Tidal stream triangle: course to steer
Point straight at your destination across a stream and you end up somewhere else. The tidal stream triangle tells you how much to allow for it. Set the course, boat speed and stream: the grey boat steers the chart course and is swept off, the blue boat steers the course to steer and stays on the line from A to B.
How to draw the triangle on the chart
From your departure point lay off one hour of stream: in the direction it sets, as long as its rate in nautical miles. From the end of that line swing an arc with your speed through the water as the radius.
Where the arc cuts the line from A to B is the end of your water track. The direction from the end of the stream line to that point is the course to steer, and the distance from departure to the cut is your speed over ground. Take the stream from the tidal stream atlas or the chart, fresh for every hour.
What matters in practice
The stream changes with the tide every hour, so on longer passages work hour by hour or average over the passage time. Wind pushes the boat to leeward as well, and that leeway comes on top. The plotter shows course and speed over ground directly: if the ground track sits on the line, the allowance is right.
