10

Dead Reckoning

Dead reckoning (DR) system of navigation is something every sailor should use.  It is a process of "deducing" your future position taking into account your present position, your speed, and your direction.  The name sounds pretty strange (if not scary!), but it is derived from the old times when the sailors couldn't use charts, because they were way too expensive, and they made educated guesses as to their future position, based on the known passages that they made before.  This was called "deduced reckoning", then it was abbreviated to "ded reckoning", and now it's just called "dead reckoning".  So don't be scared - this is something that might once actually save your life!

The beautiful thing about dead reckoning is that you don't really need to use any electronics (aside from a good compass) to get from one place to another.   Nevertheless, you can still use your GPS.  This will give you an opportunity to assess the drift and the current, as well as any mistakes that the electronics are giving you.

How to plot a DR?

1. Begin at a point when you're passing an object with a known position, such as a buoy.   Mark this point on a chart.
2. Measure the speed of the boat.
3. Extend the DR plot for one to two hours based on the course you are steering, and the speed of the boat.

4. Your next mark for the position you figured you should be at in an hour should be a dot with a half circle around it.  Put down the hour as well. (You might want to write DR beside the hour to keep things simpler for yourself and others.)
5. After you've traveled the distance, check your true position.

When you travel in large waters, the drift and the current will probably make your predicted data not too accurate.  You might want to make a vector plot at this point.  

1. After traveling for an hour (or whatever other amount of time you choose), check your position.  Mark it on the chart as a fix, making a dot and a full circle around it, and connect it to your predicted position.  Check the angle and the distance between the two.
2. Now you can plot an estimated position (EP) using this new information about the current and the drift.
3. Check the difference in direction between your DR course and your true course from the last hour.
4. Locate the point on the chart where you want to go, and find the direction in which you have to head in order to get there.
5. Add the difference caused by the drift and the current to your needed direction.
6. Mark your EP on the chart, making a dot and a square around it, together with an hour.

All this is best studied on water.  If at all possible, take one weekend or so only for practicing DR.  And if you're planning to go on a longer trip on an ocean, definitely learn this method before you go.  You must be absolutely sure, that you know how to do this, because in case of even a slight mistake, it may mean that you lost a lot of hours going in the wrong direction.  This might be very dangerous on sea.   So take your time!  It's worth it!

Lesson 11: "Sea Sickness"

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