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Discussion Starter · #1 ·
I have a new boat with a pair of Plastimo Offshore 135 compasses. They seemed off by a considerable amount so I researched adjusting them. I followed the instructions where by you follow a known North course and zero the compass, then head south and calibrate it to half of the remaining deviation, splitting the diff so to speak. Same for East/West. They seem much closer now, within 3 to 5 degrees, but I do need to create a deviation card.

My question is this. Was I wrong in trying to calibrate them? When I mentioned this to a sailing instructor I was chastised for touching them, stating that the factory calibration should not be tampered with. :confused: Does it not make sense to calibrate as best you can to each individual installation, then develop a Deviation Card based on the remaining variation? Also I'm to assume that the manufacturer of a production boat is unlikely to take the steps to calibrate, let alone create a Dev Card.
 

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The "factory calibration" was probably worthless the moment the compasses were installed. Even a GRP (fiberglass) boat has enough metal bits and pieces to give it its own magnetic environment. That environment has to be compensated for by adjusting the compass(es) after installation.

You might try fine-tuning the calibration by anchoring away from any docks or other boats (i.e., away from anything else that might have an influence on the compass). Then using a GPS and charts to find some landmarks as close to the cardinal compass points as possible (but as far away from your location as possible), swing the boat around to the necessary headings and use a second (and maybe even a third) anchor to steady it as you follow the adjustment procedure recommended by the manufacturer. You may have to swing the boat through 360˚ a couple-three times to get the deviation as small as possible. Once you're satisfied, swing it one last time and note the error every 45 or 90 degrees. That's your deviation card. Don't worry too much about deviations for intervals less than 45˚, that's what GPS is for. You only really need a complete deviation card (every 15˚) if you are exclusively using traditional navigation techniques.

One last thing. Try swinging the compass with everything turned off, and then with everything (including the engine) turned on. You might find that you need two deviation cards; one for sailing, and one for motoring.
 

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Discussion Starter · #3 ·
The "factory calibration" was probably worthless the moment the compasses were installed. Even a GRP (fiberglass) boat has enough metal bits and pieces to give it its own magnetic environment. That environment has to be compensated for by adjusting the compass(es) after.

One last thing. Try swinging the compass with everything turned off, and then with everything (including the engine) turned on. You might find that you need two deviation cards; one for sailing, and one for motoring.
Yes that's exactly what I thought. 9000lb keel, 1000lb engine, 600lb generator, autopilot, electronics, etc. Factory cal would be meaningless.

As for the steps I took. I did it in calm open water under power, using a combo of GPS, Range Markers, and landmarks. I will try it with and without everything on. As for the Dev Card? I have an issue with completion, I'll get around to it at some point.
 

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I have a new boat with a pair of Plastimo Offshore 135 compasses. They seemed off by a considerable amount so I researched adjusting them. I followed the instructions where by you follow a known North course and zero the compass, then head south and calibrate it to half of the remaining deviation, splitting the diff so to speak. Same for East/West. They seem much closer now, within 3 to 5 degrees, but I do need to create a deviation card.

My question is this. Was I wrong in trying to calibrate them? When I mentioned this to a sailing instructor I was chastised for touching them, stating that the factory calibration should not be tampered with. :confused: Does it not make sense to calibrate as best you can to each individual installation, then develop a Deviation Card based on the remaining variation? Also I'm to assume that the manufacturer of a production boat is unlikely to take the steps to calibrate, let alone create a Dev Card.
Apparently that particular sailing instructor is either not familiar with the concept of deviation, or does not believe that you are competent to do it.

The factory calibration is meaningless once the compass is removed from the factory. Part of the proper installation of any compass is to compensate for any deviation in the magnetic field which may be caused by proximity to ferrous material, or electro-magnetic fields, in every individual vessel.
 

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I never thought about it, but it's certain that deviation can be different with engines running vs engines off. Food for thought.
 
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