Dead Reckoning

See Bowditch, chapter 7, for information on dead reckoning.

From reading Leg 57 we know the following:

Compass variation and deviation are normally used to convert magnetic course to true course, but the text of leg 57 gives us true course directly. We can ignore the compass errors.

The first step is to compute the elapsed time. Subtract the previous date and time from the new date and time.

1999-03-24 23:40:01 1999-03-24 00:10:23 -------------------- 00 23:29:38 Convert to decimal hours. This is almost exactly 23.5 hours.

To compute a dead-reckoning position, it is a simple matter of vector addition. On a plotting sheet -- ideally the one you used to plot your last fix -- make a mark at the last known location of the boat.

From this mark, draw a line on vector 119.3. Multiply 10.3 knots by the time spent cruising -- 23.5 hours. On my slide rule, this comes to 242 nm.

Next, from the end of this vector, draw a second vector to represent the current.

Finally, determine the coordinates of the end of the final vector: lat = 53°38.4S lon = 74°44.6W

Note that as you sail near the poles, errors can accumulate rapidly. Bowditch addresses these points in some detail.

Next: Prepare a Plotting sheet