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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Compasses and radios

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skrenz
Captain

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USA
351 Posts

Initially Posted - 06/10/2008 : 11:02:34

Do VHF radios produce a magnetic field that would interfere with the functioning of a compass? If so, how far away should you be to be safe? Would the interference change if sending and receiving as opposed to just being on? I want to mount a bulkhead mounted compass on the cabin-cockpit bulkhead. Down below is my VHF radio mounted under the shelf about 24" away. Before I start cutting a 4" hole through my bulkhead, I would like to be sure that this isn't going to be a problem.

Steve Krenz
`Elan
1978 SR/SK #482
Santa Fe, New Mexico

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 06/10/2008 : 11:08:21

Steve,
I'm pretty sure my radio manual says to install it no closer than 3' from the compass. Easy way to tell is to set the compass relatively close to the radio, let it settle down, then turn on the radio (or other equipment) and watch for any needle movement.

FWIW, my compass is on the port side of the boat, and my radio is on under the starboard lip of the hatch opening.

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Dave5041
Former Mainsheet Editor

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USA
3754 Posts

Response Posted - 06/10/2008 : 23:23:30

All electrical equipment produces a magnetic field when on, and some contain magnets. Have somebody hold the compass where you want to mount it and turn things on to see if the card moves. Even wires create mag fields when current flows, so your best test is trial and error. But remember that on a small boat you're doing good to consistently steer within 5 degrees, so don't get hung up on barely perceptible fluctuations.

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KD4AO
Navigator

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USA
202 Posts

Response Posted - 06/11/2008 : 06:58:44

The primary reason for the mounting distance between the radio and the compass is probably due to the magnetic field of the speaker in the radio.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 06/11/2008 : 07:38:11

A fellow C25 owner once asked me if I knew anything about compasses because the card on his commpass was stuck in one position. When I looked at it, I said, "Well, that stereo speaker mounted just above it might be the problem!"

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aeckhart
Master Marine Consultant

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USA
1709 Posts

Response Posted - 06/11/2008 : 11:40:51

You should have some deviation from the cabin top winch if you are putting the compass on the cabin bulkhead. It's certainly not as bad as the radio would have been but....it would be there. After you have the compass mounted, make a deviaton table to establish how much error is imparted in the compass from the boat in general.

On our boats, which get bounced around a lot in wind and wave anyway, a 3 degree error is generally not a problem, though you should know that it is there if you are going any great distance out of site of land. A professional compass adjuster might get the 3 degress adjusted out if you don't feel you want to bother with it when navigating.

Personally, now that I have a chart plotter I refer to my compass very seldom except to occasionally verify the compasses accuracy on a given heading.

For what it's worth,

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Stardog
Captain

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USA
319 Posts

Response Posted - 06/11/2008 : 14:10:04

A good way to find out if your electronics effect your compass is to set the compass where you want to mount it, then have someone else turn on your power, have them turn on every device in succession, meanwhile you watch the compass and observe whether or not the needle jumps when something is turn on. Repeat as many times as necessary in order to find a good location.

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KD4AO
Navigator

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USA
202 Posts

Response Posted - 06/11/2008 : 14:47:36

By the way, that magnetic field around the speaker in a radio is not dependent on power, it is there all the time. And the error it will cause is not constant but varies by the direction of travel, just put a radio within a couple of feet of the compass and try it.

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jerlim
Master Marine Consultant

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USA
1484 Posts

Response Posted - 06/11/2008 : 15:09:39

Our radio is mounted into the top of the starboard aft berth, below the bulkhead with the external compass. I find no influence.

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skrenz
Captain

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USA
351 Posts

Response Posted - 06/11/2008 : 22:48:34

Thanks for all the great suggestions. What I was worried about most was not a constant deviation, which could be tuned out, but one that varied with the radio on or off, I tried the suggestion of putting the compass in place and turning on the radio. I tried all its buttons and setting including send. The card never varied.

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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.