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Old 08-09-2007, 19:58   #1
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Compass & Vhf

I am/was planning on mounting a compass in the front center of the cockpit in the aft side of the bridge deck. I don't really have another good place for one. Then it srtuck me that the VHF which is just under the companionway steps might affect the compass. Is the compass going to go nuts everytime the radio makes a sound or someone keys the mic? I don't really have another place for the Vhf either.
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Old 08-09-2007, 21:28   #2
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Yes the compass will probably go nuts, especially when you transmit.

Metallic structures and the "magnetic hum" of your boat will affect the compass. The compass has compensating screws to adjust out, or actually, balance out the errors. Compass compenstating should be done with all "standard" equipment running including the VHF.

Any magetic field or metallic object introduced after compensating or any variable field, like the VHF when transmitting, will affect it. How much you won't know until you trial install. It might not be too bad.

So with the VHF running in "idle" near the compass you could compensate the field out. It will likely swing big time when you hit xmit but the real killer in the VHF is the antenna and presumably this is mounted on the mast.

You may be able to live with this because the amount of time you are transmitting is insignificant when considering how much time you are navigating.

You could jimmy them up and see what happens.
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Old 09-09-2007, 01:23   #3
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A minimum distance of 1m(3ft) is usual practice of seperation. But not always possible on a boat. However, I would never compensate a compass to an item that can be changed from an on/off state. It would be better to move the set to a safer distance.
The other question would be, Do you have a "swung" compass anyway?? If it has not been officially swung, then it won't be accurate enough to rely on real navigation with anyway.
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Old 09-09-2007, 04:35   #4
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However, I would never compensate a compass to an item that can be changed from an on/off state.
Coming from the aviation world I have swung a compass or 10. The cockpit is an almost impossible environment to get 3 ft of clearance from the electronic stack so there may be some differences.

Most boats have the luxury of being able to stick the compass on the binacle far from electronics but it doesn't have to be that way. You can compansate for any steady state influences and build a card.

You should absolutely swing the compass in the "cruising" configuration. If cruising configuration is radio and refrigerator on then that's the way you should do it.

If you are going to rely on the compass for passage making you should have an accurately swung compass with a complete deviation card and you might want to make a comparison with the boat powered up and not powered up.

BTW - If you motor sail 90% of the time, your iron genny is under the cockpit and your compass was swung statically or under sail you could be quite off...

Compasses have tons of limitations. There are whole books on compass navigation.
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Old 09-09-2007, 06:15   #5
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Hey, how about holding the compass in place, note the heading, then turn the radio on. Did it move? Hit the PTT button. Did it move? If this indicates no problem and your still worried, try it on a couple of other headings.

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Old 09-09-2007, 20:24   #6
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I plan to try something like that. I won't be able to do it as well as I would like as the boat is on the hard. The mast isn't stepped either but I can do that when I get a little further along. I should have the compass in hand in a week or so, it is ordered now. If I have to I will do something different I just don't know what.
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Old 09-09-2007, 21:45   #7
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Thumbs up

does this thread not validate the statement, "A good navigator never knows where he's going 'til he gets there... then he's STILL not sure."

hehehe... great topic guys.
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Old 22-08-2008, 05:43   #8
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I have a plotter mounted very close to the compass and I swung the compass and made a card for power up plotter and shut down plotter ( not very much difference)however with a flux gate compass in the nav system the Compass on the binical is only used for checking headings now and for the old emergency when all else fails.
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Old 22-08-2008, 07:12   #9
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Don't forget that a fluxgate compass is effected by magnetic fields in the same way as a traditional compass.
Another sometimes forgotten aspect is that the speaker in a radio will affect the compass whether the radio is turned on or off.
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Old 22-08-2008, 07:12   #10
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Probably an extremely dumb question: Wouldn't a GPS "compass" act as a comparison when setting an "analog" compass?
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Old 28-08-2008, 14:26   #11
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I plan to try something like that. I won't be able to do it as well as I would like as the boat is on the hard. The mast isn't stepped either but I can do that when I get a little further along. I should have the compass in hand in a week or so, it is ordered now. If I have to I will do something different I just don't know what.
If the boat is stationary on the hard I suggest that is the time to test if the radio will influence the compass card. Once afloat you will not know if it is the water or the radio is moving it.

Try moving the compass fore and aft, port and stbd whilst keying the PTT (remember to select a quiet and little used channel first) within the limits that you have. Then as any compensators are on the fixed portion of the compass slowly turn it 360 and watch for any drastic change in the compass card.

You will know if there are any problems after that.

If it is to be your main steering compass swing it once the boat is fitted out.

Good luck.
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Old 28-08-2008, 14:47   #12
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An alternative is to have both a standard compass and a steering compass. The standard compass you can put anywhere so long as it is away local magnetic fields and large pieces of iron such as the engine. The standard compass is the compass that you would compensate and create a deviation table for.

The steering compass you can put where it is most convenient to steer by. Its then a simple matter of having someone at the standard compass letting you know when you are on course while you are looking at the steering compass at the same time. This is how it was done on ships. They of course use gyro repeaters but still have backup magnetic compasses.
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Old 28-08-2008, 14:50   #13
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Probably an extremely dumb question: Wouldn't a GPS "compass" act as a comparison when setting an "analog" compass?
I was thinking the same thing. For sure it'd be a lot less time consuming than a series of known course out and backs. Has anyone done this? Any thoughts? It seems too easy, so there must be a problem...
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Old 28-08-2008, 15:23   #14
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With a few exceptions, the 'compass card' on a GPS is driven by running calculations of your course over the ground. It has no knowlege of which way the boat is pointing. Come to think of it, a wet compass has no idea of which direction you are moving, it just shows, for better or for worse, which way you are facing! However, if you have a chartplotter with NMEA input, your GPS can probably be set to show what a remotely mounted electronic compass is saying. This little black wart can be mounted below decks, far away from any magnetic field generating objects such as engines, motors, speakers or just nearby power cables. Here I allow for the obligatory caveate than it is and electronic device, dependant on enthusiastic and participating electrons.

Option two is a remotely mounted VHF radio with a RAM mic, haning on a hook somewhere more than 3 feet away from the compass. A word of caution, many wheel steering systems use wire rope something like a motorcycle chain to connect the wheel to the rudder quadrant. Some bycycle chain is made from a grade of stainless that can be magnetized by the ships field over a long period of time, so when you are checking the compass of an older boat you should turn the wheel lock to lock and look for a change in compass heading.

In airplanes we had to consult a compass correction card because the error was not the same on every heading. We apparently don't have as bad a problem as airplane cockpits, where you couldn't hear a radio speaker five feet away.
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Old 28-08-2008, 15:32   #15
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There is such a thing as a GPS compass. It uses two GPS antennas and compares the differences in position which generates a heading. This of course is not the same thing as Course Over Ground generated by a single antenna GPS, which as Sandy says is not the same thing as heading.
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