Marine Compass dip correction

Talbot

Talbot

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My fine vessel has a plastimo 105 offshore compass which doesnt work very well / mainly because some dummy installed the big vtronix VHF speaker about 2ft away from it. I was looking at the problem of correction etc when I recognised that my future plans for sailing in the southern hemisphere demanded a compass that would allow dip correction.

Anyone know of any that allow this



(the only solution for the vtronix is to move the darn thing)
 
Lady Campanula said:
There's a 'lead' to sound, reliable advice on the Plastimo website here.;)
Click to expand...

When the advise is to buy another compass designed for the southern hemisphere / I am not sure that it could be termed useful.

I know that the Suunto 135 and 165 will both automatically compensate for dip, but these are too big and too expensive. Just wondered if these was a single compass that met my requirements.
 
Talbot said:
When the advise is to buy another compass designed for the southern hemisphere / I am not sure that it could be termed useful.

I know that the Suunto 135 and 165 will both automatically compensate for dip, but these are too big and too expensive. Just wondered if these was a single compass that met my requirements.
Click to expand...

Compasses are made for 3 zones IIRC. They put a weight (non-magnetic) on one side of the needle to compensate for the needle trying to point into the earth in the direction of the poles. Since the magnetic pole is inside the earth and not at the surface, that is where the needle is trying to point. If, for example, you were (on the Earth's surface) at one of the magnetic poles, it would be trying to point into the Earth to line up with the magnetic lines of force. Think here of the lines of force coming from space, having circled the Earth. Think also of iron filings on a sheet of paper and the magent underneath - in the vicinity of the polar regions a compass needle on the sea will point to the pole but also into the Earth.

So there are 3 zones they make compasses for, according to the weight that is put onto the needle. They also make a 'global compass' such as this handheld: http://www.amazon.com/gp/product/B0...mp=1789&creative=9325&creativeASIN=B000FEUCRW

A little internet searching will find you the model numbers of the Silva or Suunto models for the other zones, or for the global versions they make. Otherwise order a compass from a retailer in the zone you are after. www.whitworths.com.au for example.


PS. Here is a link to Suunto's page about the 5 zones (I thought there were only 3). It also talks about the horizontal and vertical components in the magnetic force. http://www.wide-screen.com/Suunto/zoneInfo.shtml

Or this page trying to persuade you there's now a 2-zone system: http://casanovasadventures.com/catalog/compass/p1408.htm

I've read that far outside your compass' zone the needle will tend to touch the glass or stick to the compass housing, and needs a tap to free it. Or buy a cheaper or smaller $50 compass for the zone you're going to.
 
Last edited:
Talbot as I am sure you know but others don't one does not normally have to buy 5 different compasses when cruising the world in order to have a compass that works in all zones, nor do they have to be adjustable for dip.

A sail boat compass card should have a weighted centre of gravity well below the card pivot point so that it resists the dip force on the card. It should also allow for plenty of rotational fore and aft boat motion so that the card does not foul during pitching (and so not from dip too).

I cannot comment on your Plastimo compass but if Plastimo cannot assure you then I suggest talking to Nexus Marine in Sweden who do Silva marine compasses now and as far as I understand are built as I described; unless this has changed in recent years were also reasonably priced. We used them, same model as still in production, some years back on our smaller boats.

Some compasses can be a problem though and, for example, I have seen a northern hemisphere corrected Ritchie compass jamb its card on southerly headings when the boat was down in the southern hemisphere - that in calm water on a 90 foot power boat :-(.
 
barnaclephill said:
Compasses are made for 3 zones IIRC. They put a weight (non-magnetic) on one side of the needle to compensate for the needle trying to point into the earth in the direction of the poles. Since the magnetic pole is inside the earth and not at the surface, that is where the needle is trying to point. If, for example, you were (on the Earth's surface) at one of the magnetic poles, it would be trying to point into the Earth to line up with the magnetic lines of force. Think here of the lines of force coming from space, having circled the Earth. Think also of iron filings on a sheet of paper and the magent underneath - in the vicinity of the polar regions a compass needle on the sea will point to the pole but also into the Earth.

So there are 3 zones they make compasses for, according to the weight that is put onto the needle. They also make a 'global compass' such as this handheld: http://www.amazon.com/gp/product/B0...mp=1789&creative=9325&creativeASIN=B000FEUCRW

A little internet searching will find you the model numbers of the Silva or Suunto models for the other zones, or for the global versions they make. Otherwise order a compass from a retailer in the zone you are after. www.whitworths.com.au for example.


PS. Here is a link to Suunto's page about the 5 zones (I thought there were only 3). It also talks about the horizontal and vertical components in the magnetic force. http://www.wide-screen.com/Suunto/zoneInfo.shtml

Or this page trying to persuade you there's now a 2-zone system: http://casanovasadventures.com/catalog/compass/p1408.htm

I've read that far outside your compass' zone the needle will tend to touch the glass or stick to the compass housing, and needs a tap to free it. Or buy a cheaper or smaller $50 compass for the zone you're going to.
Click to expand...

Thank you for listing the data on which I had based my enquiry! :rolleyes:
 
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