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Last summer I was navigating a large boat (Ok, it was the 12 Meter INTREPID) using the Narragansett Bay racing chart. In prep for the weekend’ racing, I had laid out the course and distance between all the various marks using my Navionics app sitting at my kitchen counter. That way, once the around-the-buoys course was announced I’d be ready to go.
Turns out it wasn’t quite that easy. It turns out that Navionics and many of the comparable navigation apps give courses based on True North. Your boat’s compass gives you headings based on Magnetic North. OOPS!
The boat captain filled me in as to why my courses weren’t accurate. I quickly went back to the nav station and looked at the paper chart for Narragansett Bay. If you look at the compass graphic on the chart you’ll see an inner and outer circle. The outer circle is True North (Star icon for north) and the inner is Magnetic (arrow icon for north). The difference, the “Variation” was approximately 14 degrees west so, after some quick math, I updated our course heading and off we went. Yup, we won the regatta.
In a bit of frustration after the race, I reached out to the folks at Navionics asking why True vs. Magnetic. They explained that the Variation is different wherever you go on the earth because the Magnetic North Pole is off center vs. the geographically calculated True North Pole. And, because Variation can vary greatly depending on where you are, they leave it to you to find your local Variation and adjust your Navionics courses manually.
Now, Deviation is not to be confused with Variation of a compass heading. Your cockpit’s magnetic compass can be influenced by metal devices around it. It could be the engine, winches, even a handheld radio. That’s why when you first get your boat, or replace your compass, you “swing” the boat to adjust it for any Deviation.
Understanding True vs. Magnetic headings and the difference between Variation and Deviation are key insights.
