Electronic plotting devices

On larger ships, at least one of the radars carried is likely to have automatic radar plotting aid (ARPA) functions. Radars on smaler ships may be fitted with either automatic tracking aid (ATA) or electronic plotting aid (EPA) functions.

ATA uses ARPA hardware but with limited functionality: no trial manoeuvre, target past position or guard zone features, and manual acquisition limited to 10 targets. EPA offers basic electronic plotting functions that are as good as a reflection plotter.

In comparison with standard radar, ARPA and ATA offer a number of automated collision avoidance features. However, watchkeepers should be aware of the dangers of being over-reliant on these devices and:

• understand the types of errors that are possible and recognise the
operational warnings that appear on the display;

• understand the limitations of the devices;

• regularly test the devices using the built-in operational test facilities.

4.2.4.1 Heading and speed inputs

Correct and reliable speed and heading inputs into the ARPA or ATA are vital if targets are to be processed correctly.

Speed and heading inputs need to be sea stabilised (water tracked) to provide the ARPA or ATA with speed and course through the water. The use of these devices in a ground stabilised (bottom tracked) mode for assessing risk of collision could be particularly hazardous in sea areas that experience significant tidal streams and currents.

4.2.4.2 Automatic target acquisition

Features such as guard zones and target acquisition footprints are commonly used for the automatic acquisition of ARPA targets.

Such features should always be used with caution, especially in sea areas where radar inconspicuous targets can be expected.

P;

Guidance to masters and nav

4.3 Steering gear and the automatic pilot

Testing of steering gear

The OOW should ensure that the SOLAS requirements for the operation and testing of the steering gear are observed (see annex A7).

Steering control

These paragraphs should be read in conjunction with section 3.4.2 of this Guide.

Steering control of the ship will comprise manual steering, probably supplemented by an automatic pilot (autopilot) or other track control system. At each steering position there should be a gyro repeater and rudder angle indicator. An emergency back-up steering position, usually in the steering gear flat, is also required.

If an autopilot is fitted, a steering mode selector switch for changing between automatic and manual steering, and a manual override control to allow the OOW to gain instant manual control of the steering, will be required.

.3.2.1 The autopilot (heading/track controller)

The role of the autopilot is to steer the ship automatically. The autopilot can either be operated independently or, in an integrated bridge, controlled by a navigation system (see section 4.8.2).

When operated as an independent system, the course to steer will need to be manually set on the autopilot and the autopilot will steer that course until a new course is entered. When linked to an integrated system, the autopilot will be able to receive cross track error (XTE) commands and track-keep automatically.

.3.2.2 Automatic track-keeping (if fitted)

Track-keeping control allows the ship to maintain its planned track, whereas course-keeping only ensures that the ship is pointing in the right direction. Wind and currents can, for example, move the ship sideways and off its track while the ship's heading remains unchanged.

For a ship to operate an automatic track-keeping system, the autopilot should be adaptive and able to perform turns automatically between track legs, using either pre-set turn radius or rate of turn values.

Turns are commenced at a wheel over position, only after the OOW has acknowledged the wheel over position alarm and is satisfied that it is safe to execute the turn.

If a malfunction occurs when track-keeping, the system should alarm and revert immediately to course-keeping mode.

If the malfunction occurs while the autopilot is on a track, the autopilot should continue to steer the pre-set course of that track. If the autopilot is performing a turn when the malfunction occurs, the autopilot should complete the turn at the pre-set turn value and take up the course of the next track.

ICS BRIDGE

An autopilot performing automatic track-keeping functions and its alarm outputs should always be closely monitored.

The ability of the autopilot closely to follow a planned track will depend upon the accuracy of the XTE information sent to the autopilot from the navigation system (see sections 4.8.2 and 4.8.3).

Off-course alarm

As part of the steering control system there should be an off-course alarm facility to warn the OOW when the ship excessively deviates from its course. The alarm should be in use at all times that the autopilot is in operation.

The use of the off-course alarm does not relieve the OOW from frequently checking the course that is being steered.

Non-activation of the off-course alarm will not always mean that the ship is maintaining its planned track. The ship may be moved off its track by wind and currents even though the heading remains unchanged.

Compass system

Magnetic compass

The magnetic compass is generally fitted above the bridge on the centreline with a periscope so that the compass is readable from the helmsman's position.

Where the magnetic compass is needed to provide heading outputs to other bridge systems, a transmitting magnetic compass (TMC) is fitted. TMC outputs should be corrected for compass error and the TMC should be tested once a week, in clear visibility.

A compass deviation card should be maintained and posted on the bridge. The compass will need to be swung at intervals during the ship's life, and particularly after major steel conversion work to the ship. Caution should be observed when using the magnetic compass on ships that carry magnetic cargoes such as iron and steel.

Compass safe distances are specified on all electrical bridge equipment and provide the minimum distances that equipment can be installed from the magnetic compass.

Gyro compass

It is recommended that the gyro compass should be run continuously. Should a gyro compass stop for any reason, it should be restarted and subsequently checked before use to ensure it has 'settled' and is reading correctly.

Speed and latitude corrections need to be applied to the gyro compass. Where
the gyro has no direct speed log or position input, manual corrections will have
to be made as required.___________________________________________

The gyro will support a number of repeaters, including a repeater at the emergency steering position. Gyro repeaters on the bridge should be checked against the main gyro at least once a watch, and after excessive manoeuvring. Other repeaters should be checked frequently.

Compass errors

As a safeguard against the gyro and gyro repeaters wandering, frequent checks should be made between the magnetic and gyro compasses.

Magnetic and gyro compass errors should be checked and recorded each watch, where possible, using either azimuth or transit bearings.

A record of magnetic and gyro compass courses to steer and compass errors should be maintained and available to the helmsman.

Rate of turn

Rate of turn measurement is used by automatic track-keeping systems to perform controlled turns. When ships are manoeuvring, particularly large ships where the distance between the bow and the pivot point of the ship is considerable, rate of turn indication provides the ship handler with feedback on how quickly the ship is turning.

Date: 2016-04-22; view: 1129