Bridge equipment

09.09.2021

Bridge equipment

This chapter describes the standard navigation and nautical package mandatory for a ship for unrestricted service.

Navigation has changed enor­mously with the introduction of the global positioning system (GPS). To determine the posi­tion of a ship the sextant was for many years the tool to use. As this method uses visual orienta­tion to the stars, planets, sun and moon weather conditions often hindered its use. With satellites and sophisticated computer systems navigation has evolved to an accurate all­weather tool.

   1 Bridge equipment

1.1  Compass systems

1.1.1  Magnetic compass

From 150 GT upwards all ships shall be fitted with a steering com­pass. The magnetic compass is the old­est and simplest. The system is us­ing the earth magnetism. Disadvantage is, that the direction of the magnetic field of the earth is different from the direction of the earth's axis of rotation.

The south pole of a magnetic bar, when suspended from a string free in the air, will point at the earth's magnetic north pole. A magnetic standard compass is still required for all ships. Magnetic compasses indicate the direction to the magnetic north pole, which is not located at the geographical north pole, but at present some 100 miles away.

The location of the magnetic north pole changes continuously. The magnetism, when observed on board of a ship, is influenced by the steel of the ship itself. The compass has therefore to be calibrated to compensate for the magnetic field of the ship itself, when commissioned, and eventu­ally later, when deviations are be­coming too high. The compass is also influenced by the cargo, when this is sensitive for magnetism

       

The magnetic standard compass and the compensation engineer at work

A view on the bridge

   1.1.2. Gyrocompass

Ships of 500 GT and upwards have to be fitted with a gyrocompass. There are 3 different types of gyro­compasses:

The gyrocompass consists es­sentially of a gyroscope, which, when spinning at a sufficiently high speed will have its axis maintain­ing a constant direction in space, regardless of how the supporting rings are tilted or turned. This property is known as the rigid­ity in space. Magnetic forces do not have influ­ence on the maintained direction.

The gyrocompass is installed in a binnacle, where the spinner is in­stalled inside a ball shaped housing. This ball floats in a special liquid, with a specific gravity keeping the ball vertically accurately inside its surrounding housing to allow the spinner to seek its direction in space. Inside the floating ball, an electric motor is installed, with the rotor as the gyro-spinner. Electric contacts are ensured by so­phisticated sliding devices.

When suitable controls are applied, the axis of the gyroscope seeks the direction of the true north. Because of the rotation of the earth, the axis of the gyro appears to move, although maintaining its direction in space. This motion is a combination of drift and tilt, together the apparent mo­tion. Drift is the horizontal devia­tion from the selected direction in space, due to the earth's rotation. The magnitude and direction of drift is depending on the latitude. By creating friction, which is al­ready there from the liquid the ball floats in, the axis points itself in the direction of the earth's axis, i.e. in the direction of the true north. Tilting is a result of the latitude. When at the equator, the direction of the axis is the same as to the horizon. When at higher latitude, the direc­tion to a point above the north pole of the earth results in a vertical an­gle with the horizontal. 

This can be adjusted by gravity, i.e. by a weight or a system with ad­justable floats in mercury. Added weights give the ball a posi­tion parallel to the horizon. Settings depend on the actual lati­tude. The ship's speed is producing an­other deviation. The gyro will adjust itself rectan­gularly to the resultant of the true course of the vessel and the east­going direction of the earth. The instrument itself also has some constant deviation.

Above deviations are corrected by various electronic devices. The binnacle is normally installed in a technical room near the wheel­house of the ship. Often at a lower deck, to reduce transversal forces due to the ship's movement. At various places repeaters are in­stalled, showing the directional in­formation wanted for navigation (or other purposes). Normally at the steering position, at both bridge wings, sometimes near the magnetic compass for easy calibration of that compass.

The principle of the dry gyro is the same as of the liquid gyro. How­ever, the big advantage is there is no maintenance required during its MTBF (mean time between failure).

1.1.3.   Fiberoptic Gyrocompass

The last development of the gyro principle, also electrical, is the Fiberoptic Gyrocompass. This is a complete solid unit, which has no rotating or other moving parts. It is based on a laser beam sent into a horizontal glassfibre coil, split in two halves when enter­ing the coil. One half goes left, the other half right. When the coil has not turned, both beams return at the entering point at the same moment. If the coil has turned, the beams do not return at the starting point at the same time, resulting in a phase difference. Three coils at the x, у and z axis, enable the calculation of the true north. The device is made in solid state and needs only an short settling time.

