Roach is the curved area of a sail that lies outside the straight line drawn from the head to the clew. On a mainsail it is the convex bulge of the leech, held out by battens, that gives the sail its elliptical outline; on a headsail with an unsupported leech the roach is usually negative, a slight hollow that keeps the leech from fluttering. New sailors see roach as free extra area. Sailmakers see it as area in the most valuable place on the sail – high up, where the wind is stronger and cleaner – bought at the price of batten support, leech load and a fight with the backstay on every tack.
How Roach Is Measured and Supported
Roach is measured as the distance from the straight head-to-clew line out to the leech at a given height, and rating rules capture it through girth measurements taken at fixed fractions of the leech. A full-batten or square-top mainsail can carry very large girths; a conventional sail with short leech battens carries less. Without battens the cloth beyond the straight line has nothing to hold it out and will curl and flog, so the amount of roach a sail can carry is fixed by how far its battens reach and how stiff they are.
The extreme case is the square-top or fathead mainsail, where the roach is so large that the head becomes a broad, near-horizontal edge supported by a diagonal batten. That planform is common on multihulls and sport boats and is spreading to cruising catamarans because it adds area without adding mast height.
Why Roach Matters for Shape and Speed
The upper part of a sail works in stronger, less disturbed wind than the foot, so area added up high produces more drive per square metre than area added anywhere else. An elliptical planform also spreads the lift more evenly along the span and reduces induced drag from the tip vortex at the head, which is why designers of everything from gliders to America’s Cup wings favour it. A sail with generous roach can therefore be designed with a shorter boom for the same power, which keeps the centre of effort lower and the boat more upright.
The cost is control. Roach loads the leech, so the battens, the leech tape and the head and clew patches all work harder. Because the leech is longer and more curved, the sail twists more readily and the trimmer needs the mainsheet, vang and traveller to manage that twist. On a masthead or fractional rig with a standing backstay the roach must clear the backstay on each tack and gybe, which in light air means the battens can hang up and need a tug on the leech or a pump of the mainsheet to flick them across.
Roach on Headsails
A hanked or furling genoa has no battens along most of its leech, so the sailmaker gives it a slight hollow – negative roach – that keeps the leech tight and quiet. Vertical battens on furling headsails, and short leech battens on hanked jibs, allow a straight or modestly positive leech and a little more area. The same is true of in-mast furling mainsails: with no room for horizontal battens, they are cut with a hollow leech and lose the roach a slab-reefing main would carry. Vertical battens, which UK Sailmakers lofts fit to many furling mains, restore some of that area and a straighter leech.
Cruising Versus Racing
Racers push roach to the limit their class or rating rule allows, because the area is powerful and the rating penalty is often modest. Full-length battens, stiff leech battens and careful leech-tension trim are the price. Cruisers generally accept moderate roach for simpler handling – the sail comes down more easily into a stack-pack, the battens clear the backstay without drama, and there is less to go wrong. A cruiser choosing in-mast furling should understand that the hollow leech gives away real area and drive compared with a conventional main.
Frequently Asked Questions about Roach
Does more roach always make a boat faster?
More roach adds power up high, which is fast in light to moderate air and on boats with enough stability. In strong wind that same area heels the boat sooner, so the answer depends on the boat and the conditions she usually sails in.
Why does my mainsail catch on the backstay when I tack?
The battens holding out the roach have to pass under or around the backstay. In light air there is not enough pressure to flick them across, so a tug on the leech or a sharp pump of the mainsheet is normal.
Can a furling headsail have positive roach?
Only with vertical battens or a very short section of positive roach near the head. An unsupported leech needs a slight hollow to keep from fluttering, which is why most furling sails are cut with negative roach.
Roach is where a mainsail earns much of its power, and it is also where the sail asks the most of its battens, its leech and its trimmer. Understand how much your sail carries and how to manage its twist, and that curve of extra cloth will pull the boat along in the wind up high that the rest of the sail never sees.