Luffing means the front edge of a sail, the luff, is fluttering or flapping because the sail is let out too far or the boat is pointing too close to the wind. To luff up, or luff, also means to turn the boat towards the wind.
The word has two related meanings.
A sail that is luffing is flapping or rippling along its front edge, the luff. It happens when the wind meets the sail too nearly edge-on, so the air pushes on the back of the sail near the luff and it lifts and flutters. There are two causes:
- The sail is let out too far for the course you are sailing. Pull the sheet in until the luff fills and the windward telltale streams.
- The boat is pointing too close to the wind. If the sail is already pulled right in, close-hauled, bear away a little until it fills.
A slight lift at the luff, just on the edge of flapping, is often where a sail is at its most efficient, which is why trimmers ease a sheet until the luff starts to curl and then pull it back in a little. A sail flapping hard, though, produces almost no drive, and the flogging wears the cloth and the sheets.
Luffing can also be used on purpose. Easing the mainsheet until the front of the mainsail luffs is a quick way to spill wind in a gust and reduce heel. Letting both sails luff completely, by heading into the wind, is how a boat slows and stops, for example when picking up a mooring.
To luff up, or to luff, means to steer towards the wind, the same as heading up. It gets its name because, if you keep turning without pulling the sheets in, the sails eventually start to luff. In racing, luffing also describes a leeward boat heading up towards a boat to windward of her, which must keep clear. The Racing Rules of Sailing limit how and when she may do it (rules 16 and 17).
More in the book: Chapter 5, Trimming the sails