Course to Steer Calculator

What is Course to Steer?

When sailing in tidal waters, a straight line on the chart from A to B (the required track) is not the heading you steer. Tidal current continuously pushes the boat sideways, so you must aim into the tide to end up tracking straight toward your destination. The Course to Steer (CTS) is the heading you point the bow at, the one that results in the desired ground track.

The vector triangle method

Navigation teaches the tidal triangle, with three components:

1. Tidal vector: direction and speed of the tidal current over 1 hour
2. Water track vector: direction the boat is steered, length = boat speed × 1 hour
3. Ground track vector: the actual path over the ground = required track

By constructing these three vectors graphically or mathematically, the CTS emerges as the water track direction needed to produce the desired ground track.

The calculation

This calculator uses the cross-track component approach:

Cross-track tide = Tidal speed × sin(Tidal direction − Required track) Tidal offset = arcsin(Cross-track tide ÷ Boat speed through water) Water track = Required track − Tidal offset Course to Steer = Water track ± Leeway

The along-track component of the tide either helps or hinders progress:

Speed Over Ground ≈ Boat speed × cos(CTS offset) + Tidal speed × cos(Tidal − Track)

Leeway, and why it has a sign

Wind pressure on the sails and hull pushes the boat sideways, downwind, so the boat crabs a few degrees off the heading you are steering. Typical values run 3° to 10°, more in a light boat or a big sea, less under power.

The correction is to steer further into the wind, and that means the sign flips with the tack. Wind on the port side pushes you to starboard, so you steer left and subtract. Wind on the starboard side pushes you to port, so you steer right and add. Get this backwards and you have doubled the error rather than cancelled it, which is why the calculator asks which side the wind is on instead of guessing.

Note that leeway does not change the tidal triangle. The triangle is drawn on the water track, the direction the boat actually moves through the water, and leeway is the last thing added on top to get the heading you hold on the compass.

Important note

This calculation gives a true course. Apply magnetic variation and then compass deviation before you steer it on the instrument. Always cross-check your position against GPS and chart landmarks, because a course to steer is an estimate built on a tidal prediction and a log reading, and neither of those is exact.