day. The instructor will conrm or state otherwise that the boat will be carrying at least that much.
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Contingency Identify safe havens, other refuges to be used in case of emergency, and/or get-home plan. The instructor should lead the students through the options by considering the eventualities that might lead to the need to seek an alternative destination.
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Set timings Timings would normally be worked backwards from slack water at the dive sites. However on this course, timings are set by the start and nish time, and the duration of the exercises to be carried out. The instructor should explain this to the students, and move on to the next task.
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Waterproof backup The voyage plan should be transcribed on to waterproof material, or laminated, or place in a suitable waterproof container such as a map case.
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Check weather The instructor will supply the latest marine weather forecast (or one can be obtained from the internet). The group should consider its impact on the planned voyage.
2. Debrief and disperse ●
Summarise any key points
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Conrm the arrangements for the following day
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Disperse
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PRACTICAL NAVIGATION AFLOAT Lesson Objectives This lesson is intended to give each student practical experience of navigating a dive boat at sea.
Achievement Targets At the end of this lesson students should have practical experience of the following: ●
using transits taken from a chart to nd a position
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taking transits and bearings to x a position
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locating a position using GPS
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following a route using GPS and visual marks
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following a route using pilotage
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locating a dive site using an echo sounder
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carrying out spiral and grid searches
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predicting and dealing with the effects of tides and weather
Equipment Needed Boat equipped with GPS and echo sounder Waterproof charts of the area Hand bearing compass Each person requires: protective clothing appropriate for the expected weather conditions at sea; lifejacket, buoyancy aid, or buoyant diving suit suitable for the conditions, food and drink for personal needs Each student needs, in addition: a waterproof copy of their voyage plan plus the means to write on it
Lesson Contents The lesson involves navigating the whole of the voyage plan prepared by the students on Day 1 of the course, carrying out various navigational exercises, detailed below, on the way. Each student should be involved in each activity. Students take it in turns to use the voyage plan to navigate & drive the boat. If local conditions prevent the exercises being performed exactly as specied, do whatever best meets the Achievement Targets. The focus of the lesson should be on navigation and techniques, as opposed to boat handling and responsibilities, or the specics of operating the particular GPS and echo sounder units on the boat. If using a charter boat or if most or all of the students are not capable of helming a boat competently, let the skipper or a helmsman drive the boat. Instructors should use every opportunity to coach the students in addition to the specic exercises detailed below (but not at the expense of the core parts of the syllabus). Concentrate on helping the students to understand how the theory they have been taught relates to real world features they can see around them. The following areas are suggested topics to coach on should the opportunity arise:
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Orientation ●
'Where is North?'; 'Where is 100°?' Encourage students to look at the chart and identify what is to the N,S,E,W of them.
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Point to features on chart (easy ones - buoys etc; harder ones - bays, inlets, entrances etc). and get students to point out the actual features.
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Demonstrate how the appearance of features change as you motor past.
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Ask 'where are we on the chart?' Look at the GPS, take bearings, etc.
Buoyage ●
Lateral, cardinal, safe water, isolated danger, special buoys. Shapes, colours, top marks & lights
Positioning ●
Transits, compass lines, GPS, latitude/longitude & echo sounder. Leading lines, leading lights, clearing lines.
Tides ●
Slack, ebb, ood, LW, HW, springs, neaps, tidal range, Rule of twelfths, Chart datum, tidal diamonds, tidal stream atlas.
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Effect of tide on boat at slow speed, at high speed.
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Overfalls, eddies and other evidence of tidal streams.
Wind & waves ●
Sea types: head sea, following sea, beam sea.
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Beaufort scale; 4 - ‘moderate breeze’, wind speed 1 -16 knots, wave height 1-1.5m. Compare with local conditions.
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Onshore/offshore winds, fetch, wind over tide.
Pragmatics ●
Demonstrate that you can't do chartwork easily in an open boat, that's why you need to do it before.
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Time and plan don't have to be followed slavishly.
