Module 10 - Chartwork
Leeway
What you will learn
Distinguish the leeward displacement used in an EP from the windward heading correction used for a course to steer.
Leeway is the angle between the vessel's heading and its track through the water caused mainly by wind and sea. The displacement and the steering correction point in opposite directions.
Wind and sea can make a vessel move sideways to leeward, so its track through the water differs from its heading. The amount depends on vessel type, loading, point of sail, speed, wind, sea state and steering. There is no universal angle suitable for every yacht or motor vessel; use the exercise value, vessel information or an estimate checked against observations.
For an EP, begin with the heading actually steered and apply the estimated leeway towards the leeward side to obtain the track through the water before adding tidal movement. With wind on the port side, the leeward displacement is towards starboard. With wind on the starboard side, it is towards port.
For a CTS, the direction reverses because the problem is solved backward. Once the required water track is known, steer to windward by the estimated leeway so the vessel's leeward slip produces that track. Continue comparing observed course over ground and position checks with the plan; changing wind or sea may require a revised estimate.
Worked example
The required water track is 078°T, wind is on the port side, and estimated leeway is 5° to starboard.
- 1Port-side wind produces a leeward track displacement to starboard.
- 2To make good 078°T through the water, steer 5° to windward, on the port side of that track.
- 3Required true heading = 078° - 5° = 073°T.
- 4Only then apply variation and deviation if a compass course is required.
Sense check: A windward heading correction must oppose the direction in which the vessel is expected to slip.
Say which direction you are converting
Two crew members both say 'add five degrees of leeway', but one is building an EP and the other is preparing a CTS.
1. Name the starting direction
For the EP it is the heading steered; for CTS it is the required water track.
2. Name the physical slip
Leeway moves the vessel's track through the water towards the leeward side.
3. Apply the correct inverse
Predict leeward from heading for EP; steer windward from water track for CTS.
Sense check: If the heading correction points towards the expected slip, it reinforces the error rather than cancelling it.
Common mistake or limitation
- A memorised instruction to add or subtract leeway fails when the wind side or conversion direction changes.
- Leeway affects track through the water; tidal stream then changes the track over the ground.
Recap
- Leeway is sideways movement to leeward relative to the heading.
- EP and CTS apply the same physical effect in opposite calculation directions.
- Use a stated estimate and revise it when observations disagree.
Optional quick check
Section 7 of 7
Wind is on the port side and the required water track is 078°T. With 5° estimated leeway, what true heading should be tried before compass correction?
Continue studying Chartwork
This public lesson is part of Revision Module 10. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.