Shape, in the sailmaker’s and trimmer’s vocabulary, is the three-dimensional aerofoil form a sail takes when it is filled with wind: how deep it is, where that depth sits, how much it twists from foot to head, and how the entry and exit curves are drawn. A beginner should understand that a sail is not a flat sheet but a curved wing, and that a wing’s curve decides how much lift and drag it produces. The experienced sailor knows that shape is designed into the cloth by the sailmaker through luff curve and broadseaming, then continuously adjusted on the water by the trimmer, and that it is the first thing an ageing sail loses.
The Elements of Sail Shape
- Depth (camber or draft): the maximum distance from the chord line to the sail, expressed as a percentage of chord length. A mainsail might carry 10 to 14 per cent, a light-air genoa 15 to 18 per cent.
- Draft position: where along the chord the maximum depth sits. Around 45 to 50 per cent back on a mainsail, 30 to 40 per cent on a headsail.
- Twist: the change in the angle of the chord from foot to head. Some twist is necessary because apparent wind is freer aloft; too much dumps power.
- Entry angle: how round or fine the luff is. A fine entry points high but stalls easily; a round entry is forgiving in waves.
- Exit: the shape of the last quarter of the sail. It should be straight or slightly open, never hooked to windward.
How the Sailmaker Builds Shape
Cloth comes off the roll flat, so shape has to be manufactured. Two tools do the work. Luff curve is extra cloth cut into the luff of a mainsail; when the mast is straight that extra cloth becomes depth in the front of the sail, and when the mast is bent to match the curve the sail flattens. Broadseaming is the tapering of panel edges so that, when the seams are joined, the cloth is forced into a curve. The designer specifies the moulded shape in software, and the panel layout and seam curves are computed from it.
Cloth behaviour is the third ingredient. Woven Dacron stretches on the bias and slowly along the threadlines, so the designed shape only holds within a wind range. That is why UK Sailmakers developed Tape-Drive®, and later Titanium™ and X-Drive®: by carrying the load in high-modulus tapes or yarns laid along the computed load lines, the designed shape stays where it was put across a far wider range of wind speeds.
How the Trimmer Changes Shape
Every control on the boat is a shape control. Halyard and cunningham tension pull the draft forward and flatten the entry; ease them and the draft moves aft. The outhaul controls depth in the lower third of the mainsail; backstay and mast bend flatten the middle and upper sections. Mainsheet and vang set leech tension and therefore twist; the traveller changes the angle of attack without altering twist. On a headsail the sheet lead does most of the work: forward for a tighter leech and rounder foot, aft for more twist and a flatter foot.
The rule of thumb is simple. Fuller shapes make power for light air, chop and acceleration out of tacks. Flatter shapes make speed and pointing ability in breeze and flat water, and reduce heel and weather helm when the boat is overpowered. See also draft, twist and camber.
A racing sailor thinks about shape all day and expects the sailmaker to deliver a specific designed shape with a known range. A cruising sailor wants a shape that is forgiving and holds for many seasons, and would rather have a sail that is slightly full and durable than one that is knife-edged and short-lived.
When Shape Goes Wrong
Cloth stretches with use, and it stretches most where the load is highest, along the leech. The result is that the draft creeps aft, the leech hooks to windward, the entry goes flat and the boat heels more while pointing less. If your mainsail needs full backstay and cunningham just to look acceptable in twelve knots, the shape has moved. A loft can often recut the sail, taking cloth out of the leech and moving the draft forward, but there is a limit beyond which new cloth is the only cure.
Frequently Asked Questions about Shape
What is the ideal sail shape?
There is no single answer; it depends on wind, sea state, boat and point of sail. The designer aims for a shape that is efficient across the sail’s intended range, and the trimmer moves it within that range.
How can I see the shape of my sail?
Sight up the sail from beneath the boom or forward of the mast and read the draft stripes: their curve shows depth, the deepest point shows draft position, and the change in angle from stripe to stripe shows twist.
Shape is the whole point of a sail. The cloth, the seams, the tapes and the controls exist to create and hold an aerofoil that drives the boat efficiently, and a sailor who learns to see and adjust shape sails faster, flatter and more comfortably in everything from a drifter to a gale.