The mainsheet is the line that controls the mainsail, running from the boom through a system of blocks to the cockpit, where it is hauled in to trim the sail and eased to let it out. For the beginner it is accelerator and brake: pull it in and the boat powers up and heels, let it out and the boat stands up and slows. The experienced trimmer knows it does something subtler – upwind the mainsheet controls leech tension and therefore twist, while the boom angle is largely set by the traveller, and getting that division of labour right is what separates a fast main from a merely full one.
Anatomy of a mainsheet system
Because mainsail loads are high, the mainsheet almost always runs through a purchase of several blocks – 4:1 on a small keelboat, 6:1 or a winch on a forty-foot cruiser. The lower blocks sit on a traveller car running across a track on the cockpit floor, bridge deck or coachroof, so the sheeting point can be moved to windward or leeward. The upper blocks attach to the boom either at its end, which gives the best leech control, or at mid-boom, which keeps the sheet out of the cockpit but bends the boom harder. Racing boats often split the sheet into a coarse tune and a high-purchase fine tune, and some use German sheeting, with both tails led forward along the boom and back to winches on either side so the sheet can be worked from the high side.
What the mainsheet actually controls
Think of the boom as a lever and the sheet as pulling down on it as well as in. With the traveller centred and the boom near the centreline, hauling on the mainsheet pulls mostly downwards, tightening the leech, closing the twist and bringing the top of the sail to windward; easing lets the top open and spill wind. That is why, upwind, a trimmer sets the boom angle with the traveller and plays the sheet to keep the top leech telltale streaming. Off the wind, with the boom well out, the sheet pulls mostly inward and the vang takes over the leech, so the mainsheet becomes a pure angle-of-attack control.
Technique and common mistakes
- Ease in a gust: when the boat heels hard and the helm loads up, ease a few inches before the helmsman has to fight. When in doubt, let it out.
- Trim by the telltales: the top leech telltale should stream most of the time and curl behind the sail now and then. Always stalled means over-trimmed; always streaming means there is more to be had.
- Do not oversheet off the wind: a tight mainsheet on a reach hooks the leech to windward, adds heel and drags the boat sideways. Ease until the luff just lifts, then trim a touch.
- Use the whole system: if you are pulling harder and harder for a flat sail, the answer is usually the outhaul, backstay or cunningham, not more sheet.
- Keep it ready to run: flake the tail so it can be released instantly in a broach or knockdown.
The mainsheet also carries enough load to injure: never wrap it around a hand, never stand in the bight of the tail, and in a gybe haul it in before the boom crosses so it swings a short arc.
Cruising vs racing
On a cruising boat the mainsheet is often set for a leg and left, and the gains come from small habits: easing when the boat heels too far, hardening the vang before easing on a reach so the boom does not lift, and cleating only in a way that one hand can release under load. The racing trimmer never leaves the sheet alone: in a breeze it is eased and trimmed constantly to keep the boat on its feet and the telltales flying. On a dinghy or sportsboat the mainsheet is in the helmsman’s hand at all times and is as much a part of steering as the tiller. From the sailmaker’s side, a main left flogging with the boom crashing about damages batten pockets and clew reinforcement, and a woven sail habitually oversheeted in a breeze stretches its leech before its time.
Frequently Asked Questions about the Mainsheet
What is the difference between the mainsheet and the traveller?
The mainsheet pulls the boom in and down; the traveller moves the sheeting point across the boat. Upwind, use the traveller to set the boom angle and the mainsheet to set leech tension and twist.
How tight should the mainsheet be when sailing upwind?
Trim until the top leech telltale streams most of the time and stalls now and then, with the top batten roughly parallel to the boom in moderate air. Ease in a gust and in light air; harden in flat water and a steady breeze.
The mainsheet is where a sailor’s hand meets the mainsail, and learning what it does at each point of sail turns trimming from guesswork into control. Set the angle with the traveller, shape the leech with the sheet, ease before the boat fights you and keep the tail ready to run, and the biggest sail on the boat becomes the easiest to drive.