Leeway
Leeway is the sideways drift of a vessel away from its intended course, to leeward (downwind), caused by the force of the wind on the hull and superstructure (and, more broadly, the combined effect of wind and current pushing the ship off track). It is the difference between the direction the ship is heading and the direction it is actually making good through the water.
Navigators must allow for leeway when steering a course, applying a correction so the ship actually travels along the intended track rather than being set to leeward of it — an important part of passage planning and pilotage, especially for high-sided ships in strong winds or sailing vessels going upwind. Underestimating leeway (and the related current-induced set and drift) can put a ship dangerously off track toward hazards. Leeway is a fundamental concept in navigation and ship handling — accounting for the wind's sideways push is essential to keeping a vessel on its intended course. It is part of how a course to steer is worked out. (Figuratively, “leeway” also means margin or room to manoeuvre.)
The wind doesn't just slow a ship — it pushes it sideways off course, and that is leeway. A navigator who fails to allow for it ends up to leeward of the intended track, potentially toward danger. Accounting for leeway (and current) is fundamental to steering a course that actually reaches the destination.
What causes leeway?
The wind pushing sideways on the hull and superstructure (with current adding to the effect), setting the vessel to leeward of its heading.
How do navigators handle leeway?
By applying a leeway correction to the course steered so the ship actually makes good the intended track.