Holy Deviation, Capt. Dan

Remember what you learned in ASA 103 about deviation, about how electrical circuits can interfere with your compass?

We were sailing a Hunter 36 last week and encountered traffic approaching from the south in the coastwise traffic lane. Two of us were taking bearings to ensure we were not on a collision course. When we compared our readings, we were about 50 degrees apart.

I was standing in the companionway and my shipmate was standing near the binnacle. The radar – positioned just above the binnacle – was on. Net result – big error in his handheld compass.

My question is – why does the ship’s compass on this boat appear to be accurate? It’s right below the radar.

While none of us is likely to do a full compass swing, getting deviation readings every 5 degrees or so, our experience shows that it’s worth checking your ship’s compass against the GPS and the autopilot compass and, from time-to-time, using a transit range to get an accurate measure of deviation. Turning the radar, radio and any other electronic devices off and checking headings against readings with the gear on would also be very instructive.

If this was sort of interesting, check out geomagnetic interference

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Holy Deviation, Capt. Dan

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