Compasses

Hand Bearing Compass

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Fig 4.12 – Plastimo version of hand-bearing compass.

The hand-bearing compass is a fantastic and extremely versatile bit of navigation equipment and every sailor should be develop the skills to use them. (Fig 4.12). Like any compass they are affected by magnetic influences but is impractical to draw up deviation cards for them as they will never be in the same position twice. If we hold the compass clear of metal work and out of range of electrical equipment, for the purposes of practical navigation we assume that the hand bearing compass is ‘free’ from deviation and only subject to variation. In other words, if properly used we can assume that a hand-bearing compass always points to the magnetic north pole and only has to be corrected for variation to convert into a bearing that can be plotted on a chart.

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Fig 4.13 – Most heand-bearing compasses have a phosphorescent compass card and can be used without affecting night vision.

There are loads of different types of hand-bearing compass but all work in a similar way. There are some things you want in a hand-bearing compass…. something robust, easy to use, that is not powered by batteries and as with the Plastimo variety in Fig 4.13 has a phosphorescent compass card that can be charged with normal cabin lights for use in the dark.

Steering Compass

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Fig 4.14 – A bulkhead mounted compass sometimes found near the mast in small racing boats.

Most steering compasses have a swinging graduated card that pivots on a small spindle and encased in fluid, historically an alcohol, which if drunk would lead to court martial! Bulkhead compasses can be mounted in a bulkhead and sometimes seen attached towards the base of the mast in some racing yachts (Fig 4.14).

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Fig 4.15 – A ships steering compass found near the helm, usually on the binnacle.

A binnacle or steering compass (Fig 4.15) is also a swinging card compass and mounted in a pedestal on the binnacle or steering position. The swinging compass card is sufficiently dampened by the fluid in the compass so that the helm can read the course though bumpy seas. Swinging Card Compasses are generally mounted near to metallic and electrical influences so will be subject to deviation and therefore should have a deviation card drawn up.

The steering compass is affected by deviation, due to its proximity to metallic material. So if we have a steering compass and a hand-held compass pointing at the same thing the difference in readings is our ships deviation value for that heading.

Fluxgate Compass

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Fig 4.16 – A solid state fluxgate compass is used to drive the autohelm and can be used with some radar equipment

Fluxgate compasses are used in a vessels navigation and auto-helm system and provides heading information to other equipment such as chart plotters and radar (Fig 4.16). Deviation can be corrected and calibrated using the compasses own software. After your perform a 360 manoeuvre, the electronics are able to build its own stored deviation card. The fluxgate unit is invariably located below somewhere and must be guarded against placing deviating influences near it. Otherwise, if properly calibrated the information from the fluxgate compass can be regarded as deviation free and only needs corrected for variation.