The leach – also spelled leech, and pronounced the same way – is the aft edge of a sail, running from the head down to the clew. On a mainsail it is the long, roached edge supported by battens; on a genoa or jib it is the free edge that passes across the mast on every tack. Both spellings are correct and in daily use in lofts and on race boats. The beginner needs to know that the leach is the edge you watch to see whether the sail is trimmed correctly. The experienced sailor and sailmaker knows that it is also the edge that carries the highest loads, ages fastest and reveals a sail’s remaining life – a leach that flutters or hooks is telling you something about the cloth.
Anatomy of the Leach
Unlike the luff, which is held in a groove or by hanks, or the foot, which on a mainsail is attached to the boom, the leach is unsupported. Its shape depends entirely on the sail’s design and the tension the crew puts on it. A mainsail leach is usually convex, its roach extending beyond the straight line from head to clew and held out by battens whose pockets are sewn into the leach. A jib leach is typically straight or slightly hollow so it will pass the spreaders and mast cleanly, unless it is a fully battened racing jib. Along the edge runs the leach tape, a folded strip of reinforcing cloth, and inside it a leach line – a light cord that can be tensioned at the clew to stop the edge fluttering. On a headsail, telltales streamed from the leach show whether the air is flowing off the sail or stalling.
Why the Leach Matters for Speed
The leach controls twist, and twist controls how a sail behaves in the wind gradient. Because the wind blows harder and slightly freer aloft, the top of a sail should be trimmed at a wider angle than the bottom; a properly twisted leach lets each part of the sail meet the wind at its correct angle. Close the leach too much – over-tight mainsheet, jib lead too far forward – and the top of the sail stalls, the boat heels, and weather helm builds without extra speed. Open it too far and the top of the sail luffs while the bottom is still pulling. On the mainsail, the leach also does most of the pointing: a firm upper leach with the top batten parallel to the boom is the classic upwind setting. The leach of the genoa and the leach of the main also interact, because the slot between them accelerates the air; a jib leach that hooks into the main backwinds it and kills the flow.
What the Sailmaker Builds Into It
Loads in a sail fan out from the clew and head along the leach, so the leach is where cloth and construction matter most. In a cross-cut woven sail the panels are laid so the fill yarns run roughly parallel to the leach, taking the load on the thread line. In a radial laminate the panels fan from the corners with the warp following the leach load. In UK Sailmakers’ Tape-Drive®, Titanium™ and X-Drive® sails, continuous high-modulus tapes or yarns run from the clew up the leach to the head, carrying the load directly. Leach hollow or roach is cut into the design, with the roach on a main limited by how far the battens can support it and by the backstay it must pass. As a sail ages, the leach is where stretch shows: the edge grows longer relative to the body, the cloth cups, and the sail develops a hooked, motor-boating leach that no amount of leach line will cure.
Trimming the Leach
- Mainsail twist: mainsheet tension closes the leach upwind; the vang takes over when the boom is eased off centreline. Watch the top batten and the top leach telltale – it should stream most of the time and stall occasionally.
- Headsail twist: move the jib lead aft to open the upper leach, forward to close it. All the leach telltales should break at about the same moment when you luff slowly.
- Leach line: tension it only enough to stop the flutter, and ease it again as the wind drops. Overdone, it hooks the leach to windward and costs pointing and speed.
Frequently Asked Questions about the Leach
Is it leach or leech?
Both are correct and mean the same thing. Leech is more common in modern rule texts and in the United States; leach appears frequently in British usage and older sailmaking literature.
Why does my leach flutter?
Light flutter is normal on a new sail in fresh air and a touch of leach line stops it. Persistent flutter that the leach line cannot cure means the edge has stretched or the cloth along it has broken down, and the sail needs a leach recut or replacement.
Watch the leach and it will tell you how the sail is trimmed and how much life it has left. Give it the right twist for the wind, only as much leach line as it needs, and sound battens, and the aft edge of your sails will keep the boat pointing and moving as the designer intended.