Auto pilot.docx

STEERING GEAR A Steering Gear is the equipment provided on ships to turn the ship to left (Port side) or to right (Starboard side) while in motion during sailing. The Steering Gear works only when the ship is in motion and, does not work when the ship is stationary. All the ships are to be provided with, an efficient main steering gear, an auxiliary steering gear and, except for very small ships, the main steering gear should be power operated. Manually operated, mechanical Steering Gears were in use during sailing ship days. Sailors with strong body were required to operate the Steering Gears. Later on, after the onset of steam engines, mechanized gears were used. Modern ships use all very sophisticated Steering Gear systems which could fall in either of the categories   

Fully hydraulic type Electro-hydraulic type Fully electric type

We also saw a general overview about these Steering Gears in a previous article. In this article we will proceed to know more about their constructional details and components. Before studying further, just take a close look at the sketch of the Steering Gear arrangement which shows the various parts and components of a dual type system i.e. electro-hydraulic type gear arrangement.

Steering Gear Arrangement

Working of the Steering Gear When the ship is required to be turned on receiving an order (say by 10° to port) from the Master or, the Duty Officer, the helmsman turns the steering wheel towards port until the rudder has reached 10° to port as read on rudder indicator. The mechanism of the Steering Gear works as under; Complete Steering Gear system consists of three main parts namely 1. Telemotor 2. Control Unit 3. Power Unit.

A brief description about the construction and working of these components is as follows

Telemotor unit comprises of two parts namely, Transmitter and Receiver. The Transmitter is located on the navigation bridge in the form of a wheel, which transmits the given order to the Receiver located in the steering gear compartment, by turning the steering wheel. The Receiver conveys this order to the Control Unit, also located in the steering gear compartment, via linear motion. The Telemotor is generally hydraulic type, electric type or, as is the case with modern steering systems, it could be electro-hydraulic type. In olden days, Telemotors were purely mechanical type consisting of linkages and chains with sprockets. As they were operated manually, they required very healthy sailors to operate them. Control Unit is the link between the Telemotor and the Power Unit. I receives signal from the Telemotor and operates the Power Unit until it receives another signal, this time from the Rudder through the Hunting Gear, to stop the operation of Power Unit. Power Unit can be any prime mover like steam engine, diesel engine or, an electric motor, directly coupled to the Rudder; it can be an electro-hydraulic unit or, an allelectric unit complete with the Telemotor.

Auto pilot An autopilot is a mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being

Auto pilot on a ship is driven by the gyro compass. The gyro compass is not magnetic but works on the principle of gyroscopic inertia and points to true north. Once set, the gyro compass will steer the ship in a straight line until turned off. It has adjustments for sensitivity in case it's too windy. You don't want to over work the steering engine in a storm.

Adaptive autopilot for marine vessels An autopilot automatically adjusts the sensitivity of a ship's steering system to accommodate changes in speed as well as sea and wind conditions. The autopilot utilizes heading error, speed and speed squared signals to produce a rudder order signal for controlling rudder position. The rudder order signal is developed in a heading keeping circuit unless a heading change greater than a predetermined threshold is commanded, in which event a programmer substitutes a heading change circuit for the heading keeping circuit. The sensitivity of the heading change circuit is automatically adjusted as an inverse function of vessel speed squared, and automatic rudder order limits are established in the same circuit as an inverse function of speed. The sensitivity of the heading keeping circuit is adjusted in accordance with a signal from an automatic gain control circuit which derives a performance index J from ship's speed, heading error and rudder order signals occurring during a given measurement interval. The performance index derived during a given measurement interval is compared with the index derived in the previous interval and a counter register is set

according to the results of this comparison. The heading keeping circuit receives heading error signals which are processed in a first proportional channel and also differentiated and processed in a second or rate channel. The attenuation in each channel is adjusted in accordance with the value stored in the counter register. The attenuation in the rate channel is made equal to the square root of the attenuation in the proportional channel. The modified rate and proportional signals are added to obtain the final rudder order signal. Emergency Steering Gear On ships equipped with electromechanical steering gear, the old-fashioned, hand-operated steering wheel is about the only recourse if the primary mechanism fails. On some small ships, a yoke can be fitted over the rudder head, and the rudder can be turned with a block and tackle. Electrohydraulic steering gear usually is provided with a standby pumping unit for emergency use. It is composed of a pump and an electric motor, identical to those shown in figure 2-6. If the steering engine being employed has a casualty, the six-way pump transfer cock is adjusted to align the ram with the standby pumping unit; the power is turned on in the standby unit; and steering is transferred over to the standby unit. Emergency steering for destroyers also uses the trick wheel, shown in figure 2-6. If a steering signal failure occurs between the steering wheel on the bridge and the receiving unit, the helmsman standing watch in after steering operates the trick wheel and receives steering orders on the sound-powered telephone. Should a power failure occur in steering aft, the rudder is moved by disengaging the running electric motor, and hand-pumping oil to the ram by means of a handcrank. This procedure is very slow. The rudder turns only a small amount for every revolution of the crank. Steering Engine Cutout A safety device is installed on every steering engine. This safety device stops rudder movement when the rudder is brought against the stops. The limit most rudders can be turned is 35° to either side of center. Full rudder on most ships is 30° right or left; the extra 5° is applied only in emergencies. Unless you are ordered to do so, never put the rudder hard over. It is possible for the rudder to jam against the stops, causing you to make circles in the ocean. Rudder Every ship is provided with a rudder located aft. When the rudder is set at an angle on a moving ship, a high-pressure area builds on the leading surface, and a low-pressure area forms on the trailing surface. Thus the water, through this difference in pressure areas, exerts a force against the leading surface of the rudder, which in turn forces the stem in the direction opposite that which the rudder is set. When the helm on an oldtime ship was moved athwartships across the deck, the rudder motion was in the opposite direction. The result was that the ship's head would go in the direction opposite that in which the helm was moved, and this still is true of any small craft steered with a tiller. On all ships equipped with steering wheels, however, the wheel, rudder, and ship's head all move in the same direction. That is, when you turn the wheel to port, the rudder goes to port, and the ship makes its turn to port. Remember, though, that the ship begins its port turn by sending its stem to starboard. The more headway a ship has, the more water piles up against the rudder under the counter, and the quicker the stem is pushed off. Consequently, a ship always turns faster and answers its rudder sooner at high speeds than at low speeds. Also, a greater angle on the rudder is required to turn a ship moving slow than one moving fast. STEERING STATIONS When a ship goes into action, no one knows where it might be hit. If a ship has only a single steering station, a hit there would put it out of the fight. For this reason, a combat ship has more than one steering station so that control can be shifted almost instantaneously to any station. A destroyer, for instance, may be steered from the bridge, after steering, or the steering engineroom. Some ships have fewer steering stations, but every ship has at least two. RUDDER ANGLE INDICATOR The instrument above and forward of the wheel angle indicator is the rudder angle order indicator-transmitter (fig. 2-7). This instrument has a dual purpose. During normal steering situations, it shows the actual angle of the rudder, which usually lags the wheel angle indicator by about 2° because of the time required for the steering mechanism to operate. For emergency steering, this instrument becomes useful in transmitting visual orders to the helmsman in after steering. By operation of the control knob, the rudder order is displayed on the instrument when the pointer 2-6