We recently did a trip from Boothbay to Marblehead where we encountered a really weird situation with our Raymarine E80 chartplotters. It’s been suggested that it could have been due to the Seguin Island magnetic anomaly. I wonder if any of you have had a similar experience or have any suggestions.

After leaving Boothbay, near the Cuckolds, I set a WPT for Mile Ledge off Seguin. All OK. At Mile Ledge, I set a WPT for Cape Ann – when we immediately noticed something was not right.

What should have been a bearing of 226 to Cape Ann showed a bearing of 096! Moving the cursor on the screen also produced incorrect bearings-to-cursor. The COG reading was wrong. However most other readings were correct: GPS positioning of the boat and the cursor; autopilot headings; distances to WPT and to cursor. Even the graphic vector line showing COG was correct, despite the incorrect readings.

We shut down all the electronics and rebooted. Same thing. Here’s the other weird thing: usually, the Ray chartplotter shows a boat icon that generally points in the direction you’re headed. But I had noticed earlier, before the Mile Ledge mark, that the icon was pointed in a different direction.

This series of problems persisted throughout the 15 hour trip home. We rebooted several times to no effect. Because the weather was clear and very calm, we continued. After a bit, I realized that the system’s bearing readings were off consistently by 130 degrees. At some point later in the day, I discovered they were off by 120 degrees. And at the end of the day, I found they were off by 130 degrees again.

To sum up: GPS OK. Fluxgate/autopilot OK. Electronic distance calculations OK. Bearing/heading readings way wicked messed up.

And now for some added fun: Went out for a day sail today, fired everything up, and all is hunky dory!

The Seguin anomaly is shown on the west side of the island; we were on the east side. I found something (on the chart notes maybe) that said it can account for an 8 degree magnetic discrepancy. But this? And affecting the system for the whole day? Ever hear of something like this?