CHAPTER-4

STEERING GEAR AND ELECTRONIC POSITION-FIXING SYSTEMS

Q.1.?How is a steering gear tested?

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Ans. After prolonged use of autopilot and before entering coastal waters the steering gear must be tested at all the manual steering positions on the bridge.

In coastal waters more than one steering gear power unit, when such units are capable of simultaneous operation should be used.

Within 12 hours before departure from a port, check and test the steering gear along with the operation of the following:

·?????the main steering gear;

·?????the auxiliary steering gear;

·?????the remote steering gear control systems;

·?????the main steering position on the bridge;

·?????the emergency power supply;

·?????the rudder angle indicator in relation to actual rudder position;

·?????the remote steering gear control system power failure alarms;

·???????the steering gear power unit failure alarms; and

·?????automatic isolating arrangements and other automatic equipment.

The checks and test should include the full rudder movement, the timing of such movement from hard-over to hard-over and the operation of the means of communication between the bridge and the steering gear compartment.

All OOW must ensure they are familiar with the operation or maintenance of the steering gear along with the change over procedures.

Emergency steering drills should take place at least in every three months. It should include direct control from within the steering gear compartment, the communication procedure with the bridge and, where applicable the operation of alternative power supplies.

The dates/ time of testing including details of emergency steering drills carried out are to be recorded in the logbook.

Q.2.?What is the difference between a track-keeping and course-keeping autopilot?

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Ans. Track-keeping autopilot allows the ship to follow the track laid for a passage, whereas course-keeping autopilot ensures that the ship points to the right direction. Wind and current for example can move the ship sideways and off its track, while the ship's heading remains same.

In a track-keeping mode the autopilot should perform turns automatically between track legs, using either pre-set turn radius or rate of turn values. If there is a malfunction in track- keeping mode, an alarm should come and the system should automatically revert to course- keeping mode.

Q.3.?How does an off-course alarm work in an autopilot?

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Ans. Off-course alarm is fitted in every autopilot to warn the OOW when the ship deviates from the course set by a pre-set amount. Non-activation of the off-course alarm will not always mean that the ship is maintaining its planned track. The ship may have moved off the track by wind or current even though the heading remains same.

Q.4.?What precautions are required to be taken while using a transmitting magnetic compass (TMC) or a gyro compass for navigating a ship?

Ans. A transmitting magnetic compass (TMC), if used to provide heading outputs to other bridge systems should be corrected for compass error through transit bearings, azimuth or amplitude observations. The TMC should be tested once a week in clear weather. If TMC is not used the magnetic compass is generally fitted above the bridge on the centreline of the ship with a periscope so that the compass is readable from the helmsman's position.

Magnetic compass uses the magnetic properties of the earth for its direction finding. There are corrections like variation, which is the difference between the true north and the magnetic north and deviation, which is the difference between the magnetic north and the compass north of a ship. Both these corrections vary from place to place and with different ship's head. They must be recorded and applied correctly to the compass course to find the true course. Every ship is provided with a deviation chart posted on the bridge. Large or unusual deviation found for a ship's head should be investigated thoroughly. It could be because of major steel conversion of the ship, magnetic anomaly in a certain place or carriage of magnetic cargoes like iron ore or steel. It is also possible that magnets inside the binnacle of the compass were tempered with. Magnetism of all electrical bridge equipment can affect the magnetic compass and the minimum distances from them are specified and should be strictly followed.

Gyrocompass uses the directional properties of a gyroscope, which is the rigidity in space and precession. Gyrocompass should be run continuously. Should a gyrocompass stop for any reasons, it should be restarted and checked for errors after it is 'settled'. Speed and latitude corrections must be applied to the gyrocompass. Where the gyro has no speed log or position input, manual corrections would have to be made.

Gyrocompass has a number of repeaters connected to it. They must all be checked for alignment with the master gyro including the repeater in the emergency steering gear compartment. Gyro repeaters on the bridge wings must be checked against the master gyro at least once a watch and after excessive manoeuvring. Gyro compass error must be checked and recorded every watch if possible. Care must be taken the error is applied correctly for all bearings etc.

Gyrocompass gives the output of direction to several bridge equipment and care should be taken that in case of failure of the gyro, the OOW is aware of contingency plans and actions required.

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Q.5.?What types of speed measurement instruments are used on the ship?

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Ans. Generally speed through the water is used for collision avoidance and speed over the ground is used for navigation. Speed made good can be measured on ships and represents the speed the ship has achieved over a period of time. Speed made good can also be measured from the charts between two position fixes and calculated and transmitted by electronic position-fixing systems.

Doppler-type logs can be single-axis or dual-axis. In case of single-axis, the speed is measured in the fore and aft direction and in dual-axis measured fore and aft and

athwartships. If coupled with a rate of turn measurement, dual-axis logs can calculate the speed and direction of movement of the bow and the stern.

It is good navigation practice to set the log at zero at the start of every voyage.

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Q.6.?What precautions are required to be taken while using an echo sounder?

