How to choose a compass
Compasses come in many different designs and when selecting one it is worth thinking about what is suitable for your use, read on to learn how to choose a compass.
An Approved Compass
When a marine compass is used for navigation it should 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 disturbed, 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 symbol denotes compliance with the directive and therefore an approved compass. Autonautic are one of the few manufacturers left of SOLAS MED approved compasses.
Putting aesthetics to one side, the most distinctive compass features to consider are the card design and location of the compass. The card will be one of three basic shapes – 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. Equally important is the compass location and therefore its design and mounting, vertically mounted, horizontally mounted, gimballed or bracket mounted.
The most critical components in the construction of a compass are:
- The Crystal dome that magnifies the card, making it more legible but also protects the compass contents from Ultra Violet Radiation.
- The jewelled pivot that ensures the card rotates smoothly and evenly.
- 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 the fore and aft line of the vessel. The main lubber line can be located either at the front or rear of the card and this positioning depends on the type of card which in turn is dependent on how the compass will be used.
Cylindrical card compasses
Cylindrical card compasses have traditionally found favour with yachtsmen, in smaller yachts, since when standing in the cockpit with the compass is mounted on the companion way bulkhead it is in line with your eyes and easier to read. These compasses are read from the aft end of the compass and is important when considering how to choose a 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 since they are easier to read from above. The flat card is appropriate where you stand at the helm and find favour in larger vessels. 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
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, and a benefit of the conical card design means that the cardinal and half cardinal points can be marked on the inner slope of the card, allowing them to be read from the bow end of the compass.On the underside of the card is the carrier for the magnets which is balanced on the jewelled pivot.
Compass Design and Location
Ideally a compass should be located on the fore and aft line in 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 (also known as bulkhead), horizontal (also known as flushmount) or gimbal mounting. The bracket mounted compass is a variation of the gimballed compass without the ability to swing freely.
Vertically Mounted Compasses
Vertically mounted compasses fit easily into a bulkhead and take up the least space. They are convenient to read and are optimised to go on a vertical surface which is a considering when deciding how to choose a compass. They will tolerate a sloping surface (say up to a few degrees from the vertical) and the move to conical style cards has allowed the slope to be less of an issue. But the issue to consider is that any incline (be it vertical or horizontal) will physically restrict the card movement in one direction – a limitation imposed by the laws of physics!.
Horizontally Mounted Compasses
Horizontally mounted compasses (also referred to as flush mounted) need space to fit them and are most popular in motor vessels where they can be located in front of the helm steering wheel. In this situation a conical card would be ideal. They also find favour in larger yachts with wheel steering and binnacles where they can be mounted in the binnacle pod. It is also possible to mount compasses in the roof with a viewing mirror underneath.
Gimballed compasses have the advantage of allowing the compass to self level as the vessels motion changes and are popular in larger vessels. 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.Bracket mounted compasses find favour in Power boats, RIBS and smaller vessels. They have the advantage of being easily removed for added security and have the smallest mounting bracket footprint. In addition bracket position can be adjusted to accommodate both side and even inclined roof mounting.
Some compasses additional lubber marks are located at +/- 45 degrees, so that in a yacht when it is heeled, the course can be more easily read. The two bracket compasses above show this feature as well as a built in inclinometer to display the angle of heel.
Vessel Construction in relation to magnetic compasses
There are generally two main classes, vessels made of steel and those that are not. In a steel vessel you will need a compass which has built in quadrantal correctors and possibly Band C.
Compass Compensation and Correction
The compass being magnetic responds to the earth’s magnetic field and 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 to the surrounding magnetic field.
When choosing a compass the main area you operate in needs to be considered. This is because the magnetic dip (from the horizontal) varies across the world’s surface and the compass card must be balanced for the area in which you are cruising.
Depending on the compass manufacturer the world is broken into between five (eg Autonautic) and seven regions. The card is balanced during manufacture for the appropriate zone. Generally our advice is if you buy within the zone where you do most if not all of you boating then you will generally be ok. If you buy outside of the region or intend to use outside of the region then check you are getting an appropriately balanced compass card. It can make quite a difference.
Once a compass is installed on a vessel the distortions that arise locally need to be compensated for and this is done by an arrangement of compensating magnets. This can range from a simple arrangement of two orthogonally mounted 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. All compasses can be corrected, but you should consider this, when a bracket mounted compass is used is correction appropriate?. If the compass can be tilted in the mounting bracket it may be that the correction is no longer valid. The issue is that you cannot guarantee returning the compass to the position the correction was applied to. This is not the case in vertically, horizontally or gimballed compasses where the compass position is always fixed relative to the magnets used to correct the compass.
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 the readings are inaccurate. Commercial vessels have a compass deviation book and owners of leisure boats should take the opportunity to regularly check the deviation as the opportunity arises.
Correction methods take one of two forms.
Either a simple system where the correction magnets are located at 90 degrees to one another and are rotated to increase or decrease their effect, these are typical on compasses used in leisure craft and typically slide into a custom made slot. Or the more complex installations in binnacles with Quadrantal correctors, a Flinders Bar, and B & C correctors.
Navigatebyus can supply a wide range of compasses designed by Autonautic Instrumental for both the leisure and commercial markets.
Web site www.navigatebyus.com
Contact is Chris Cole +44(0)1494677751