The sideways drift of a boat through the water caused by the wind, the angle between where the bow points and where the boat goes.
Leeway is the boat's sideways slip through the water, pushed by the wind. The sails don't only drive the boat forward; part of their force pushes it to leeward, and the keel can't resist all of it. Leeway is measured as the angle between the heading and the course the boat actually makes through the water.
When it's large
- Close-hauled, when the sideways force is greatest. On a reach or a run there's hardly any.
- In strong wind and big waves, especially with too much sail up or a lot of heel.
- At low speed, because the keel works better the faster water flows past it.
- On boats with shallow keels, centreboards raised or lots of windage.
Upwind it's typically a few degrees; in heavy weather it can be considerably more.
Correcting for it
To make good a course, steer that many degrees to windward of it. With the wind from port and 5° of leeway, a desired course of 090° means steering 085°. Leeway affects the course through the water; the current then adds its own offset on the way to the course over ground. In dead reckoning, forgetting leeway is one of the most common sources of error.