Compass deviation, who’s responsible?
The compass adjustment trip that I missed the other day was aboard a nearly brand new, high quality 30’ power boat. But the steering compass, a decent Ritchie SS1000, turned out to be 45° off on some headings and “limp” if you were pointed north! Jeff Kaufmann tells me that this is just an extreme example of a common situation. Boatbuilders may install the compass but don’t pay attention to the deviation, leaving that for whoever installs all the electronics, who may not pay attention either. Jeff says his fee is sometimes paid by a yacht broker trying to complete a deal, because no one else will take responsibility.
At any rate, Jeff took this boat out into Penobscot Bay and used a GPS bearing to a fixed and charted object to get one solid bearing. With that reference he could then use his directional gyro (lower left in the Bloomquist photo above, home built from old military parts) to check the compass on multiple headings. Then he used that little magnetometer in his hand to look for the culprits. The main problem turned out to be a large steel hydraulic steering control mounted almost directly under the compass (the yacht in the picture is not the same boat, by the way).
Using well developed intuition plus trial and error, he figured out where to position an external magnet to negate the offending one. That took a big bite out the worst deviations, plus made the compass lively again on North headings. Then he used the SS1000’s small built-in correction magnets to get the deviation nearly to zero on all headings. Finally, he checked for possible transient problems and found one in the windshield wiper motor, which he couldn’t fix, of course, but did note on the deviation card. Jeff has come across a lot of odd compass problems over the years; I think at this point he can look around a boat and visualize 3D magnetic fields emanating from wiper motors, power cables, buzzers, hunks of steel, CRT tubes, etc. etc.
Ritchie, incidentally, has a good guide to compass basics that they will send you for free or let you download as a .pdf.