   1.1.4. Fluxgate compass

A fully electrical compass is the Fluxgate compass. Two coils under 90° produce an electric current by the magnetic flux passing through the coils. From the difference in measured current the direction of the mag­netic north can be calculated.

   1.2.  Off-Course Alarm

When a ship, whilst on passage changes course unwanted, an alarm has to sound. Often this is a device coupled to the gyro. Also the magnetic compass must be used for this purpose. 

Allowed degrees off course are to be set. When coupled to the gyro this can be done automatically.   

A gyrocompass opened up. The grey cylinder in the center contains the gyro spinner. Cooling is provided by liquid

Circular line shows the apparent motion of the axis of a gyroscope around the pole star in the absence of a pendulous mass. 
The addition of the pendulous mass (lower drawing) converts the circu­lar motion into an ellipse; the el­lipse can then be damped out
and the gyroscope becomes a gyrocom­pass pointing to true north

   1.3.  Radar

A RADAR (Radio Detection and Ranging) with automatic plotting (ARPA) function and rotating trans- mitting/receiving aerial, usually the X-band (frequency 8-12 GHz). For ships bigger than 3000 GT a second radar has to be provided, usually an S-band radar in the fre­quency range of 3-4 GHz. The reason to select two radars with different frequency bands is their different capabilities to cope with the environmental conditions such as fog, rain, sea clutter. 

A radar installation comprises a transmitter/receiver, and a rotating antenna. A display shows the outcome. The transmitter/receiver is a box mounted directly under the an­tenna. The antenna or scanner, is installed in the radar mast, usually on top of the wheelhouse.

The scanner is rotating. A very short pulse is sent from the raytube to the scanner mirrors and leaves the scanner as a narrow beam. When this beam bounces on an object, part of it can be received in the scanner. From the timespan between send­ing and receiving, the distance to the object can be calculated.

The direction is given by the posi­tion of the scanner, relative to the ship's centerline. The bounced pulse is seen as a dot on the display.

The reach of the radar is deter­mined by the height of the scanner and the height of the target.

Sensible precautions

If radar equipment is to be worked with under power in port, sensible precautions would include ensuring that:

 Any work carried out on such equipment should be carried out by competent persons, operating a safe system of work, so that they put neither themselves nor others at risk.

   1.4. Global Positioning System, GPS

GPS is simple to use and so reliable that nearly all ships, from small yachts to the largest ships at sea, are fitted with one or more GPS re­ceivers. 

GPS is an independent auto-posi­tion fixing system, with omnidi­rectional aerial. The input data are produced by satellites. The system was originally designed for the US defence department but has been made available for civil­ian use. Europe is working on an alternative independent system, Galileo.

DGPS or Differential Global Posi­tioning System, is a more accu­rate GPS, by the installation of an additional signal from a reference transmitter. The location of this transmitter is accurately known, so improving the outcome of the posi­tion calculation. 

Due to the limited reach of this additional transmitter, this is a local improvement.

Global positioning systems operate on low power signals, transmitted by a large number of satellites, which orbit the earth at an altitude of 20,000 kilometres. Normally there is input from some 8 satellites at every moment.

This (D)GPS gives not only the ac­tual position in coordinates, but when the receiver (the ship) is moving, it calculates also speed and course over the ground.

   1.5.  Autopilot

   1.5.1.  Automatic course function

Automatic pilots are control devic­es that compare the actual course on the gyrocompass with the set course, and take corrective meas­ures if the actual course is deviating from the set course. Most of these control devices are now adaptive, which means that it adapts to the ship's characteristics by applying minimum rudder angle to get back to the set course. Autopilots can be adjusted for gain, maximum rudder angle and maximum rate of turn. 

The modern autopilots are so sen­sitive that they operate the rudder at a minimum deviation of the set course before the helmsman would notice. This way steering a more straight course than a helmsman would do. A straighter course saves fuel and time.

Display GPS

   1.5.2.  Autotrack function

GPS positioning giving course and speed via ECDIS or GPS over the bottom makes it possible to steer according to a planned track. Way points can be added and at the way points the vessel will slowly turn to the next track, after a warn­ing and being acknowledged.

