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024 - MagVar

We were traveling down one of the straighter sections of canal when a faster sailboat roared by us at 7kts versus our 6kts.  Even under power it’s hard to lose to another sailboat.  But he wasn’t keeping a tight helm and I could imagine the pilot yacking away and periodically looking up and making course adjustments, so I felt better about losing out.  When he did a strong S-curve up ahead underneath a bridge, I assumed he swerved to avoid a log or something in the water.  Ha, no such thing, now I think he was using his auto pilot and it was simply searching for the magnetic heading.  Because as we traveled under the same bridge our autopilot did the same thing to our boat!  The iron in the bridge added about 20deg variance to the heading and then swung back minus 20 as it oscillated back to the original heading.  This became entertainment for me measuring the MagVar as we transited different bridges.  The old railroad bridges had the most magvar, the new road bridges the least.  Likely the result of modern highway using steel versus iron.  Checkout the video below on a different bridge.


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