Luffing is the fluttering or flapping of a sail along its leading edge, the luff, when the angle at which the wind meets the sail becomes too small for the airflow to stay attached. It happens when the boat is steered too close to the wind, when a sheet is eased too far, or when the wind shifts ahead faster than the trimmer reacts. The word also describes the manoeuvre of turning towards the wind – to luff, or luff up. For a beginner the essential fact is that a luffing sail is producing little or no drive, and the cure is to bear away or sheet in until it fills. The experienced sailor adds two refinements: a sail on the very edge of luffing is usually the fastest sail on the boat upwind, and a sail allowed to flog for long is quietly being destroyed.
What is happening in the air
A sail drives because air bends around the leeward side and speeds up, dropping the pressure there, and that only works when the wind meets the luff at a positive angle of attack. Reduce that angle by heading up or easing the sheet and the pressure difference near the luff falls to nothing; the windward telltales stream upwards as the flow on that side stalls. Reduce it further and the wind gets around the leading edge to the wrong side, pressing the cloth behind the luff to windward. That backwinding bubble is the visible luff, and if the sail is eased further still the whole thing flogs.
Luffing as a trimming tool
Upwind, the aim is to sail with the luff on the point of lifting. A helmsman steering to the genoa keeps the windward telltales streaming with an occasional flick upwards, which means the sail is at the highest angle it will hold, and the trimmer works the sheet to keep the telltales flying at all heights. If the whole luff breaks and the boat slows, the helm has gone too far and must bear away to re-attach the flow before heading up again.
Luffing is also a way to shed power. In a gust, easing the mainsheet until the front third of the main luffs while the leech still draws is faster and safer than fighting the helm. Cruisers luff deliberately for other reasons: heading up until the sails flap takes the load off for a reef, slows the boat for a mooring pick-up, and is the first step in heaving to.
Why sailmakers dislike flogging
A sail that flutters gently along the luff for a moment is not harmed. A sail that flogs – snapping back and forth with the sheet flying or the boat head to wind in a breeze – is taking a beating. Each snap shock-loads the fibres, breaks down the resin finish on woven polyester and works at the bond between film and scrim in a laminate; batten pockets, leech tapes and clew stitching suffer most. Ten minutes of hard flogging can take more life out of a sail than a season of ordinary use. UK Sailmakers’ advice is simple: a sail that is not driving should be sheeted in or lowered, and sail changes and reefs should be planned so the flogging lasts seconds rather than minutes.
Curing an unwanted luff
- Bear away: if the whole leading edge lifts and the boat feels dead, the course is too high. Turn a few degrees off the wind until the sail fills, then work back up slowly.
- Sheet in: if the course is right but the sail lifts, the sheet is too far out. Trim until the telltales stream, watching that the leech does not close too far.
- Check the twist: if only the top luffs, the sail is over-twisted. Move the jib lead forward or tighten the vang and mainsheet to close the upper leech.
- Look at the wind: a sail that luffs with nobody touching anything means the wind has shifted ahead. Follow it or tack.
Frequently Asked Questions about Luffing
Is a luffing sail always a bad thing?
No. Sailing with the luff just on the point of lifting is the fastest upwind trim, and a deliberate luff is a useful way to depower in a gust or slow the boat. Prolonged flogging, however, damages the sail.
What is the difference between luffing and backwinding?
Luffing is the flutter when a sail’s own angle of attack drops too far. Backwinding is the bubble in a mainsail when the headsail’s exhaust presses on its leeward side, cured by trimming the headsail or moving the lead, not by bearing away.
Can I luff another boat in a race whenever I want?
Only within the rules. A leeward right-of-way boat may luff a windward boat but must give her room to keep clear, and she may not sail above her proper course if the overlap was established from astern. Luffing matches also slow both boats while the fleet sails past, so pick the fights that matter.
Luffing is the sail’s way of talking to you. A flicker at the luff says you are on the edge of the wind and sailing well; a full flog says something needs to be trimmed, steered or lowered. Learn to read the difference and act on it quickly, and both your speed and your sails will last longer.