Course to Steer Calculator
Calculate the compass heading to steer to counteract tidal current and leeway.
Find the actual course made good and estimated speed over ground.
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 water-track heading — the direction you point the bow — that results in the desired ground track.
The vector triangle method
Navigation teaches the tidal triangle, with three components:
- Tidal vector — direction and speed of the tidal current over 1 hour
- Water track vector — direction the boat is steered, length = boat speed × 1 hour
- 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) CTS offset = arcsin(Cross-track tide ÷ Boat speed through water) Course to Steer = Required track − CTS offset
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
Wind pressure on the sails and hull pushes the boat to leeward (downwind). Leeway angle (typically 3°–10° depending on boat type and wind strength) must be added to the CTS by steering further into the wind. Total correction = CTS offset + Leeway.
Important note
This calculation gives the magnetic or true course to steer through the water. Add or subtract compass deviation and magnetic variation to get the compass course to steer on your instrument. Always cross-check your position against GPS and chart landmarks — CTS is an estimate, not a guarantee.