   1.  Gyro repeater

    1.6.  Speed and Distance (Log)

On ships over 500 GT the speed and distance through the water has to be measured. One log with speed and distance indication through the water has to be installed. This can be for in­stance an electromagnetic log. In shallow water the so-called Doppler log can measure speed through the water and over the ground, water track or ground track. This can be chosen at the display. 

Dual-axis logs measure speed in forward and aft direction as well as transverse movements. The latter for very large ships (tankers, bulkcarriers), to control the impact forces on the jetty dur­ing mooring.   

   1.7.  Rudder angle indicator

The physical position of the rud­der has to be shown on a display. Normally this is displayed on a deckhead-mounted indicator vis­ible from everywhere in the wheel­house.

   1.8.  Rate of turn indicator

Rate of Turn Indicator has to be in­stalled on ships of 50,000 GT and upwards. The rate of turn is impor­tant for large ships, to determine the time needed to come to a de­cided course.

In advance of a turn, the helm has to be moved in the position to get the ship turning. Especially large ships need time to start to react.

In the bridge console there are dis­plays for RPM and turning direction of the propeller. Or the pitch in case of a controllable-pitch propeller.

Displays are also installed on the bridgewings, as these parameters are very important during manoeu­vring and mooring.

   1.9.  Wind and sound

Ships with an enclosed wheelhouse, which are vulnerable to wind during manoeuvring, are to be fitted with a wind indicator and a sound recep­tion system. The latter consists of microphones outside and a speaker system inside enabling to establish the incoming direction of the out­side sound.

   1.10.  Echosounder

The water depth under the ship is measured with an echosounder. A transducer in the ship's bottom sends a sound pulse downward, and receives the bounced pulse. The distance between ship's bot­tom and seabed can be calculated from the time between sending and receiving.

The speed of the pulse through the water is more or less constant. Ad­justment settings can be made for the ship's draft. An alarm can be set at any depth below the trans­ducer. The sent sound beam has a conical shape, with the top of the cone at the transducer.

   1.11.  Daylight Signal Lamp

All ships over 150 GT, must have a daylight signal lamp. The source of electric power has to be independ­ent of the main power supplied to the wheelhouse equipment. Often an ordinary battery is used,

   1.12.  Navigation Lights panel

In the wheelhouse an alarm and indication panel is to be installed to control and monitor the naviga­tion lights. Most of the time next to this panel is a control panel for the signal lights like NUC (Not Under Command) lights.

Doppler log display showing speed in bottom track mode and sideways speed bow and stern

Echosounder display showing depth under the kee

   1.13.  Voyage Data Recorder

Passenger ships and ships other than passenger ships of 3000 gross tonnage and upwards constructed on or after 1 July 2002 must car­ry voyage data recorders (VDR, Black Box) to assist in accident investigations. Details can be found in SOLAS. 

Such a unit consists of a data ac­quisition unit, acquiring all neces­sary data from the various instru­ments and a data capsule. The device records information re­garding course, speed, communi­cation, alarms, alterations, engine particulars and what has been said in the wheelhouse. Data can, if wanted, be transmitted to the shorebase of the vessel.

Like the black boxes carried on air­craft, VDRs enable accident inves­tigators to review procedures and instructions in the moments before an incident and help to identify the cause of any accident.

The data acquisition cabinet is nor­mally installed in or near the wheel­house, the data capsule on the wheelhouse top. The latter has to be installed so, that it floats up in case the ship sinks. The device has to be tested yearly by an approved company.

   1.14.  Electronic Chart Display (Ecdis)

Instead of paper charts, the infor­mation is displayed on a comput­er screen. On this screen also the ship's position is shown.

The charts can be raster-type, which means that they are scanned paper charts, or vector-type, fully digital. The last type has advan­tages. The electronic chart can be com­bined with AIS and Radar, this means that all information can be made visible on one screen. Updates of the charts are carried out digitally. A second system has to be provid­ed for back-up. Paper charts also can be the back-up, but this means that they have to be corrected. 

Raster-type charts are not ap­proved for paperless sailing.

Above the AIS displayed on the radar screen. Below the ECDIS display of the same area. The ship is displayed on both screens  

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