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.
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.
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).
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 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.