Two ways, use electronic compass with the sensor about 2m above the deck, or invest in the special steel boat compass, the ones with the round balls on either side, and have it adjusted and corrected and use the deviation card that the compass adjuster will give use. If it is a new boat, check the compass after a few months because the magnetic forces seem to settle down, but we always check our compass anyway, don't we! I have to do some serious metal arithmatic on some course on my boat, but you get used to it, it almost becomes second nature, eventually.
Good advice.. I had a Suunto , which again was a steel boat compass. The professional adjuster actually said not to worry about the two black balls.. he said they were there as a decoration or if seious , then they had to be regularly adjusted. I left mine on and checked and altered my deviation chart in many differnt locations Its ok if you sail regularly in the same area, but an annual check would be advised in this case.
Funny, when I did my Yachtmasters the examiner just hapened to have a magnet in his pocket.... dirty trick. The point is however always make sure there is nothing nearby that will play havoc with the compass.
Compass ideally is installed away from the largest amount of metal, & where it will be less affected by heeling errors, which means not up the mast, so the common comprmise on larger vessels is on the wheelhouse roof, & viewed through mirrors etc.
As others have said, buy the correct one for the job & have it swung professionally.
I agree with the others, get a good quality steering compass and have it professionally set up first time round.
BUT ... Unlike on a GRP yacht, don't expect deviation to remain constant for very long! You really ought to check it a couple of times yearly. The best way is to use transits to check headings, then draw a deviation curve. (Get a really large scale local chart, use fixed transits not buoys!). Periods ashore doing maintenance work to the hull, and severe thunderstorms, are things that change deviation.
Don't waste money on a handbearing compass, quite useless on a steel yacht. For fixing position, use a pelorus on the main compass, or get a sextant. (Most of us just rely on GPS).
A fluxgate compass is also affected by magnetic deviation, but if the compass unit is mounted a little way up the mast, with the repeater in the cockpit, will be less influenced by the yacht's field. Do not confuse with a gyro compass, which is not affected by magnetism but unsuitable for use aboard a small yacht.
Are we at cross purposes here, I was not suggesting siting a conventional compass up the mast, rather the sensor for an electronic one. Whilst I agreee that the further from the pitch and roll centre the worse the gimballing for the fluxgate compass gets. However Raytheon state in their installation guide that sensors can be on the mast to get it away from the steel. They say 1.8m from deck level.
The heeling errors which may be introduced in such a position when I tried this on a GRP boat were insignificant and would certainly be overshadowed by the general overall better performance. It seems to work for me, but in bumpy rolling situations the compass can be bounced out of it's gimbals realetively easily.
You say.. The heeling errors which may be introduced in such a position when I tried this on a GRP boat were insignificant....
Well they would be, on a grp boat, try again with a steel boat & it is a different story
Think of being on a ladder & coming down to look through a shop window (grp)
Coming down the same ladder & looking through a milk bottle bottom (steel), after all when the compass heels its position relative to the magnetic disturbance & the earths field changes, & if the disturbance is greater the greater the effect. In the Grp boat the biggest effect will be in round terms from the engine, where as in the steel boat, there is more metal spread around
As to electronic compasses if the manufacturer says put it 6 feet up the mast, that is where it is to go, after all he wants satisfied customers, & in their view it is a best compromise position
Others have mentioned the need for regular reswings, was told by a adjuster working in Rotterdam for Observator (Dutch arm of Kelvin Hughes) A steel yacht settles down after a couple of years, after which there should be little change unless welding has been done on the boat.