Streams, set and drift
Tidal stream information comes from tidal stream atlases and from the tidal diamonds printed on the chart, tabulated for each hour relative to high water at a standard port. 'Set' is the direction the stream flows toward, and 'drift' or 'rate' is its speed in knots, both interpolated for springs and neaps on the day. On top of the stream, 'leeway' is the downwind slip caused by the wind blowing on the boat, larger in a sailing boat heeled and making to windward. Both the stream and leeway must be allowed for to know where the boat really is and where she is really going.
Working an estimated position (EP)
- From the last fix, plot the course steered (as a true bearing) and the distance run through the water.
- Apply leeway by offsetting the water track a few degrees downwind.
- From the end of that track, plot the tidal set and drift for the period.
- The end of the tidal vector is the estimated position; mark it with the EP symbol (a dot in a triangle).
NOTE — Course to steer works it backwards To find the course to steer to make good a desired ground track, plot the tide first from the start, then find the heading through the water that brings you out on the track. It is the EP construction run in reverse.
WARNING — Ignore the stream and you will be set down-tide A boat that steers the straight-line course to a mark, ignoring a cross-setting stream, ends up down-tide of it — sometimes onto the very danger the mark was warning of. Always allow for the set.