Freeboard Calculation

ICLL Regulation 3 Length (L) means 96% of the total length on a water line at 85% of the least moulded depth measured from the top of the keel, or the length from the foreside of the stem to the axis of the ruder stock on the waterline, if that be greater. In ships designed with a rake of keel the waterline on which this length is measured shall be parallel to the designed waterline. The length (L) shall be measured in meters. Depth for Freeboard (D) (a). The depth for freeboard (D) is the moulded depth amidships, plus the thickness of the freeboard deck stringer plate, where fitted, plus T x (L-S)/L if the exposed freeboard deck is sheathed(berselubung), where T is the mean thickness of the exposed sheathing clear of deck openings, and S is the total length of superstructures as defined in sub-paragraph (10)(d) of this Regulation. (b). The depth for freeboard (D) in a ship having a rounded gunwale ( gunwale is the top edge of the side of a boat) with a radius greater than 4 per cent of the breadth (B) or having topsides of unusual form is the depth for freeboard of a ship having a midship section with vertical topsides and with the same round of beam and area of topside section equal to that provided by the actual midship section.

(7)Block Coefficient. The block coefficient (Cb) at 0.85 is given by: Cb = Dis / ( L x B x d1) Dis is the volume of the moulded displacement of the ship, excluding bossing, in a ship with a metal shell, and is the volume of displacement to the outer surface of the hull in a ship with a shell of any other material, both taken at a moulded draught of d1; and where d1 is 85 per cent of the least moulded depth. (9). Freeboard Deck. The freeboard deck is normally the uppermost complete deck exposed to weather and sea, which has permanent means of closing all openings in the weather part thereof, and below which all openings in the sides of the ship are fitted with permanent means of watertight closing. In a ship having a discontinuous freeboard deck, the lowest line of the exposed deck and the continuation of that line parallel to the upper part of the deck is taken as the freeboard deck. At the option of the owner and subject to the approval of the Administration, a lower deck may be designated as the freeboard deck provided it is a complete and permanent deck continuous in a fore and aft direction at least between the machinery space and peak bulkheads and continuous athwartships. When this lower deck is stepped the lowest line of the deck and the continuation of that line parallel to the upper part of the deck is taken as the freeboard deck. When a lower deck is designated as the freeboard deck, that part of the hull which extends above the freeboard deck is treated as a superstructure so far as concerns the application of the conditions of assignment and the calculation of freeboard. It is from this deck that the freeboard is calculated.

ICLL Regulation 31 Share

Correction for Depth 

(1). Where D exceeds L/15 the freeboard shall be increased by

(D - L/15) x R millimetres, where R is L/0.48 at lengths less than 120 metres and 250 at 120 metres length and above, or (D - L/15) x R inches, where R is L/131.2 at lengths less than 393.6 feet and 3 at 393.6 feet length and above. 

(2). Where D is less than L/15 no reduction shall be made except in a ship with an enclosed superstructure covering at least 0.6 L amidships, with a complete trunk, or combination of detached enclosed superstructures and trunks which extend all fore and aft, where the freeboard shall be reduced at the rate prescribed in paragraph (1) of this Regulation.



(3). Where the height of superstructure or trunk is less than the standard height, the reduction shall be in the ratio of the actual to the standard height as defined in Regulation 33.

ICLL Regulation 39 Share

Minimum Bow Height 

(1). The bow height defined as the vertical distance at the forward perpendicular between the waterline corresponding to the assigned summer freeboard and the designed trim and the top of the exposed deck at side shall be not less than: 

. for ships below 250 metres in length, 56



L x (1 - L /500) x 1.36 / (Cb+0.68) millimeters;

for ships of 250 metres and above in length,

7000 x {1.36/(Cb + 0.68)} millimeters; where L is the length of the ship in metres, Cb is the block coefficient which is to be taken as not less than 0.68 

or, . for ships below 820 feet in length,

0.672 x L x (1 - L/1640) x 1.36 / (Cb + 0.68) inches; 

. for ships of 820 feet and above in length,

275.6 x {1.36/(Cb + 0.68)} inches; where L is the length of the ship in feet, Cb is the block coefficient which is to be taken as not less than 0.68 

(2). Where the bow height required in paragraph (1) of this Regulation is obtained by sheer, the sheer shall extend for at least 15 per cent of the length of the ship measured from the forward perpendicular. Where it is obtained by fitting a superstructure, such superstructure shall extend from the stem to a point at least 0.07 L abaft the forward perpendicular, and it shall comply with the following requirements: 

(a). for ships not over 100 metres (328 feet) in length it shall be enclosed as defined in Regulation 3 (10), and



(b). for ships over 100 metres (328 feet) in length it need not comply with Regulation 3 (10) but shall be fitted with closing appliances to the satisfaction of the Administration.

