How to Trim the Mainsheet on a Gunboat 68

Reaching under an A3 spinnaker with the first reef tucked in, daggerboards well out — two metres on the leeward board, two and a half on the windward — because the apparent wind is still forward enough to want to push the boat sideways, even with the true wind well aft. A good moment, mid-passage, to break down how mainsheet trim actually works on a boat like this.

Two separate controls, not one

The mainsail on this boat runs through a track-and-traveller system operated entirely by push buttons from the helm — a line driver moves the traveller car port and starboard, and the mainsheet itself runs into the boom, driven by a hydraulic ram. What matters isn’t the hardware so much as what each control actually does, because they’re doing two genuinely different jobs.

The mainsheet controls twist — how open or closed the leech sits, adjusted independently of everything else. Ease the mainsheet, watch the leech open and go slack; sheet back on, and the leech stands back up. That’s the shape control, set first and mostly left alone once it’s right.

The traveller controls power — how much the mainsail is picking the boat up and loading the rig, and it’s the number one control on a big multihull, arguably more important than the mainsheet itself. Push the traveller up (toward the centreline) and the boat gets more power, and more flighty — the windward hull starts to lift and load builds in the windward shroud. Drop the traveller down and the power comes off immediately. A load cell on the windward shroud, with an audible alarm at a set threshold, gives a direct read on how hard the rig is being pushed — genuinely useful at two in the morning when there’s no visual reference for how loaded the boat actually is.

Why separating the two controls matters

This is the real advantage of a track-and-traveller mainsheet system over the twin-mainsheet A-frame arrangement found on some cruising cats: on this boat, twist and power are two entirely separate, independent adjustments. Set the twist with the mainsheet, then power the boat up or depower it purely with the traveller — no interaction between the two.

On a double-bridle A-frame system, the same two elements — twist and power — end up controlled by the same single input. Ease one side of the bridle to depower, and the boom lifts and the leech opens at the same time, changing the twist whether that’s wanted or not. Getting the shape back then means pulling down the other side of the bridle to compensate — a genuinely difficult, fiddly process compared to a single clean adjustment on a proper track-and-traveller setup. Some larger cats improve on the basic A-frame with two push buttons instead of manual winch work on both sides, which helps, but the fundamental limitation is still there — the two functions still aren’t independent.

From push-button cruising to full-crew racing

In cruising mode, this system is about as simple as sail trim gets — drop the traveller a set distance, and the equivalent adjustment on a bridle system would take real time and effort to replicate. But the same setup scales up for racing, too: with a full crew aboard, the traveller can be run off a clutch and winch rather than the push button, with one person handling winch grinding and another playing the traveller by hand — fast, sensitive control, the kind needed when actively flying a hull rather than sailing flat. Not what today’s passage called for, but exactly the setup that makes it possible when it is.

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