Luff carries three meanings, all in daily use. 1. The luff is the forward edge of a sail, running from head to tack and attached to the mast, the headstay or a furling foil. 2. To luff, or a sail luffing, describes the sail fluttering along its leading edge because the boat is heading too close to the wind or the sail is not trimmed in enough. 3. To luff, or luff up, means to alter course towards the wind. Beginners meet the second and third meanings first, when the helm calls “we’re luffing” and turns the boat away. The experienced sailor and the sailmaker spend more time on the first, because the shape and tension of the luff decide where the draft sits and how the whole sail drives.
The luff as a sail edge
The luff is the edge that meets the wind first, and everything downstream depends on how it is built. On a mainsail it is attached to the mast with a bolt rope in a groove or with slides or cars on a track, and the sailmaker cuts luff curve into it – extra cloth along the edge that pushes depth into the sail when the mast is straight and is pulled flat when the mast bends. Matching luff curve to mast bend is the heart of mainsail design. A headsail luff is held by hanks, by a luff tape in a furler foil or, on a free-flying Code Zero, by a torsional cable inside the luff, and its shape is designed around the headstay sag the sailmaker expects.
The entry, the first foot or so behind the luff, decides how forgiving the sail is. A fine entry points high but stalls easily and demands concentration; a rounder entry gives a wider steering groove and more power in a chop at the cost of a degree or two of pointing. UK Sailmakers designers adjust the entry for the boat and its likely helmsman, which is worth discussing when ordering a new sail.
Luff tension and draft position
Halyard tension, and on racing sails the cunningham, set the tension along the luff, and luff tension sets where the deepest part of the sail sits. Pull the luff tighter and the draft moves forward, the entry rounds and the leech opens slightly; ease it and the draft slides aft, the entry flattens and the leech tightens. The sail tells you what it wants: horizontal wrinkles running back from the luff mean too little tension, a hard vertical crease along the luff means too much. As the wind builds the load blows the draft aft, and more luff tension pulls it back where the designer put it. This is the most under-used control on most cruising boats, where the halyard goes to the same mark in every wind and stays there; racers ease it on the run and harden it before the leeward mark.
Luffing as a sail behaviour and a manoeuvre
When the boat is steered too close to the wind, or the sheet is eased too far, the angle of attack at the luff drops until the airflow no longer bends around the leading edge. The cloth just behind the luff lifts, flutters and then backwinds, and the sail is luffing; windward telltales streaming upwards are the early warning. Sailors use this deliberately close-hauled: steering up until the luff just lifts and then bearing away a fraction is how the helmsman finds the highest course the sail will hold.
Luffing up, the third meaning, is the manoeuvre of turning towards the wind. A cruising skipper luffs up to reduce heel in a squall, to slow for a mooring or to start a tack. In racing the luff is a weapon: a leeward boat may luff a windward boat to defend her position, within rules that limit how sharply she may alter course when another boat is close. Both meanings share the same root – a sail that luffs has been headed up into the wind, whether by the helmsman or the breeze.
Frequently Asked Questions about the Luff
How tight should the luff of a mainsail be?
Tight enough to remove horizontal wrinkles along the luff and no tighter. Increase tension as the wind builds to hold the draft forward, and ease it in light air to keep the entry fine and the sail powerful.
What is the difference between luffing and pinching?
Pinching is sailing so close to the wind that the boat slows even though the sails may not visibly flutter; luffing is the visible flapping of the leading edge when the angle of attack has dropped too far. Pinching often leads to luffing.
Why does my genoa luff sag to leeward?
Headstay sag increases with load. Tightening the backstay on a masthead rig, or the runners on a fractional rig, straightens the headstay and flattens the sail’s entry. Some sag is designed in; too much makes the sail full and the boat heel.
Whichever meaning you reach for, the luff is where the wind arrives and where good trim starts. Watch it for the first flutter, tension it to suit the breeze, and understand how its curve was cut to match your rig, and you will get far more out of every sail you own.