Course to steer in a tidal stream

How it works

A cross-stream pushes the boat sideways. If you point the bow straight at the waypoint, the ground track curves and the distance grows. The course to steer (CTS) is the heading that, once the stream is added, makes the boat follow the line A-B exactly. You find it with the tidal triangle, a drawing in four moves on the chart. What set and rate mean is in the manual: here the method matters.

First move: draw the wanted track from A to B (ground track) on the chart and read it with the protractor. In the example it is 186°T, distance 6 miles, and the boat makes 9 knots through the water. Second move: from the tidal diamond or the atlas take the set and rate for the right hour and draw, starting at A, a segment as long as the rate in miles (the stream for one hour) in the direction of the set. In the example set 033°T, rate 2.3 knots: a 2.3-mile segment towards 033°.

Third move: open the dividers to the boat's speed through the water, that is the distance of one hour's sailing (9 miles). Put the point on the end of the stream segment and swing an arc until it cuts the line A-B. Fourth move: join the end of the stream segment to the point where the arc cuts the track. The direction of that line, read with the protractor, is the CTS: 193°T. Never join the end of the stream vector to B by eye: you need the arc.

The distance from A to the point where the arc cuts the track, measured on the latitude scale, is the distance you make over the ground in one hour: the SOG (speed over ground), here 6.9 knots. It is lower than the boat's 9 because the stream is partly against you. Time is always worked with the SOG: 6 miles at 6.9 knots is 6 ÷ 6.9 × 60 = 52 minutes, not the 40 that 9 knots would give. Leaving at 10:00 you arrive at 10:52.

The drawing gives a true course. The helmsman needs a compass course, so convert: first variation, then deviation for that heading. From true to magnetic, westerly (W) variation is added: 193 + 7 = 200°M. From magnetic to compass, easterly (E) deviation is subtracted: with 3°E, 200 − 3 = 197°C. The stream changes every hour, though: if the leg lasts longer, or the stream varies, use the mean set and rate or split the passage into one-hour legs, with a CTS for each.