A short page on the one thing that makes a headsail different from every other sail, and why the sail’s shape parameters are not offered as choices.
The forestay moves
A mainsail’s luff is attached to a mast. A headsail’s is attached to a wire under tension, and a wire loaded sideways sags to leeward — more in more wind, and differently depending on backstay tension and rig tune.
The sail gets fuller as the forestay sags, because the sag shortens the straight-line distance from head to tack and the slack that appears becomes depth in the front of the sail.
So a headsail is cut against an assumed range of sag rather than against a fixed geometry. That is why two identical sails behave differently on two boats tuned differently, and why we ask about the rig rather than only about the sail.
What you would notice if it were wrong. A sail that goes full and stops pointing as soon as the breeze builds, or a flat, lifeless sail when there is no load in the rig.
The sheet lead belongs to the sail
Where the sheet meets the deck decides how the sail’s load is shared between its leech and its foot. Lead further aft and the foot flattens while the top opens; lead further forward and the top closes and the sail powers up.
The consequence worth knowing: the correct lead position depends on the sail’s own geometry — its LP and its clew height — not only on the boat. A new sail with a different overlap or a different clew height wants a different lead, and a boat whose car is already at one end of its track may not have it.
This is why the lead is worth photographing before a headsail changes size, and it is also why a partly rolled genoa is awkward to trim: the clew has moved and the car has not.
What you see, and what we own
You see the outline and its girths, LP and clew height, the cloth, the panel layout, the luff system, the corner fittings and the protection.
We own the assumed sag, the luff curve cut against it, and how depth is distributed up the sail.
As with any sail, the reason is that these only mean anything together and against a particular rig.
Configuring this at SurgeSails
Nothing on this page is a field.
The configurator asks for the boat and rig, the dimensions, the luff system and the visible choices. If you are changing the overlap or the clew height, say so in Design Note — that is the case where the sheet lead needs checking before the sail is built.