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Ans. A navigational echo sounder is expected to work up to a depth of 200m (approximately 110 fathoms). Care should be taken as follows:

·?????units of sounding of the echo sounder is the same as those used in the chart;

·?????when comparing the echo and chart soundings, allowance must be made for the draft of the ship or any tidal effects;

·?????errors caused due to aeration, double reflection of echo are taken into consideration;

·?????echo sounder is always used in coastal waters and when making a landfall;

·?????shallow water alarm system if fitted should be set to an appropriate depth to warn of approaching shallow waters.

Q.7.?What are the electronic position-fixing systems used on the ship and what care should be taken while using the same?

Ans. Except Loran C, other electronic position-fixing systems like Decca and Omega is being phased out. Loran C has a basic range of 1200 miles and corrections need to be applied for the variations of the conductivity of the earth's surface.

A global navigation satellite system (GNSS) today provides continuous worldwide position, time and speed information. The USA operates global Positioning System (GPS) and the Russian Federation operates Global Navigation Satellite System (GLONASS).

GPS provides an accuracy of 100 metres. Differential GPS (DGPS) is corrected by terrestrial monitoring systems and can be as accurate as 10 metres.

Following care should be taken when using electronic position-fixing systems:

·?????OOW must understand the capabilities and the limitations of all electronic position-fixing systems;

·?????the integrity and the quality of the data transmitted must be safe guarded by pre-set quality limits to monitor the quality of the fix;

·?????comparing all positions to identify and reject any false position;

·?????comparing electronic position-fixing with estimated position calculated by taking observation with input from log, gyro etc.;

·?????checking the status of the data and ensuring only the valid data is used.

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Q.8.?What is Integrated Bridge Systems (IBS)?

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Ans. Integrated Bridge Systems (IBS) are designed to combine systems, which are interconnected to allow centralised monitoring of sensor information and control of a number of operations such as passage execution, communications, machinery control, safety and security.

IBS is not mandatory. Various designs are offered by classification societies and factors taken into account are the design of the bridge, type of equipment fitted and the layout of that equipment on the bridge.

Design of the IBS should be such that failure of one sub-system does not cause the failure of another and the OOW knows about the failure immediately. In a navigation management

system, the link is provided between charts, position-fixing system, the log, the gyrocompass and the autopilot.

The IBS has an alarm system provided to warn the OOW if a potential dangerous situation arises. The alarm system is connected to radar, gyro, autopilot, position-fixing systems, ECDIS, the steering gear and the power distribution system. There should be watch safety or fitness alarm to transfer the alarm to cabins within 30 seconds, if the OOW fails to acknowledge the alarm. An interval timer for setting alarm intervals of up to 12 minutes should be part of the system. A number of alarm acknowledge points, each with a pre- warning alarm to give the OOW notice that the alarm is about to be activated should be available around the bridge. If the fitness time interval expires, an alarm should sound away from the bridge.

Clear guidelines are to be written in the shipboard operation procedure manual with advice as to when to commence and when to suspend the use of IBS. Over-reliance on automatic systems with OOW not paying proper attention to visual navigational watchkeeping techniques can be very dangerous.

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Q.9.?What is ECDIS and how is it used for navigation on ships?

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Ans. Electronic Chart Display and Information System (ECDIS) include the display of electronic vector and/or raster charts overlaid with the position of the ship and its track. It has facilities to route plan and automatically update chart corrections using digital notices to mariners. Sensors from GPS, log and gyro are connected to ECDIS to provide position of the ship. Autopilot may be connected to provide an integrated bridge system. Some ECDIS has radar-input display overlaid on the chart. This can be either selected targets or a full radar picture. Care should be taken in differentiating target vectors based on ship's speed through the water when OOW tries to overlay the same in a chart that is displaying speed over the ground. Electronic chart display systems can be categorised as ECDIS, RCDS (Raster Chart Display System) or ECS (Electronic Chart System). ECS must have a complete set of paper charts to supplement it on board.

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Q.10.?What is the difference between the electronic chart format of Vector and Raster charts?

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Ans. Vector chart has its features stored in a layered digital format and each feature has a set of typical values. Storage in the database allows the chart to be displayed as a seamless chart. The layers can be added or removed enabling fields of data required or not required respectively.

Chart features can be expanded to get any additional information on any charted object. Vector charts are inherently "intelligent" and thus allows three-dimensional monitoring while sailing. Chart depth contours and air draught clearances around the ship can be automatically monitored both at the planning stage and while the ship is on passage. Alarms will be automatically triggered if a safety zone around the ship is breached at any time.

Raster charts are exact copies of a paper chart and are produced by digital scanning techniques. Information on raster charts cannot be layered and the move from one chart to another cannot be seamless. Raster charts have to be individually selected and displayed.

Raster charts have no inherent "intelligence" and the chart data itself cannot generate automatic alarm systems, unless fed manually by the user during the route planning. Datum and projections will be different in case of raster charts and care must be taken to take account of such differences.