The common failure point
Candidates often confuse course to steer with course made good. In an oral, if asked to work a tidal or leeway problem and the candidate states only the direction of their intended track, they have answered the wrong question. The examiner wants the heading the helmsman steers — the one that accounts for all vectors acting on the vessel.
What course to steer actually means
Course to steer (CTS) is the compass heading ordered to the helmsman so that, after all external forces are applied, the vessel tracks along the desired ground track. The vectors you must resolve are:
- Leeway — windage pushing the vessel bodily to leeward. Applied into the wind; add the leeway angle to the course made good on the windward side.
- Tidal stream / current — a set and drift that displaces the vessel. Resolved graphically (vector triangle) or by calculation.
- Variation and deviation — the CTS is ultimately expressed as a compass course. Work the sequence: True → apply Variation → Magnetic → apply Deviation → Compass. Use the mnemonic TVMDC (adding West, subtracting East for true-to-compass).
A complete, exam-standard answer states: "I would construct the tidal vector triangle over the planned ground track, find the water track, then correct for leeway and convert to compass using the vessel's deviation card."
ETA calculation
ETA is derived from speed over ground (SOG), not speed through water, because the tidal stream affects your rate of progress along the ground track.
- Resolve the vector triangle to obtain SOG along the ground track.
- Divide the distance along the planned track by SOG to get passage time.
- Add passage time to departure time (use UTC or state the zone).
Common exam trap: a candidate uses engine speed (STW) over the total distance and ignores a foul tide — the ETA can be significantly wrong. Examiners probe this by introducing a tidal stream that changes mid-passage; you must calculate each tidal-hour leg separately and sum the times.
Practical exam pointers
- Always state whether your answer is in True, Magnetic or Compass — an unlabelled heading is incomplete.
- If deviation is unknown at the required heading, state you would consult the deviation card and interpolate.
- ETA should account for planned waypoint alterations; state you would review and update ETA at each waypoint or after each tidal-hour calculation.
- Confirm all inputs — chart datum, tidal atlas or tidal stream diamond — are for the correct date and location.