Compass Problem CD-22

R

RichardAK

New member
In getting my 2000 CD-22 ready for a boating season in Prince William Sound it seems that the compass is greatly in error.... about 60 degrees off where it is presently mounted. The boat is now on the trailer. When I remove the compass from its mount and move it about the boat I can get good readings if the compass is away from all the instruments, electronics, and helm wheel. The compass is mounted on the flat surface in front of the wheel and just aft of the front window. The previous owners most likely relied on the GPS for navigation. Past experience as a small plane pilot has taught me the importance of a good, reliable compass. I searched the forums but didn't find any info about this problem. Any ideas on another compass location or how to fix the problem.

Richard in Fairbanks, AK
 
With the compass in it's mount, theer should be a way to adjust the reading (known as swinging the compass).

This assumes that the magnetic variation at the mounted location stays consistant. There is obvioulsy something near the compass at the helm that is causing the error.

Got any photos of your helm so we can see what might be interfering. It may be possible to move something for better compass perfromance.
 
Richard,

I'd be interested to see how it reads when on the water instead of on a metal trailer. Or possibly, move the trailer 90 degrees and see if the error diminishes or increases..
 
I've never had to correct out 60 degrees; no idea if that is even possible. I've been on boats in the past where the compass was more of a "decoration" than a trusted instrument. :crook Assuming you've tried checking the compass with all the instruments on to see if there's any difference? Is there any place you can move it around the helm where you can still see it (and you don't get the interference?). Ours is mounted in the center, above the entrance to the v-berth... not the optimum for viewing, but using the 30º index line works.
 
Well, I gave this issue some thought when locating my compass. I placed it just over the hump on the Port side. It is out of the way 99% of the time and for the other 1%, the reading will be unaffected by all of the other stuff forward of the wheel. This includes all of the electronics on the forward side of the helm station.
 
I checked the distance from the compass to the windshield wiper motor is 7" and as the compass is moved closer to the the motor the compass really reacts. This may be the main problem. The speaker from the radio/CD player is 22" from the compass. The GPS and Radar receivers are mounted over the entrance to the V-berth and the compass seems to be better in that location but it would take some doing to move the receivers. When I'm seated at the helm I did find an area at about knee height on the extreme starboard side where someone mounted a drink holder that the compass seems pretty close. The back of the GPS receiver notes the compass safe distance... STD 0.8M, Steer 0.6M. The GPS and Radar receivers seem to have a small effect on the compass. I took photos of the area but don't know how to post. I appreciate all you guys help on this.

Richard in Fairbanks, AK
 
The windshield wiper motor with its magnets is the problem. The 25 and the 255 Tom Cat have the wiper motor at the top of the window--and that may be a good solution for you also to move the wiper motor.

You will have to mount the compass away from the wiper motor. This may mean in the center of the "dash" or even further from the compass motor There will be some parallax, with your being set off to the side. You might also consider an electronic compass with a flux gate sensor which can be placed remotely. I picked up a discontinued one about a year ago for about $40.

There are also "tell tale" compasses, which can be mounted overhead. This is inconvient on the C Dory.

You will have a very difficult time getting that compass compensated, or even making a good deviation table or Napier diagram (the amount of deviation every few degrees) with a deviation of 60 degrees.

Deviation is the influence of magnetic fields on the boat, or in the close proximity of the boat, and will change as the boat swings in a circle. Deviation may be ferrous metal or electrically induced magnetic fields. That is why you need to make compensation tables or Napier diagrams for the compass with all circuits on and all circuits off.

Variation (commonly called declination) is the influence of the earth's magnetic pole being different than the geographical pole. In the Fairbanks area this is somewhere near 20 degrees east. That will be fairly constant, unless by some quirk you are over a highly magnetic field.

There is also dip. Most compasses sold in the US are made for the zone of 30 degrees North--this is usually not a problem until you go to 30 degrees South. But Fairbanks is almost 65 degrees N.--and for a very accurate compass, you might want to take dip into consideration..This is not done on small boat compasses.

You can compensate compasses, both using the built in compensators, and by using external magnets. But 7" from a magnetic motor is going to be too close to compensate.

One also has to be sure that the compass is aligned exactly parallel to the fore and aft plane of the boat.

My favorite instrument to compensate a compass is the sun compass. It is simple and easy to use. Plus you can check the compass in a few minutes if there is any question about putting in new electronics.

I like to use a hand bearing compass (puck type is easiest to use)--and check for deviation in various parts of the boat before mounting the steering compass.
 
Bob makes some excellent points, (of course) plus the trailer doesn't help, also PW Sound has variation of 22 degrees with local variations along that coast of up to 14 degrees more in places. Not sure if you are taking that into account?

Regards,
 
You guys are great. Thanks for all the help. Problem definitely is the magnets in the wiper motor. I knew I couldn't compensate out 60 degrees of error. Dr. Bob's idea of an electronic compass may be the best idea rather than relocating the wiper motor. Meanwhile I will temporarily locate the compass on the starboard side of the helm. Thanks again for all your help.

Richard in Fairbanks, AK
 
Richard,

On my 1983 22' Classic I had to mount my Richie compass on the starboard side of the helm at knee level to eliminate the wiper motor effect. I did this 28 yrs. ago and it has worked perfectly and is easily visible once you get used to it.
Good luck.
 
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