Adjusting the compass on a steel boat

[merlin]

Does anyone have any information or tips on how to adjust the compass on a steel yacht?

Any tips or sources of information gratefully received.

Thanks,

Merlin

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[kenpatterson]

hi merlin
This is a tough one. I installed a compass on my home built tucker 32 yacht a couple of years ago. i settled for a silva on price. I rang Salva for advice about sighting it and ajustment. I was told you needed to have 5 years training behind you to set it up correctly. The Silva compass appears to be a standard unit with about eight washers attached each side. A friend who knew a little about the subject had a go at setting it. I found it important to check where to place the compass to the obtain the least interference. I ended up placing it on the headcloth just under the steel sliding hatch. this works ok when the hatch is open but not when closed. On attempting to place on cockpit bulkheads it was about 45 degrees out. To correct the deviation error I swung the boat at sea away from interferce and made a card. I tried to ajust the compass by removing or adding the washers to either side but found this made little difference. Except on a couple of points of sail it is quite accurate 5 to 10 degrees out. To have this done by an expert I would think would be sore on the wallet.

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[ongolo]

Hi,

I found 2 or 3 sites on the web that explain how to do it, it is really simple if your compass has movable magnets below. The biggest problems you are going to face is to get true north or any other accurate reference.

One way of obtaining true north is with a sun compass or a wire on two pillars and a line below. With your LHA your will know when it is noon and where exact north is when the shadow from the wire and the line below are matching.

And once you have true north reference, the rest is just patience, trail and error and common sense.

I would not let my compass be adjusted by anyone but me. Where is the adjuster in mid atlantic when you have a problem?

regards ongolo


<hr width=100% size=1>1000 dream about it, 100 talk about it, 10 start and 1 completes it

 

[AndrewB]

In principle its no different from on a plastic yacht, but the deviations involved are typically much larger. My compass had a deviation of up to 40 degrees when first fitted, but with some patience that can be reduced to 5 degrees.

You may need additional needle magnets beyond the correcting magnets fitted. The correcting magnets themselves need to be replaced every 5-10 years. A company like SIRS can supply them.

I assume you are familiar with how to adjust a compass and establish a deviation curve (do a search, it has been explained here in the past). Personally I prefer to avoid making adjustments unless the deviation exceeds 10 degrees.

If possible, try to establish separate deviation curves for under power, under sail upright, under sail heeled on port and starboard tacks. With luck all these won't be too different.

Redo, or at least check, your deviation every six months. Lying ashore for any length of time or even a fierce thunderstorm can have quite an impact. Sometimes the deviation will slowly return to normal afterwards - sometimes not.

 

[ongolo]

Hi Merlin,

E.S.Ritchie had a site with instruction for adjusting a compass. I have saved itm but in doing so it does not display the address anymore.

Then if you google compass compensation, you should find a group or message board that has also explicit instructions how to do it. I have both these saved, but not sure where I got them from. I could try and send them to you if you dont come right. Or maybe I should put it to a yahoo group we are using as a archive.

Ritchie also have a pdf file with operating and parts list, you might find this first, but that is NOT what I am talking about, search beyond that.

regards ongolo


<hr width=100% size=1>1000 dream about it, 100 talk about it, 10 start and 1 completes it

 

[MainlySteam]

Most boats I deal with have gyros or are constructed of aluminium so my experience with magnetic compasses on steel boats is limited to my own and a few others. Fortunately I had been involved with a few others before we had our own steel sail boat built.

For those with the compass placed on a pedestal in the cockpit none required soft iron correctors and were all adjustable to well within the resolution of the compass and helmsman's accuracy (so no deviation card required) with just the compasses E W standard correctors. Everyone I have known who has added soft iron or other magnets and not put the compass on a pedestal seem to have complicated the situation for themselves. The key is to have the compass mounted so there are no great unbalanced masses of steel close to it - on a pedestal is good, on a bulkhead or steel deck is bad.

So when we had our boat built it had a fluxgate compass and mounted a conventional magnetic compass on the steering pedestal. The fluxgate was set up as normal and was proven against the GPS. The steering compass is a Silva pedestal type which is available with or without soft iron correctors, so on past experience we omitted the soft iron. I set it up against the fluxgate using just the standard E W magnets as standard fitted, double checked against some bearings from a known position and against the GPS - it is within 1 or 2 degrees on all points for which on a smallish yacht (41 foot) is not worth worrying about. Any even remotely reputable compass will come with instructions how to do this.

Interestingly, a handheld fluxgate seems to be quite accurate (never investigated how but certainally usable) down below on our steel yacht.

There are a whole library of books describing how to adjust compasses (Mary Blewitt's book on coastal navigation has an excellent section written by her husband) - just don't be tempted to make it too complicated for yourself, most compasses for smallish yachts are only graduated in 5 degree increments, and if finer than that no one is going to be able to steer to the accuracy of the graduation.

John

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