Posted by: fullandbye | January 22, 2009

dead reckoning

I am almost done co-teaching my first keelboat skipper class. It really is such a privilege to teach.

There was very little wind yesterday so we decided to do some compass work and then some navigation exercises. First order of the day was to make a compass deviation card.

As everyone is aware, in most places in the world there is a local variation between true north and magnetic north. What many people are not aware of though is that generally speaking magnetic north and the north shown by a magnetic compass are also not the same thing. They are much closer if you are using an orienteering compass in the woods and are not anywhere near ferrous rock, but on a boat, with the compass frequently mounted somewhere near an engine, with a keel that could well be made of iron, and hardware all over the place, the compass deviates from magnetic directions. This difference (called deviation) is describable and generally is expressed in a sin curve described through the 360 degrees of the compass. Most sailing ships (and all airplanes, I am told) have a document on board that is this sin curve drawn through a line representing the spread of the compass.

It is possible to adjust a compass to minimize the deviation, but it is nearly impossible to completely resolve it. Minimizing deviation involves moving two magnets within or very near the compass to compensate for deviation throughout the swing of the compass. On a large binnacle, there are two balls of solid iron mounted to horizontal slides on either side of the compass. These balls are to create a local magnetic field powerful enough to compensate for any other nearby disturbance, but not so powerful that the compass cannot rotate.

No matter, our first order of the day was to motor around using a GPS to give us very precise magnetic compass headings, and as we swung through 30 degree course changes, the deviation was determined by marking a compass heading at the moment that the boat was on a course determined by the GPS. What we found was that the compass deviated from magnetic north by as much as 11 degrees. A pretty big deviation indeed. We made a card so that we could correct for deviation and then proceeded to navigate.

With a compass card made, we broke the students up into two groups. One group stayed below deck with the charts, and the other group would drove the boat on the compass heading at the speed and for the time specified by the group below. The group above would also take fixes when called to do so to give the group below some idea of where they were and if their estimates were good. The first group was to motor from Webster Point to Yarrow Bay, and then to alter course and go to the Magnuson Park boat ramp. At the end of about an hour of motoring, the students below estimated that they had arrived at their location and told the students driving the boat to stop. Poking their heads above deck, they were astonished to find that they were about 200 yards offshore, with the Magnuson park boat ramp quite literally dead ahead.

The groups were switched and the new navigators were told to direct the boat handlers to motor back to Webster Point by way of Hunts Point. After an hour again of motoring, they estimated their position to be right at the mouth of the Montlake cut and came above decks to discover that indeed they were about 100 yards south of their estimated position, but still very near Webster Point.

Although it was not foggy when we did this exercise yesterday, the skills they learned are precisely the skills used when navigating in fog. Although they were allowed to take a fix once or twice throughout their journey, for the most part they were navigating on instruments alone. As it was, their dead reckoning was so thoughtful that every fix placed them more or less exactly where they guessed they were. They knew their heading, they made very good estimates of speed, and they marked time. Knowing direction, speed, and time traveled they were able to chart their course and navigate on instruments alone.


  1. That’s pretty awesome.

  2. It was great to see them do so well. But it is amazing how well this works.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s


%d bloggers like this: