An estimated position (EP) is your best position short of a fix. To turn a DR into an EP it is necessary to make allowances for tidal stream, currents, leeway, and the pricking of your thumbs, as shown in Figure 11.5.
Working out the expected rate and direction of the tidal stream should be part of your normal passage planning and it is a good idea to note this down rather than store it electronically. The actual tidal stream may be slightly different from that predicted, but an estimate based on good data is better than a wild guess.
If you are making an ocean passage, work out the rate and direction of any currents you are experiencing. These can differ from the figures in the pilot chart. We tend to assume the North Atlantic Drift is a smooth conveyor belt carrying you across the Atlantic from west to east. Not only can its rate vary but it is full of eddies and whirls that turn it back on itself.
Finding the actual current is easy when the GPS is working. Once a day, use the GPS to compare the course and distance made good over the last 24 hours with the course steered and distance sailed through the water. The difference is a mixture of leeway and current. Done as part of your noon position each day, it builds up a picture of the direction and rate of the current.
Otherwise it is back to rule of thumb. Currents are children of the wind. It is usual to assume that the rate of the current is 2% of the average wind speed over the last 24 hours. The Coriolis effect means that currents are deflected to the right, occasionally by as much as 30°, in the northern hemisphere and to the left in the southern hemisphere.
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