How to Choose a Magnetic Compass for your Boat
Magnetic compasses come in may different designs and when selecting one it is important to consider how and where you will use it.
When a marine compass is used for navigation purposes it must always be approved to the SOLAS-MED standard. What the approval does is to establish certain characteristics, such as how fast the card settles when distrurbed, the marked graduations, the size of vessel that it is approved for and other performance related parameters. Marine compasses are referred to as either Class A - used in larger commercial vessels and Class B used primarily in leisure vessels and lifeboats. Practically all modern marine compasses are liquid damped to ensure they perform consistently and according to the apprpriate directive. The use of the wheelmark symbol denotes compliance with the directive and an approved compass. So first look for the wheelmark symbol.Putting aesthetics to one side, the most distinctive compass features to consider are the card design and location of the compass. The compass card will be one of three basic shapes - cylindrical, conical or flat. We take reading the compass for granted, but at the end of a long watch, when you are tired, possibly cold and maybe wet, the ease of reading the card is important for safety. During a watch a compass card will be read many times so card choice is crucial. Equally important is the compass location and therfore its design and mounting. Mounting choices range from vertical to horizontal with gimballs and brackets offering even more choice.
The most crucial components in the construction of a compass are:
The crystal dome that magnifies the card (making it appear larger than it really is) making it more ledgible but also protecting the compass card and keeping the fluid in place.
The jewelled pivot that ensures the card rotates smoothly and evenly with many years of service.
The underside corrugated expansion diaphragm that compensates for the expansion and contraction of the fluid with variations in temperature.
The damping fluid that is specially chosen to give the correct degree of damping as the compass card rotates.
A compass card is read at the lubber line which is located on or parallel to the fore and aft line of the vessel. The lubber line can be located either at the front or rear of the compass card and this positioning depends on the type of card which in turn depends on how the compass will be used. Lubber lines can also be offset to either side of the aft lubber line so that the compass is more easily read when, for example a yacht is heeled.
Cylindrical card compasses have traditionally found favour with yachtsmen, especially in smaller yachts, since when standing in the cockpit with the compass mounted on the companionway bulkhead it is in line with your eyes and easier to read. These compasses are read from the aft end of the compass. Currently there is a move away, by manufacturers such as Autonautic Instrumental, from this style of card design to a convex profiled side to the card or even a conical card since it is easier to print the card and cheaper to manufacture. They will often have a fore and aft lubber line and lubber lines at 45 degrees to make reading easier when the vessel is heeled.
Flat card compasses need a higher viewing angle since they are read from above. The flat card is appropriate where you stand at the helm and find favour in larger vessels or where a binnacle is used. All large compasses, such as Class A compasses, have flat cards. The Autonautic flat card compass with a blue card is particularly striking. The flat card is read from the forward end of the compass and has the added advantage that if there is an aft end lubber mark it is possible to get the reciprocal bearing.
Conical cards find favour where you sit at the helm e.g. in a motor vessel so that you quickly glance down to read the compass heading. Conical cards are read from the aft end of the compass and a benefit of the conical card is that cardinal and half cardinal marks can be marked on the inner slope of the card, allowing them to be read from the fore end of the compass.
On the underside of any compass card will be the carrier for the magnets which is balanced on the jewelled pivot.
Compass Design and Location
Ideally a compass should be located on the foreand aft line of the vessel. It should be as far away as possible from any large pieces of ferrous metal (eg the engine(s), keel) and any electric wiring. But, it's a compromise largely governed by vessel size, construction and aesthetics. The case design is based on one of three basic mounting configurations - vertical (or bulkhead), horizontal (or flushmount) or gimbal mounting. The bracket mounted compass is a variation on the gimballed compass without the ability to swing freely.
Vertically mounted compasses fit easily into a bulkhead and take up little space. They are convenient to read and are optimised to go on a vertical or near vertical surface. The move to conical cards has allowed the inclination from the vertical to be less of an issue but probably 10 degrees off the vertical is the maximum - this issue becomes one of restricting the movement of the card in one axis when the craft/vessel is pitching.
Horizontally mounted compasses need either bench space to fit them (and increasingly this space is at a premium in motorcruisers today) or a binnacle. They are located in front to of the helm and offer two reading options - aft or forward read depending on the construction. In forward read compass the cover is normally built up to shade the lubber line making it easier to read in bright sunlight.
Gimballed compasses have the advantage of allowing the compass bowl to swing freely in any direction as the vessel motion changes. They are very popular in larger compasses and all Class A compasses must be able to swing freely through 360 degrees! They are always flat card compasses and can be read from both the foreward lubber line (the heading) and the aft lubber line (the reciprocal heading).
This is a special case of the gimbal and finds favour in day boats, ribs and smaller vessels - since it can be easily removed for safe keeping. In addition bracket positions on the Autonautic Instrumental compasses can often be adjusted for mounting on a sloping surface or even on the roof of the wheelhouse.
Having selected your new compass the final task is to ensure the card is corerctly balanced for the region in which you use your vessel. Manufacturers balance the card so that the effect of the magnetic dip (which changes across the surface of the earth) does not adversely the compass when in use. The manufactures generally split the world into five zones (some use 7) and generally if you are in the Northern Hemisphere Zone A applies. The manufacturer or their distributor/ retailer will be able to advise accordingly.
Compass vs GPS
And on a final note, many people think that they only need GPS, but consider this. GPS can only give a position so the heading or direction you are travelling in, is only calculated your vessel moves. I get many enquiries from people who ask why is their GPS heading not correct when they are moored up - with normal GPS it never can be - this is one reason why a magnetic compass is still useful. Another is that it is totally independent of any electrical power for it's operation.