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Consider what is worth writing down and what isn't. How could voyage details be made more useable at sea. How detailed do plans need to be?
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Pros and cons of electronic chart systems.
Distance off ●
Estimate distance off by size of objects (see Appendix B)
1. Prepare & launch boats (45min) This section assumes that the boats will be trailer launched. Modify the tasks as appropriate if using boats from a berth or mooring. ●
Review an up-to-date weather forecast & reconsider impact of the weather/tide.
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Prepare boats for launching. Deal with fees, permits, etc.
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Demonstrate how to operate/use GPS and echo sounder. Enter route details (this task could be done in advance, provided the students are still involved). Consider handing out copies of the operating manuals before the course, or at the end of Day 1.
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Make sure that someone ashore knows your plans and knows what to do should they become concerned for your well being.
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If using more than one boat, swap VHF callsigns and agree channel (or use DSC).
2. Navigate towards eercise area 1 ●
'Buoy Hopping’ (Pilotage) (30min) The position of Exercise area 1 should be chosen so that at least part of the passage towards it can be accomplished using pilotage (the marks do not have to be buoys) alone. Demonstrate the use of leading lines (transit of landmark/buoy). If no seaward mark is available use a back bearing.
3. Eercises in area 1 ●
Transits (30min) Select a suitable random point, take transits, move away and demonstrate the return procedure. The leading transit should be into the tide/wind, whichever is greatest. Check position relocation accuracy with GPS.
●
Searches (20 min) Deploy a small shot and demonstrate spiral and grid search patterns. Remind students of the pros and cons of the methods (see Finding Dive Sites lesson), and point out how the tide, wind, sea state, sonar beam width, expected nature of target, and availability of visual marks inuence the way in which searches should be carried out. Students will get an opportunity to practice search techniques themselves after lunch.
4. Eercises around 'Mster Point' ●
'Mystery Point' location (30min) Locate ‘Mystery Point’ using transits (check accuracy with GPS).
●
GPS point location (20min) Select a suitable random position with the GPS, record or store the lat/long. Move away & demonstrate return procedure using the lat/long numbers & compass only.
●
Plot current location (10min) Select a suitable position & take transits/bearings then plot the location on a waterproof chart. Check the lat/long position accuracy with GPS.
●
Dead reckoning (20min) Run a 4 leg square course approximately 30 seconds per leg, turning 90° each time and returning to start point. Check accuracy with the GPS and/or plotter screen.
5. Eercises on passage to lunch ●
GPS route (20min) Follow the route pre-entered into the GPS. Use a GPS moving highway, cross track error (XTE), or similar display to maintain course. Demonstrate the distance to go and time to go/ETA displays and the arrival alarm function.
●
Lunch To be taken at anchor or ashore as necessary. Instructors should provide coaching on the events so far, or on additional theory/practical topics, if appropriate.
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6. Location of dive sites A normal diving expedition would be timed to arrive at a tidal dive site a comfortable time before slack, but this may be impractical on the course. Teaching the course syllabus takes priority. The choice of location techniques offered in the following exercises reects the wide variation in dive sites and their surroundings at different locations around the UK. ●
Location of dive site 1 (45min) Navigate to the site using the voyage plan, and locate it by echo sounder using, in descending order of preference either transits, pilotage or GPS. Do not shot. Practice grid search technique. Compare the state of the tide and weather with the predictions, and discuss the implications.
●
Location of dive site 2 (45min) Navigate to the site from dive site 1 using the voyage plan and locate it by echo sounder using, in descending order of preference either GPS, pilotage or transits. Shot with a small datum shot (once only) and practice spiral search technique. Use a grid search instead if conditions dictate. If different from dive site 1, compare the state of the tide and weather with the predictions, and discuss the implications.
●
Return navigation (45min) Use the relevant parts of the voyage plan pre-entered into the GPS to navigate back to shore. Take visual xes as required.