ICLL Regulation 40 Share

Minimum Freeboards Summer freeboard (1). The minimum freeboard in summer shall be the freeboard derived from the tables in Regulation 28 as modified by the corrections in Regulations 27, as applicable, 29, 30, 31, 32, 37, 38 and, if applicable, 39. (2). The freeboard in salt water, as calculated in accordance with paragraph (1) of this Regulation, but without the correction for deck line, as provided by Regulation 32, shall not be less than 50 millimetres (2 inches). For ships having in position 1 hatchways with covers which do not comply with the requirements of Regulations 15 (7), 16 or 26, the freeboard shall be not less than 150 millimetres (6 inches). Tropical Freeboard (3). The minimum freeboard in the Tropical Zone shall be the freeboard obtained by a deduction from the summer freeboard of one forty-eighth of the summer draught measured from the top of the keel to the centre of the ring of the load line mark. (4). The freeboard in salt water, as calculated in accordance with the paragraph (3) of this Regulation, but without the correction for deck line, as provided by Regulation 32, shall not be less than 50 millimetres (2 inches). For ships having in position 1 hatchways with covers which do not comply with the requirements of Regulations 15 (7), 16 or 26, the freeboard shall be not less than 150 millimetres (6 inches).

Winter Freeboard (5). The minimum freeboard in winter shall be the freeboard obtained by an addition to the summer freeboard of one forty-eighth of summer draught, measured from the top of the keel to the centre of the ring of the load line mark. Winter North Atlantic Freeboard (6). The minimum freeboard for ships of not more than 100 metres (328 feet) in length, which enter any part of the North Atlantic defined in Regulation 52 (Annex II) during the winter seasonal period, shall be the winter freeboard plus 50 millimetres (2 inches). For other ships, the Winter North Atlantic Freeboard shall be the winter freeboard.

Fresh Water Freeboard (7). The minimum freeboard in fresh water of unit density shall be obtained by deducting from the minimum freeboard in salt water: Δ / 40T centimeters (inches) where Δ = displacement in salt water in tons at the summer load waterline, T = tons per centimetre (inch) immersion in salt water at the summer load waterline. (8). Where the displacement at the summer load waterline cannot be certified, the deduction shall be one fortyeighth of summer draught, measured from the top of the keel to the centre of the ring of the load line mark.

Freeboard calculation Share

The main purpose of the calculation is to find the freeboard draught (maximum allowable draught). The rule reference is the International Convention of Load Lines, 1966. The first step in freeboard calculation is deciding the type of freeboard. There are two kinds, Type 'A' and Type 'B' (defined in ICLL Regulation 27). Usually Type 'A' ships are the ones designed to carry liquid cargoes in bulk, where the cargo tanks have small assess openings. Because of small access to the cargo and lack of floodable volume after damage in case of a loaded ship, such ships can have a low freeboard compared to Type 'B' ships. Type 'B' ships are all the other ships other than Type 'A', and these have larger freeboard (or lower maximum draught). Oil tankers are usually Type A ships. But sometimes a tanker can be taken as a Type B ship, with larger freeboard when the cargo capacity is the limiting factor. In such tankers with a normal density cargo the maximum dead weight would be with a lower draught than the Type A ship, and near the Type B ship. In choosing Type B notation, the stringent damage stability calculation by ICLL need not be satisfied.

One example is taken for calculation purpose. In this a 70,000 tons deadweight oil tanker is taken. Our aim is to find the maximum summer draught allowed, and to check the bow height. Once the summer draught is decided Contents [hide]

1 Principal Particulars 2 1. Type of Ship: 3 2. Dimensions 4 3. Freeboard calculation 5 4. Summary 6 5. Minimum Bow height 7 6. Freeboard mark

Principal Particulars LBP = 264.00 m BREADTH (MLD) = 48.00 m DEPTH (MLD) = 23.10 m

1. Type of Ship: Freeboard : 'A" type of ship; Freeboard deck (ICLL Regulation 3): Upper deck

2. Dimensions Freeboard length (Lf) = 265.152 m (ICLL Regulation 3) LWL (Length on waterline) at 85 % depth (mld) = 270.468 m LWL x 0.96 = 259.649 m Length from the fore side of the stem to the axis of the rudder stock on a waterline at 85 % depth (mld) = 265.152 m Amidship = Freeboard length / 2 = 132.576 m Freeboard depth (Df) = 23.119 m (ICLL Regulation 3) Design depth (mld) = 23.100 m Thickness of freeboard deck (stringer plate) = 0.019 m

Depth at 85 % depth (mld) = 19.635 m Thickness of keel plate = 0.021 m Volume (mld) at 85 % depth (mld) = 207,733 cu.m Block coefficient (Cb) at 85 % depth (mld) = 0.8313 (Regulation 3) Cb = Δf0 / (Lf x B x 0.85D) = 207,733 / (265.152 x 48 x 19.635) = 0.8313

