We can think of variation as an ‘error’ in the earths magnetic field not being aligned with the rotational axis and true geographic north pole. You can think of a boats have its own magnetic field and ‘deviation’ as another magnetic error between the vessels own magnetic field and the earths magnetic field. A boats deviating influence on the earths magnetic field is affected by the amount and position of ferrous metal and electromagnetic equipment. Lots of things on yachts can influence a boats magnetic field and subject the sips compass to deviation:
Ferromagnetic iron (iron and steel)
Objects such as the engine, keel, mast, boom, standing rigging, pulpit, push-pit, stanchions, guardrails, batteries, food and drink cans, metal frames, glasses, etc.
Electromagnetic influences from wire carrying current, such as radar, VHF radio and mobile phones
Magnetic influences from magnets found in loudspeakers and magnetic instruments.
Deviation is caused by the interaction between the boats and the earths magnetic field. Basically we have a boat loaded with ferrous metal and electromagnetic generating its own magnetic field navigating on the surface of the earth surrounded which is surrounded by the earths magnetic field (Fig 4.10 a,b,c,d). As the boat changes direction the the interference between the magnetic fields deflect the ships compass needle away from the magnetic north pole and is called deviation.
If there is zero deviation the ships compass would be pointing towards the magnetic north pole, but like variation, deviation can have direction and the compass can be deflected to the west or east of the magnetic north pole. At first it is mind boggling to think about deviation as having a direction and best to think of it as being another ‘error’ with a simple east or west value.
Lots of things can affect the amount of deviation on a vessel. We could install new electrical equipment, replace the engine or even move the kedge anchor from one locker to another., all have some effect on the deviation values of the vessel. Each vessel has its own deviation ‘signature’ and can affect the accuracy of the ships compass readings. It is imperative to us that we are aware of how much deviation the compass is subject to individual headings so that we can work it into our navigational calculations.
The steering compass can be assessed for deviation by a compass adjuster who will measure the amount of deviation the compass experiences on different headings. Of course if you move the steering compass or make significant movements of any of the deviating ‘metal & electrical’ influences on the vessel the values will change. We perform an operation known as ‘swinging the compass’ where we steer the vessel on different known true headings and note the steering compass reading to determine amounts of deviation experienced on that particular heading. Compass adjusters can correct the deviation by placing tiny magnets around the steering compass to take some of the error out, and smooth the line on the deviation card. However, what is important is that we are aware that there may be some deviation on certain headings and in concert with variation may produce significant cumulative errors.
Ships Deviation Card
The RYA Training Almanac has a deviation card printed on Page 97 (Fig 4.11). We can use the curve to determine the value if deviation on any heading. It may be necessary to interpolate between values. e.g. if we were to point the vessel on 011° we would be around halfway between 4°W and 2°W so we could interpolate the deviation experienced on this heading as 3°W. Whenever you go on a new boat you should have a look at the deviation card and check the steering compass bearing against certain known headings to determine if there are any serious spikes in deviation that may give erroneous compass readings.