7. Recover and close This section assumes that the boats are trailer launched. Modify the tasks as appropriate if using boats from a berth or mooring. ●
Recover boats onto trailers (45min) Inform persons ashore of safe return Recover boat Wash down, ush engine, rell fuel/oil etc or make arrangements for these tasks to be done later Prepare boat and trailer for towing
●
Student debrief/Open Forum (15min) See theory lesson notes
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APPENDIx A - TAILORING THE COURSE This is a practical course which demands that students use actual data from ofcial nautical reference materials to plan and execute navigational exercises at sea. This means that instructors must tailor parts of the course to the area in which the practical work will be done. These notes explain what tailoring is required.
Location The practical day may be run at any location that enables students to meet the achievement targets of the relevant lesson. Preferably, the location should be one that gives convenient access to known dive sites for which information is available in guidebooks etc. Failing that, there should be suitable submerged targets that can be located by echo sounder as if they were dive sites. In either case try to use targets which will show up clearly on the sounder, and above all make sure that the instructors can nd the targets themselves without fail. However, with advanced students, more challenging (from a navigational point of view) sites could be chosen. In bad weather, it may be necessary to divert to a more sheltered area as a backup. It should still meet the above criteria. The planning exercise includes using published information sufcient to identify and locate two known dive sites, which are then located by echo sounder during the practical aoat. This should be the highlight of the course, so instructors should think carefully about which of the following options would be best if using a backup site: ●
students plan for real dive sites, but use a substitute plan supplied by the instructors for the afternoon practical aoat
●
students plan for the backup sites using 'dive site' information prepared by the instructors and use their plan for the af ternoon practical
●
the afternoon practical is deferred until a later date when it can be carried out at the real dive sites.
Having selected a location, instructors must do the following: ●
collect information as appropriate o
o
●
up to date navigational materials: charts, tide tables, almanacs, pilots, etc, dive guides, magazine articles, local information booklets, etc. If practicable, students could also be pointed at information on the internet.
prepare handouts as required
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Assign waypoints for the practical exercise as shown in the table below. Entries in the Type column have the following meanings: ●
location: a description which can be used to identify a position on the char t
●
position: a lat/long
●
name: a description which can be used to look up reference information about a dive site but which does not directly identify a position on the chart (eg the name of a shipwreck)
Waypoint
Lat/long
Type
Notes
Launch
location
1st waypoint
location or position
simple x preferred
1st exercise area
location or position
simple x preferred
Mystery Point
position
students must nd transits on chart.
1st exercise area should be a nearby area suitable for transit exercises: not too much boat trafc, free from dangers for at least 0.5ca all round, tide but not too much
Choose point that will enable this. Otherwise criteria as for 1st exercise area lunch spot
location
anchorage or ashore
dive sites 1 & 2
name
named choose two local dive sites, suitable for locating by echo sounder, for which reliable information exists. Wherever possible, one of the sites should be capable of being located using transits or pilotage, and one using a GPS x. If possible at least one of the sites should require slack water for diving (it need not actually be slack when visited on this course).
Recovery
location
normally same as launch there is no teaching reason why the course needs to end up where it started, but logistics would usually dictate an out and back voyage
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Eercises/VAs Some exercises will need tailoring to the location, as shown in the following table. All items requiring tailoring and all links to the Voyage planning exercise are marked in red on the VAs. Note that the VAs also contain a few items in red that are neither links nor items needing tailoring.
Item
Action needed
Chart basics VA23-24
Replace questions by similar ones using local charts. Q5a must be the launch location for the practical day
Measurement basics VA7
Replace {rst waypoint on chart} by 1st waypoint location/position
Tides VA10
Optional - replace or supplement the rst four questions with local examples.
Tides VA13
Tailor the questions to the local area or replace with similar ones. Supply answers.
Position xing methods VA9
Insert the lat/long of 'Mystery Point'
Voyage planning VA19
Insert the locations/positions for the practical day
CPF09_exercises handout.doc
Update the Word le to include the above tailored exercises, then print a handout for each student.