3. Freeboard calculation For Lf = 265,152 mm and Df = 23,119 mm and Type 'A' freeboard, refer the freeboard table A. Ft = 3,101 mm at Lf = 265,000 mm Ft = 3,102 mm at Lf = 265,152 mm Ft = 3,106 mm at Lf = 266,000 mm. Tabular freeboard (Ft) = 3,102 mm Correction for Cb (ICLL Regulation 30) = 3,447 mm Correction = Ft x (Cb + 0.68)/1.36

= 3102 x (0.8313 + 0.68)/1.36 = 3,447 mm

Correction for depth (ICLL Regulation 31) = 1,361 mm (Df - Lf/15) x R = (23.119 -265.152/15) x 250 = 1,361 mm Correction for Sheer (ICLL Regulation 38) = 846 mm Aft half (standard), As = 25(Lf/3 + 10) x 1 + 11.1(Lf/3 + 10) x 3 + 2.8(Lf/3 + 10) x 3

= 2460 x 1 + 1092 x 3 + 275 x 3 = 6561

Aft half (Actual), Aa = at A.P x 1 + at Lf/6 x 3 + at Lf/3 x 3 + at miship x 1

= 683x 1 + 30 x 3 + 0 x 3 + 0 x 1 = 773

Fore half (standard), Fs = 5.6(Lf/3 + 10) x 3 + 22.2(Lf/3 + 10) x 3 + 50(Lf/3 + 10) x 1

= 551 x 3 + 2184 x 3 + 4919 x 1 = 13124

Fore half (Actual), Fa = at miship x 1 + x 1 + at Lf/3 x 3 + at Lf/6 x 3 + at F.P x 1

=0 x 1 + 0 x 3 + 7 x 3 + 854 x 1 = 875

Deficiency of sheer After sheer = (As - Aa)/8 =(6561 - 773)/8 = 724 mm Forward sheer = (Fs - Fa)/8 = (13124 875)/8 = 1531 mm Total deficiency of sheer = (724 + 1532)/2 = 1128 mm Correction for variations from standard sheer profile = (deficiency of sheer -sheer credit) x (0.75 - S/2Lf) = (1128 - 0) x (0.75 - 0/(2 x 265.152)) = 846 mm

4. Summary (i) Tabular freeboard with correction for Cb = 3447 mm (ii) Correction for depth = 1361 mm (iii) Correction for superstructure and trunk = 0 mm (iv) Correction for sheer = 846 mm (1) Summer freeboard = (i) + (ii) + (iii) + (iv) = 5654 mm (2) Deck plate = 19 mm (3) Moulded depth = 23100 mm (4) Keel plate = 21 mm (5) Summer draft (mld) = (3) + (2) - (1) = 17465 mm. This is the maximum possible. In this case a lower scantling draft is chosen to reduce the weight of the steel. (6) Scantling draft (mld) = 17050 mm (7) Scantling draft (ext) = (6) + (4) = 17071 mm (8) Proposed summer draft(mld) = 17050 mm (9) Summer freeboard = (3) - (8) + (2) = 6069 mm

5. Minimum Bow height From ICLL Regulation 39, required bow height = 7000 x {1.36/(Cb + 0.68)} for Lf >= 250 m = 7000 x {1.36/ (0.8313 + 0.68)} = 6299 mm Length of sheer / Lf = 0.186 (>0.15) The actual bow height on the ship = (Moulded depth at amidship) + (Forecastle height at FP) + Sheer at FP) + (Thickness of deck plate at FP) - (Summer draft) = 23100 + 0 + 854 + 15 - 17050 = 6919 mm

Actual bow height > Required minimum bow height

6. Freeboard mark

Once the summer draft is decided, the other (winter, fresh water, tropical fresh water) drafts can be calculated using ICLL Regulation 40 as: Tropical Freeboard = Summer freeboard - (Summer draft (mld) / 48 ) = 6069 - 17050/48 = 6069 - 355 = 5714 mm Winter Freeboard = Summer freeboard + (Summer draft (mld) / 48 ) = 6069 + 17050/48 = 6069 + 355 = 6424 mm Fresh water Freeboard = Summer freeboard - Δf0 / (40 x TPC) = 606.9 - 18265.5 / (40 x 118.29)= 606.9 - 38.6 = 568.3 cm = 5683 mm Tropical Fresh water Freeboard = Fresh water freeboard - (Summer draft (mld) / 48 ) = 5683 - 17050/48 = 5683 - 355 = 5328 mm Now from the above data, and using the Classification symbol (AB for American Bureau of Shipping here), the load line mark (plimsoll mark) is drawn. For other Classification societies it is NV for Det Norske Veritas, LR for Lloyds Register of Shipping, NK for Class NK.