Compasses 20 degrees out

dgadee

dgadee

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Installed two new plastimo 101 contest compasses either side of hatch and > 1 metre from engine. No steel anywhere near but they are both reading 20 degrees out.

What would you do?
 
Does that infer you've done a compass 'check swing'? If so,what accurate direction reference are you comparing them with? Or do they differ from each other by 20 degrees?

What's stored on the bulkhead, inside? P'rhaps a handbearing compass, like one of my friends, or binos with a compass built-in. Or.....?

Is there a large deviating influence, such as a heavy ground chain, under the boat? Do the perceived errors change when you run the engine and switch on the lights and instruments?

Is the wiring to the compass lights 'twisted' on itself, as it should be, or straight-parallel, which can induce deviation?

Have you sighted the boat's heading, using the HBC, standing up on the transom - and compared that with the Plastimo compasses?

Are the Plastimo compasses fitted to the bulkhead using magnetic screws, as were all the RAF Puma fleet's French standby compasses in the 70s?

Is there an aged navigator in your yacht club who, if you cross his palm with Heineken, could resolve all these conundrums for you...?

;)
 
Salty John said:
Do they agree with each other?
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Yes, they did - exactly agreed. I had wondered why I ended up to the south of where I was going. I then checked them against the 3 gps I had on board, each of them giving identical headings (ie 20 degrees out of kilter with compasses).

Not sure they have any means of adjustment - though I think you can purchase inserts which allow this (http://www.youngsboatyard.co.uk/product/723/1537/Plastimo-Compass-Compensation-Box). But is it worth it? Should I just draw up a table of errors?

Wire remains twisted as per bought.

There is wiring behind and below one compass, but surely that would only affect one. Nothing near the other.

Compasses both fitted with the plastic nuts and bolts which came with them.
 
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PCUK said:
A compass is useless until it's swung!
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Perhaps in the days when there was no gps that was true. Not so sure now - it just points where you need to point the boat, which can be decided by the gps.

I think I will get the compass adjusters and see what happens and whether I need to find an old and salty compass swinger.
 
dgadee said:
Not sure they have any means of adjustment - though I think you can purchase inserts which allow this (http://www.youngsboatyard.co.uk/product/723/1537/Plastimo-Compass-Compensation-Box). But is it worth it? Should I just draw up a table of errors?
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Those are the ones I have. I don't remember the details, it was a long time ago. I replaced the compasses because the lenses were worn and cloudy, the inserts were installed with the old ones. Without the inserts both new ones were a long way out. I reinstalled the inserts and eventually got both to read accurately, although it was a long job.
 
Before doing anything I'd check that assumed error by as many ways as I could; compare to the compass on a boat alongside, find a charted range, use a handheld compass. Only then would I think about compass correction. To have two compasses 'out' by the same amount sounds odd to me.
 
Twister_Ken said:
Aged navigator? Heineken? That'd never work. Aged navigators run on higher octane fuel.
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Do I hear a higher bid? :D


There will be an Ancient Mariner along any minute asking you to book the local Degaussing Range for this weekend, and a convoluted discussion of Flinder's Bars.

You could always try taking them back to the vendor and seek replacements under the Sale of Goods Act, but someone who believes that a GPS receiver offers anything vaguely comparable to a 'compass heading' is likely to be tied in knots by any good Plastimo Technical Rep.

Lady Campanula
By Appointment to HM Queen Elizabeth II - Compass Calibrator Extraordinaire ( Vulcan, Canberra, Puma, Jaguar - ret'd. )

:D
 
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Twister_Ken said:
Aged navigator? Heineken? That'd never work. Aged navigators run on higher octane fuel.
Click to expand...

Barry Jones said:
Yes usually finest Jamaica or best highland.
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I'd happily buy a bottle of either if someone could render KS's compass accurate and trustworthy.

I have a deviation card made by the previous owner, with some quite large numbers on it, and I don't trust it any more anyway. Keep meaning to go out and try to minimise the deviation and record what's left, but it seems like a fiddly and frustrating job and I'd rather just go sailing.

Pete
 
Do the compasses alter when the engine is in operation, sometime the rotary mass of a flywheel can affect the magnetic fields.
I often wondered what corrections were made on transatlantic ventures when the supply of tin caned food is depleted during the voyage, i've never read of any adjustments being made.

ianat182
 
Both are out identical amounts in the same direction.

The engine does not have a flywheel. The effect is there whether the engine is on or off.

Odd, isn't it?
 
dgadee said:
Both are out identical amounts in the same direction.

The engine does not have a flywheel. The effect is there whether the engine is on or off.

Odd, isn't it?
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If the 20° "deviation" is caused by some magnetic influence on the boat it will vary with the boat's heading. Does it or is it always the same regardless of the heading?

If it does not alter with the heading I think I would be suspecting something strange about the compasses.

I think I would take them off and take them well away from any magnetic influences and see if they are still out by the same amount.

How are you measuring this discrepancy. Against a hand bearing compass held well away from anything magnetic or against bearings or transits on charted objects or what?

Not only is this odd but so too is an engine without a flywheel what ever is it?
 
VicS said:
If the 20° "deviation" is caused by some magnetic influence on the boat it will vary with the boat's heading. Does it or is it always the same regardless of the heading?

If it does not alter with the heading I think I would be suspecting something strange about the compasses.

I think I would take them off and take them well away from any magnetic influences and see if they are still out by the same amount.

How are you measuring this discrepancy. Against a hand bearing compass held well away from anything magnetic or against bearings or transits on charted objects or what?

Not only is this odd but so too is an engine without a flywheel what ever is it?
Click to expand...

Not sure about all headings. Coming back across North Channel was where I worked out there was a problem. As I said, this was checked against 3 gps devices rather than a different magnetic compass. Haven't had time to investigate fully but was just down at it earlier this evening and note that it seems 15 degrees out as against a boat moored alongside (and at 90 degrees to heading where I noted the problem) but that boat's compass could be wrong too ...

Yes, could remove.

I meant no large outer flywheel (as with older volvo engines). It obviously has a lighter flywheel inside with the starter ring on it. I presumed that flywheel's magnetic effect would be limited.
 
3° could be due to variation if your GPS is displaying True rather than Magnetic headings. The other 17° through mistaking COG for heading!

The proper way to swing a compass is with transits and known bearings lines like leading lights.
 
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