(may have version number sufx - use latest available)
The above table shows those instances where tailoring is essential if the course materials are to work as intended. At their discretion, instructors may replace other components of the course materials which are used as illustrations, such as the chart extracts used on VAs, with local examples. However, note that UKHO permission is required to reproduce charts, and that the Course Manual is produced centrally by HQ, and cannot readily be changed. Do not underestimate the effort required to make changes beyond the essential ones listed above.
Model voage plan At the instructor's discretion, the model voyage plan may be replaced by one based on a dive site and/ or area familiar to the students. This will entail replacing VA14 and VA15 in the Voyage Planning lesson, and replacing the le CPF09 VPexample.doc, and if necessary the le CPF09 VPexample_annotated .doc (these lenames may be sufxed with a version number: use the latest available).
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APPENDIx B - ESTIMATING DISTANCE Range
Appearance
6M
large houses, small apartment buildings, towers recognisable
2M
chimneys on buildings visible, windows are dots, can see vehicles moving
1M
people = dots, trunks of large trees visible
0.5M
people = posts, larger branches on trees visible
0.25M
people's head and body visible, leg movement seen, clothing colour visible
250m
face/hands/clothing detail = blur but visible
100m
eyes = dots
50m
eyes/mouth clear
Notes In bright illumination objects appear closer than the table suggests. In murky weather objects appear more distant than the table suggests. Radar is much better at measuring distance, if you have it.
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APPENDIx C - TIDE DIFFERENCES Add or subtract time difference to High Water DOVER to get approximate local HW. Time differences are hours and minutes in the format ±hhmm.
England — South Coast
Orkney and Shetland I
Isles of Scilly
–0635
Kirkwall
–0040
Rosslare
–0525
Penzance
–0635
Lerwick
–0010
Wicklow
–0020
Falmouth
–0610
Dublin Bar
+0015
Fowey
–0555
Outer Hebrides
Lough Carlingford
+0010
Plymouth
–0540
Stornoway
–0420
Strangford Lough
+0200
Salcombe
–0535
Castle Bay
–0510
Belfast
+0010
Dartmouth
–0505
Scotland — West Coast
Londonderry
–0300
Torquay
–0500
Ullapool
–0415
Lough Swilly
–0455
Exmouth Approaches –0445
Portree
–0440
Killybegs
–0530
Lyme Regis
–0450
Fort William
–0505
Broadhaven
–0533
Portland
–0430
Tobermory
–0510
Galway
–0510
Portsmouth
+0020
Oban
–0510
Tarbert Island
–0535
Campbeltown
+0045
Bantry
+0555
Rothesay
+0100
Cobh
–0600
Stranraer
+0055
Newhaven
0000
England — East Coast Ramsgate
+0020
Sheerness
+0130
Ireland
England — West Coast
Burnham–on–Crouch +0115
Barrow–in–Furness
+0025
Brightlingsea
+0050
Liverpool
+0005
Harwich
+0040
Lowestoft
–0145
Isle of Man
Hunstanton
–0455
Douglas
Spurn Point
–0550
Wales
Bridlington
+0545
Holyhead
–0050
Whitby
+0500
Aberystwyth
–0330
North Shields
+0430
Fishguard
–0400
Milford Haven
–0500
Swansea
–0500
Scotland — East Coast
+0015
Leith
+0340
Dundee
+0350
England — West Coast
Aberdeen
+0220
Avonmouth
–0410
Inverness
+0100
Ilfracombe
–0525
Wick
+0015
St. Ives
–0610
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APPENDIx D - CHART FOR OFFSETTING TIDE AGAINST COURSE Correction angles 90
80
70
60 e l g n a 50 n o i t c e 40 r r o C
30
20
10
0 1
2
3
4
5
Speed ratio
Angle of tide to boat heading 90
86
6
75
60
45
30
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APPENDIx E - MAGNETIC VARIATION Magnetic variation in the UK in the early 21st century is sufciently small (and becoming smaller still) that small dive boats can be navigated satisfactorily without worrying about it, Consequently the calculation of variation and conversion between true and magnetic bearings are not taught on this course. However, there are parts of the world where BSAC is represented and variation is much larger. At the instructor’s discretion, the following notes may be used in conjunction with VA16-19 of Measurement basics to teach how to use variation.
