Course to Steer (CTS)

The course to steer is the compass course you need to follow to make good a desired ground track, after allowing for tidal stream, leeway, and compass errors. It involves constructing a vector triangle on the chart.

The vector triangle has three sides: (1) the tidal stream vector — direction and distance the tide moves in one hour; (2) the boat's water track — the direction and distance through the water (using boat speed); (3) the ground track — the desired line from start to destination. You plot the tidal vector from your starting position, then swing an arc of your boat speed from the end of the tidal vector to cross the ground track line. The direction from the end of the tidal vector to where the arc crosses the ground track is your water track — your course to steer (in true, before applying variation and deviation).