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Old 17 November 2009, 03:53   #11
J S
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Originally Posted by alystra View Post
I thought I would have problems with the electronics, but there is no discernable movement of the compass when I switch on all my electronics.
Things like your vhf still have fairly large magnets in them for the speakers and this will deviate your compass weather it is on or off. I know of someone who fitted speakers in his console and the deviation was 180 Deg

James
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Old 17 November 2009, 04:59   #12
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Locked away where?
For a magnetic compass to be effective the binnacle must be such that any compensatory soft/hard iron correctors are already in position?
Inside the console on it's bracket. Ready to come out if needed.
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Old 17 November 2009, 05:00   #13
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Good compass but the brackets are rubbish they always fall out with the slightest chop. Every time i went out they would end up in the back of the boat rolling around. These have a better bracket with a clip http://www.mailspeedmarine.com/hand-...ss667393.bhtml

James
Ah thanks for that James, I'll look into that one instead.
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Old 17 November 2009, 14:40   #14
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That's quite impressive. Does that apply at all points of the compass?

I ran an Advanced exam earlier this year on a client's own boat, and he was getting up to 20 degrees deflection on East-West headings on his steering compass (largely from the speaker on his VHF) ... but didn't realise. When I switched his GPS off and asked him to navigate to the next waypoint on his compass he soon found out!
Yes, it seems so. I did have to adjust it a bit (it has compensation magnets on screwed rods, and came with a non ferrous screwdriver thingy) It's not perfect by any means, but I think it's within 5 deg or so. Must confess, I haven't swung it properly - just observed the difference between gps co. and the compass reading on various courses and knocked off 9 degs. worth of variation. I'm sure I don't get as much as 20 deg deviation though.

Perhaps I should pick a nice calm day next season and do a proper deviation card. Be an interesting exercise and then I'd be really sure.
Tony.
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Old 17 November 2009, 14:56   #15
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Yes, it seems so. I did have to adjust it a bit (it has compensation magnets on screwed rods, and came with a non ferrous screwdriver thingy) It's not perfect by any means, but I think it's within 5 deg or so. Must confess, I haven't swung it properly - just observed the difference between gps co. and the compass reading on various courses and knocked off 9 degs. worth of variation. I'm sure I don't get as much as 20 deg deviation though.

Perhaps I should pick a nice calm day next season and do a proper deviation card. Be an interesting exercise and then I'd be really sure.
Tony.
What method are you using for the compass swing?
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Old 18 November 2009, 11:37   #16
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This one is a ritchie and it has been out by 45 degrees.
I have tried adjusting it, but as I have discovered, I need a plastic screwdriver.
The screwdriver that Ritchie supplies with their compasses is non-ferrous: copper, I believe.

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Old 18 November 2009, 11:41   #17
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What method are you using for the compass swing?
Pick a calm day, flat water no tide (easy to find round here). Using the GPS. steer 000. 045, 090, 135, etc. and on each heading note the compass reading on that heading. (The GPS should be set to show courses in True.)

Apply the Variation (from a chart), which in the UK is W'ly so you subtract it from the compass reading, and compare the result with the True heading. The difference is the Deviation of the compass and is plotted on a graph, W'ly if the compass reads more than it should do, E'ly if it reads less. You get a curve, usually S shaped. If the deviation is very bad (20 degs, say), then you may change heading tens of degrees without the compass moving.

There are lots of other ways to do the job, like using a hand bearing compass well away from any magnetic fields on the boat and comparing its reading with the steering compass, but some more traditional methods are impractical on a RIB - taking a bearing of the sun with the steering compass, for example, or using a pelorus.

Now, adjusting the compass to minimise the deviation (and it's important to do so) is an altogether separate black art. My compass came with basic instructions of how to do it (lost) and I managed quite well.

You may of course already know all that.
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Old 18 November 2009, 11:42   #18
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I got a big Plastimo compass fitted when my rib was built. It's nice and accurate and doesn't seem to be affected too much by nearby electronics. You can see my console here:



www.Flickr.com/photos/gj0kys
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Old 18 November 2009, 12:15   #19
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I got a big Plastimo compass fitted when my rib was built. It's nice and accurate and doesn't seem to be affected too much by nearby electronics. You can see my console here:



www.Flickr.com/photos/gj0kys
Thanks for the info. Is that an Offshore 90, 105 or 135?

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Old 18 November 2009, 16:30   #20
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Pick a calm day, flat water no tide (easy to find round here). Using the GPS. steer 000. 045, 090, 135, etc. and on each heading note the compass reading on that heading. (The GPS should be set to show courses in True.)

Apply the Variation (from a chart), which in the UK is W'ly so you subtract it from the compass reading, and compare the result with the True heading. The difference is the Deviation of the compass and is plotted on a graph, W'ly if the compass reads more than it should do, E'ly if it reads less. You get a curve, usually S shaped. If the deviation is very bad (20 degs, say), then you may change heading tens of degrees without the compass moving.

There are lots of other ways to do the job, like using a hand bearing compass well away from any magnetic fields on the boat and comparing its reading with the steering compass, but some more traditional methods are impractical on a RIB - taking a bearing of the sun with the steering compass, for example, or using a pelorus.

Now, adjusting the compass to minimise the deviation (and it's important to do so) is an altogether separate black art. My compass came with basic instructions of how to do it (lost) and I managed quite well.

You may of course already know all that.
Interesting, sounds complicated.
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