Module 10 - Chartwork
Leeway
What you will learn
Apply leeway to leeward when predicting an EP and apply the inverse windward correction when converting a required water track into a heading.
Leeway is the sideways difference between heading and track through the water. EP predicts the physical slip; CTS works backward to oppose it.
Wind and sea can move a vessel sideways to leeward so the track through the water differs from heading. The amount depends on vessel type, loading, point of sail, speed, wind, sea state and steering. There is no safe universal angle for every yacht or motor vessel; use the stated exercise value or a vessel-specific estimate that can be checked under way.
For an EP, begin with the heading actually steered and apply estimated leeway towards the leeward side to obtain track through the water before adding tidal movement. Wind on the port side produces leeward displacement to starboard; wind on the starboard side produces displacement to port.
For CTS, reverse the reasoning. Once the vector construction gives the required water track, place the heading to windward by the estimated leeway so the resulting slip produces that track. Then apply variation and deviation if a compass course is needed. Monitor COG, SOG and fixes because a changed wind, sea or speed can invalidate the estimate.
Worked example
The CTS vector gives a required water track of 078°T. Wind is on port and estimated leeway is 5° to starboard.
- 1Port-side wind is expected to move the water track to starboard of heading.
- 2To achieve 078°T through the water, place the heading 5° to windward, on the port side.
- 3Required true heading = 078° − 5° = 073°T.
- 4Apply variation and deviation only after that true heading is established.
Sense check: The windward heading correction must oppose, not reinforce, the expected leeward slip.
Stop saying only 'add leeway'
One crew member is building an EP from a steered heading while another is converting a CTS water track to a heading; both propose adding 5°.
1. Name the starting direction
EP begins from heading; CTS begins from required track through the water.
2. Name the physical effect
The vessel's water track slips towards leeward of its heading.
3. Apply the inverse correctly
Predict leeward from heading for EP; steer windward from water track for CTS.
Sense check: If the CTS heading correction points towards the expected slip, it increases the error instead of cancelling it.
Common mistake or limitation
- A memorised add-or-subtract rule fails when wind side or conversion direction changes.
- Leeway changes track through the water; tidal stream then changes movement over the ground.
Recap
- Leeway is physical displacement to leeward.
- EP and CTS use the same effect in opposite calculation directions.
- Use an explicit estimate and revise it from observed progress.
Optional quick check
Section 10 of 13
Wind is on port, required water track is 078°T and estimated leeway is 5°. What true heading should be tried?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
- RYA Yachtmaster Offshore examination scope
- RYA Coastal Skipper/Yachtmaster Offshore Theory Course
- MCA MGN 610 Amendment 1 — navigation and SOLAS Chapter V
- MCA MGN 379 Amendment 1 — use of electronic navigational aids
- IHO S-4 chart specifications, edition 4.10.0
- RIN guidance for safe use of electronic navigation on leisure vessels
- USCG Boat Crew Handbook — Navigation and Piloting
- GPS Standard Positioning Service Performance Standard, fifth edition
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.