Leeway is the sideways drift of a sailing boat away from the wind, so that she moves through the water at a small angle to the direction she is pointing. It is greatest when sailing upwind, and is usually a few degrees for a yacht.
Most of the force produced by the sails acts sideways. The keel, or a dinghy’s centreboard, resists it, but no keel can stop the boat sliding sideways completely. She moves slightly crabwise, pointing in one direction and travelling in another, a few degrees to leeward of where the bow is pointing. That sideways slip is leeway.
You can see it in the wake, which trails off slightly to windward of the line of the boat instead of straight astern. The keel needs a little leeway to work: meeting the water at a small angle is what lets it produce the sideways lift that balances the sails.
Leeway is greatest when sailing close-hauled, because that is when the side force is largest, and it is small on a run. It increases when:
- The boat is going slowly, for example just after a tack, because the keel works better at speed.
- You are pinching, steering too close to the wind, which slows the boat and adds side force.
- The boat is heeling too much, which makes the keel less effective.
- The waves are big, or the boat has a shallow keel, or a dinghy’s centreboard is partly raised.
To reduce it, keep the boat moving at full speed, sail her fairly upright by easing sails or reefing when she heels too far, and bear away a little rather than pinching.
Leeway matters for navigation. When you work out a course to steer, or plot your position from your heading and speed, allow for leeway as well as any tidal stream, especially upwind. Otherwise the boat will end up downwind of where you expect, which can be serious near a lee shore or a hazard.
More in the book: Chapter 3, How sailing works