A trysail replaces the mainsail when the mainsail should not be up — including when it cannot be lowered or has been damaged. The decision that matters is not how it is cut but how it goes up, because that is what decides whether it is usable.
What it is for
The mainsail is too much sail. Past the deepest reef, a mainsail is still large and high-aspect with its area well up the mast. A trysail is smaller, lower and cut flat, so the boat can be held at an angle to the wind and steered.
The mainsail cannot be used at all. This is the case a deep reef does not cover — a tear, a jammed track with the sail partly down, a broken batten through the leech, a failed halyard. No number of reefs helps.
So a trysail is a complete substitute: its own sail, its own attachment to the mast, its own sheets to the deck. Nothing about setting it depends on the mainsail being usable. That is what makes it different from a fourth reef.
It does need a working halyard, and few boats carry a dedicated one — so in practice it is the spare, or the main halyard if that is not what failed. Worth establishing which before you need it.
It works independently of the boom
A trysail is normally sheeted to the deck rather than to the boom, and that is not incidental. If the mainsail is lashed down the boom is occupied; if the boom is broken or thrashing it is a hazard rather than a resource. Sheeted to strong points on both quarters, the trysail works regardless of what has happened to the boom.
Which means the sheeting arrangement has to exist and be known before it is needed — which points, what length, where they lead. That is settled when the sail is ordered.
How it goes up
Two answers, and this is the question to settle first.
A dedicated trysail track — a second short track on the mast, separate from the mainsail’s. The point is that the trysail can be left permanently attached: slides in the track, held by a stop, sail in a bag at the mast foot. Setting it is open the bag, attach the halyard, hoist. Nothing is threaded, and the mainsail stays exactly where it is in its own track — which matters, because the situation may be that the mainsail cannot come out of its track. Fitting one is a rigging job.
The mainsail’s own track — the mainsail has to come out and the trysail be fed in, at the gate, on a pitching deck, in the dark, with the mainsail’s slides trying to escape. It is possible and it is much harder than it sounds. With this arrangement, a good track gate and a way of retaining the mainsail’s slides matter a great deal.
How it is built
Small, flat, and heavily built — heavy cloth, substantial reinforcement at all three corners, rings and webbing well above what the area alone suggests. The corner loads in the conditions it is used in are far higher than its size implies.
In a high-visibility colour, usually orange, so it identifies the boat’s situation and makes the boat visible.
Two things worth doing
Hoist it on a calm day, exactly as it would be set for real. That answers what matters: does it go up without threading anything, do the sheets reach and are they long enough, can the bag be got at without emptying a locker, is everything aboard.
Stow it where it can be reached — at the mast base if there is a dedicated track, otherwise somewhere reachable without moving anything else, with its sheets stowed with it.
If you race offshore
Offshore racing rules set requirements for storm sails — area, colour, attachment, and often a demonstration. Tell us which rule before the sail is designed; the requirements are specific and they govern.
Configuring this at SurgeSails
A trysail is not one of the standard mainsail configurations and is not specified through the same fields.
Tell us in Design Note: whether the boat has a dedicated trysail track or the sail must go into the mainsail track, what the deck sheeting points are, and whether an offshore racing rule applies. Photograph the mast at the gooseneck showing any separate track and its gate, and the deck points you intend to sheet to.
Cloth, corner construction, reinforcement, slides and colour follow from the boat and the arrangement.