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Old 12 March 2002, 03:07   #11
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Jono

You will proabably be left with the same problem I have - which is apparently easily solved - and that is that the compass needs to be adjusted once in situ. This takes account of any deviations caused by surrounding equipment.

Mine is currently some 20 degrees out - will hopefully be able to sort this when we get to Cardiff and have a nice flat area inside the breakwater where we can do this.
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Old 12 March 2002, 03:11   #12
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Cheers Graeme

Is it a case of just sticking some metal bits around the compass?
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Old 12 March 2002, 03:36   #13
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Jono

From what I can see there are 2 metal bars under the compass. These are both connected to a screw head fitting. The instructions tell you to face N or S, then adjust the compass one way, then face E or W and do the same until you match either your GPS reading, or another hand held compass kept wll away from the instruments.

Apparently this has to be done on all compasses when fitted to boats - and apparently should only take a few minutes
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Old 12 March 2002, 05:37   #14
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Deviation chart

As the compass will always deviate from the actual magnetic bearings (compensated or not) the only proper way to go about it is to make a deviation chart to go with the (fixed) compass.
The chart should give you the deviation for every 22,5 degrees.

If used for general headings only, this is not so important, then just compensating will do, but mind you that the deviation after compensation can, for some bearings, still be up to 10 degrees!

Succes!

Eddy
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Old 12 March 2002, 07:23   #15
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That bar is likely to be stainless steel, not chrome. As decent stainless steel tends to be non-magnetic you may find that it's not actually a problem.

Before you start fiddling with anything in the compass, do check to make sure it's necessary.

Another point to bear in mind, is if you are only using the compass to keep on a bearing provided by the GPS it doesn't actually matter if it points in the correct direction.

Consider this:

The GPS course reads 020, you get on course via the GPS and want to use the compass to stay on course, but it reads 045. What to do? Just keep it on 045 and you won't go far wrong!

John
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Old 12 March 2002, 10:12   #16
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.... until the power fails, the GPS goes off and you have to guess the deviation from then on in ......
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Old 12 March 2002, 11:08   #17
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On the PoB we have the ship's compass adjusted every year or so.

This involves getting a "Compass Adjuster" on the ship and then taking the ship out into the Bristol Channel (to avoid interferance from other ships / building in the harbour)

Once in the channel we advise Avonmouth Radio that we are having our compass adjusted, then raise 2 flags (can't remember which) to notify other ships that our compass is being adjusted.

The compass adjuster then uses a Sun Compass on the deck - it has to be a sunny day - to find our true heading. He then places small magnets of various strengths arround our compass in special "pods".

This is repeated at different headings untill he gets the compass as near as possible.

He then draws up a compass deviation chart showing any differences.

eg

N - 0 degrees
E - +1 degree
S - -1 degree
W - +2 degrees

etc. - you get the idea
The Maximum deviation on our compass was 2 degrees - and only for a very limited heading.

The deviation differs depending on the heading - so you can't say its always +5 degrees for example.

The compass has to be done regularly (especialy in a steel ship) because all sorts of things arround the compass change there magnetic strength over time.

Things magnitise as they are moved through the earths magnetic force - a bit like stroking a nail with a magnet will turn the nail into a magnet. - Have you ever noticed that cutlery on planes (before they were replaced with plastic) were quite powerful magnets!!
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