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In this age of modern electronics, satellites and software a compass still has a use. Compass manufacturers such as, Autonautic Instrumental continue to innovate investigating the use of modern materials - so I suspect the story is far from over. The big advantage of the compass is it needs no power from the vessel to work. Compasses are so refined now and the quality is such, that they last a very long time.
Compasses come in many different designs and when selecting one it is worth thinking about what is suitable for your use.
Marine compasses should always be approved to a standard (e.g. SOLAS-MED). 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 approved compasses are referred to as either Class A - used in larger commercial vessels and Class B used in leisure vessels and lifeboats. Practically all modern marine compasses are liquid damped to ensure they perform consistently and according to the appropriate directive. The use of the wheelmark denotes compliance with the directive and therefore an approved compass.
Putting aesthetics to one side, the most distinctive compass feature is the card design - it can be cylindrical, conical or flat. Think about this - we take reading the compass for granted, but given that every watch it will be read many times - the card design chosen is important.
Cylindrical card compasses have traditionally found favour with yachtsmen since when standing in the cockpit and the compass is mounted on the Companion Way bulkhead and is in line with your eyes it is easier to read. These compasses are read from the aft end of the compass. Currently there is a move away from this card design to the conical card, largely to simplify the card manufacture and keep the card cost down.
Flat card compasses need a higher viewing angle so they are easier to read from above. The flat card is appropriate where you stand at the helm and find favour in larger vessels. In fact all large compasses have a flat card. As the photo shows the Autonautic Instrumental flat card compass with its blue card is particularly stylish and suited to a smaller boat. The flat card is read from the bow end of the card, so you look into the darkened end of the compass cover, this works well in bright sunlight.
Conical card compasses find favour when you sit at the helm e.g. in a motor boat and need to glance down to hold a course. They are read from the Aft end of the compass, however the conical card design means that the cardinal and half cardinal points can be marked on the inner slope allowing them to be read from the bow end of the compass.
Gimballed compasses have the advantage of allowing the compass to self level as the vessels motion changes. These will usually have a fore and aft lubber mark, so that the heading is read from the bow end with the advantage that the reciprocal bearing can be read from the aft mark.
Additional lubber marks are sometimes located at +/- 45 degrees so that in a yacht when it is heeled the course can be held. The very top compass photo shows a yacht compass that has a built in inclinometer to display the angle of heel.
Class A compasses are more finely engineered and graduated every degree, they have large flat cards (to accommodate the degree graduation) and can often be read from both above and below. They are usually gimballed and can be be modified to allow a sensor to be added so that you can electronically read the heading. This is known as a TMC (Transmitting Magnetic Compass) and we supply a version with a NMEA0183 output for feeding into navigation equipment.
Locating a compass is frequently a compromise, especially as more and more space is taken up by navigational displays. Ideally a compass should be mounted on the fore and aft line, at the center of gravity of the boat. The fore and aft line is not that difficult to achieve but usually compasses are located higher than is ideal. With modern compass damping this is rarely an issue.
The compass being magnetic is affected by any lump of ferrous metal on board, from steel used for the hull in commercial vessels to the humble can of baked beans or tools left in a locker adjacent to the compass on a yacht. The bigger the lump and the closer to the compass it is, the more the distortion. These distortions need to be compensated for and this is done by an arrangement of compensating magnets. This can range from a simple arrangement of two magnets located centrally below the compass to more complex systems when a binnacle is involved. Compass adjusting is done by a compass adjuster and is a process that in experienced hands can be done quite quickly. The objective is to minimise the deviation to an acceptable limit - normally 5 degrees.
Compass adjustment needs to be done when a new compass is fitted, after any major work to the vessel or when it is noticed that it is now inaccurate. Commercial vessels have a compass deviation book and owners of lesiure boats should take the opportunity to regularly check the deviation as the opportunity arises.
Once adjusted a Deviation Table is drawn up so that bearings can be corrected for navigation purposes. Most recently Autonautic Instrumental have developed their CMI compass for commercial vessels. This automates the deviation process. In large vessels the whole process of creating the Deviation Table (which is a mandatory requirement) is expensive and time consuming. Who knows we might even see a version for the leisure market one day.
Autopilot
Magnetic Compass
NMEA
Satellite Compass
Weather Instrument
Autonautic Instrumental Cylindrical Card
C09-0030 Bulkhead Compass
RRP £92.60 (Ex delivery and VAT)
Autonautic Instrumental Flat Card Horizontally Mounted C12/110-0013 Compass
RRP £89.84 (Ex Del and VAT)
Autonautic Instrumental Conical Card
C12-005 Bracket Compass
RRP £97.45 (Ex Del and VAT)
Autonautic Instrumental Flat Card Gimballed CHH-0096 Compass
RRP £180.78 (Ex Del and VAT)
Autonautic Instrumental Flat Card
C20-00128 Class A Compass
RRP £870.00 Ex Del and VAT)
Autonautic Instrumental CMI Compass
RRP TBA