The foretriangle bounds a headsail without defining it. This page separates the rig’s dimensions from the sail’s, sets out what LP and the girths actually describe, and explains why a headsail’s leech is cut the way it is.
The rig’s dimensions
I is the height of the foretriangle, measured up the front of the mast to where the forestay meets it. J is its base, from the front of the mast to the forestay at deck level.
Together they describe the space the headsail works in. They are not the sail.
The sail is smaller than the foretriangle in every direction, and necessarily so. The tack fitting and the furling drum occupy height at the bottom. The head fitting and the top swivel occupy height at the top. There has to be halyard travel left to tension the luff after the sail is up. And the sail has to clear the deck, the pulpit and whatever is on the foredeck.
I and J are where a sail dimension starts. They are never a finished sail dimension.
LP and overlap
LP is the perpendicular distance from the clew to the luff — not the length of the foot, which is longer, but the shortest distance across the sail.
It is the measurement that describes how much sail there is, because it captures the sail’s width independently of how the foot happens to be cut. Expressed against J it gives the percentage that separates a jib from a genoa.
Why the perpendicular rather than the foot: two sails with the same foot length can have quite different areas depending on the clew height and the shape of the foot, whereas LP describes the sail’s actual reach.
Girths
The width of the sail at intervals up its height — recorded as HHB at the head and then at the upper, three-quarter, half and quarter heights — describes the outline in a way the three edges alone cannot.
Two sails can share luff, leech and foot and still be different sails: one with its area low down and a full lower section, one carrying more width high up. The girths distinguish them, and they are what a rating rule measures where one applies.
The leech is cut hollow, and here is why
A mainsail’s leech is cut convex, outside the straight line from head to clew, and battens hold the roach out. A headsail’s leech is usually cut the other way — concave, inside that line.
The reason is support. With no battens, a convex leech has nothing holding it out: it curls, it flutters, and it destroys its own tape and stitching. Cutting the leech hollow removes the cloth that would otherwise be unsupported.
So roach and battens go together. A headsail that carries roach carries battens to hold it out — which is why roach appears on self-tacking and high-clewed jibs and not on a plain furling genoa.
Clew height
Where the clew sits changes several things at once and is worth deciding rather than inheriting.
A low clew puts more area low down where the cloth is doing useful work, and gives a slightly more efficient sail. It also means the foot lies along the deck, visibility to leeward is poor, the sail scoops water on the foredeck, and the sheet lead has to be well forward.
A high clew gives up some area and buys visibility, a foot that clears the guardrails, and better behaviour when the sail is partly rolled — the clew comes in and up rather than dragging along the deck.
The second consideration is what pushed cruising headsails toward higher clews, and it interacts directly with overlap and with the sheet lead the boat actually has.
Area
As with any sail, there is more than one area, and they are different numbers.
The actual area of the finished sail, and the rated area calculated by a rating rule’s own formula from the girths, are different numbers. Any statement about a sail’s area should say which one it means.
The configurator also shows a Pricing Area, which is a simplified figure used consistently across all sails. It is neither of the above and should not be read as either.
Where the flying shape comes from
Everything here describes the outline — the sail seen flat. The three-dimensional shape it takes comes from the luff curve, how the panels are shaped, how the cloth behaves, and critically how much the forestay sags under load. That is How Headsail Shape Works.
Configuring this at SurgeSails
Luff, Leech and Foot are in the main form, with LP, LP% and Pricing Area shown as calculated results. In the Pro view, I and J can be entered as the rig reference they are, and the girths become available for owners who need the outline specified precisely or who hold a rating certificate.
Clew height is a consequence of the dimensions rather than a separate field. If you are deliberately changing it, say so in Design Note, because it moves the sheet lead.