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MAGNETIC VARIATION (OPTIONAL) Use these notes in conjunction with the four VAs which follow the black (end of lesson) slide in Measurement basics.
Compass Rose One or more compass roses are printed prominently on every chart. They can be used to measure the angle relative to north of any line drawn on the chart (students will practice doing this shortly). A compass rose consists of an outer circle graduated in degrees, with 0° aligned to true north. Many charts also have a similar inner circle, rotated so that 0° is pointing to magnetic north as it was when the char t was printed. Variation is shown in the special format, as in the following example:
4°30’W 2003 (9’E) where 4°30’W was the variation in 2003 and variation is changing by (9’E) each year. Using this information it is easy to work out the current variation and use it to convert between true and magnetic bearings.
One or more on ever chart True bearings on outer circle Magnetic bearings on inner circle The magnetic rose may be omitted on some charts.
Variation at given date plus annual change
Calculating variation Work through the example on the VA.
variationnow= variationchart±(earselapsed x changeannual)
Variation is 4° 30’ W in 2003 (9’ E) what is it in 2010? ● variationchart = 4° 30’ W ● yearselapsed = 2010 – 2003 = 7 ●
changeannual = 9’ E
● yearselapsedX changeannual= 63’ E = 1°3’ E ● variationnow= 4° 30’ W – 1°3’ E = 3° 27’ W
Eercise: get variation from our chart & write on our voage plan
Bearing conversion Bearings have to be converted back and forth between true and magnetic. This is a simple procedure which involves adding or subtracting the variation to or from the original bearing. There are several different mnemonics in common use to help mariners remember in which direction to apply the corrections. To avoid confusion this course just teaches one: CADET, Compass to True Add East. 'Compass' is equivalent to 'Mag88
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netic' (‘compass’ includes deviation, but he distinction is irrelevant for small boats). [Portland plotters often have a degree scale on them which can be used to apply variation. Some people nd it helpful. If appropriate mention this when introducing the plotter on VA12.]
Chart ●
bearings recorded are true °T
●
bearings measured are true °T
Compass ●
bearings displayed are magnetic °M (or °C)
●
magnetic bearing required for steering
Conversion
‘ CadET ’ ● Compass to True Add East ●
or subtract West
● vice versa for °T to °M (or °C) C to T
+E or -W
T to C
-E or +W
Bearing conversion eamples Work through the example with the students.
Bearing 330°T, variation 4° W, what is °M? ●
compass to true add east/subtract west
●
true to compass subtract east/add west
●
330°T + 4° W = 334°M
Bearing 170°M, variation 4° W, what is °T? ●
compass to true add east/subtract west
●
170°M - 4° W = 166°T
359°T, variation 4° W, °M? ●
003°M
117°M, variation 3° E, °T? ●
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APPENDIx F - MODEL VOyAGE PLAN The model voyage plan is published separately as CPF09 VPexample.pdf on on the website along with the other instructor materials for this course.
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APPENDIx G - FURTHER READING The following reading list appears in the Course Manual issued to students.
Adlard Coles Book of Navigation Exercises A Noice & J Stevens ISBN 13: 9780713663235 Adlard Coles Book of Electronic Navigation Tim Bartlett ISBN 10: 0713657154 The Expedition Manual: a BSAC manual due for publication April 2010 GPS Aoat Bill Anderson ISBN 13: 9781898660934 Reeds Skippers Handbook Malcolm Pearson ISBN 13: 9780713683387 Safe Diving: the BSAC safe diving reference booklet. Seamanship: A Guide for Divers, 2nd edition : a BSAC manual ISBN 0953891976
All the above titles are available from the BSAC Shop.
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