Choosing the Right Mainsail

It used to be simple: you ordered a mainsail, got something with a moderate roach, a few battens, maybe a couple of reef points, and off you went. But modern sailmaking is anything but simple and choosing the right mainsail is important. Today’s sailors are faced with a bewildering array of options—from square heads to furling booms, inflatable battens to membrane laminates. So, how do you go about choosing the right mainsail?

First, consider your sailing style. Weekend harbour cruiser? Offshore adventurer? Your sail should reflect your needs. The back end of the mainsail interacts with your keel to provide lift upwind, while overall sail area drives performance downwind. More roach generally means better performance, but fixed backstays often limit how much roach you can carry—hence the rise of square-top mainsails on modern rigs without fixed backstays.

As pointed out in The Art and Science of Sails, triangle-shaped (or “pinhead”) mainsails are aerodynamically inefficient. U-shaped tops, like those found in square-headed sails, produce more power and better performance—especially downwind.

Once you’ve chosen a sail profile, fabric and engineering come next. The golden rule: measure a sail’s life not just by how long it stays in one piece, but by how long it holds its shape. Budget-minded cruisers might lean toward cross-cut Dacron—durable and time-tested. But serious sailors are increasingly turning to high-tech options like laminated or membrane sails, where fibers are laid along precise load paths for better strength-to-weight ratios.

Membrane sails are engineered as one integrated piece, unlike panelled sails where fabric is cut and joined afterward. This results in lighter, more efficient sails that reduce weight aloft—important for offshore passage-making, where pitching and heeling fatigue the crew over time.

Beyond fabric, details like corner reinforcements and leech/foot tapes impact durability. Occasional sailors can make do with simple patches, but blue-water sailors should demand more robust reinforcements.

As for trim features, the cunningham adjusts luff tension, allowing you to shift the sail’s draft forward when the breeze builds. The foot can be loose or have a shelf; these days most sailors opt for the simplicity of a loose foot, eliminating hardware and friction.

A flattening reef helps depower the sail by neutralizing curvature near the tack, while leechlines< reduce flutter along the trailing edge. On large boats, overhead leechlines let you adjust tension from the tack rather than the clew—useful in big seas or strong breeze.

Reefing setups depend on your sailing grounds. Daysailors might only need one reef; ocean voyagers should plan for at least two or three. Deep first reefs suit cruisers seeking simplicity; racers prefer small first reefs for fine-tuned performance. Remember, reefing patches and lines add cost and weight—especially near the leech where it can affect light-air trim.

Battens deserve special consideration. Short battens allow flexibility in shape but can let the sail flog, wearing it out. Full-length battens support roach, reduce flogging, and ease flaking or stowage—especially with lazy jack systems. But they add compression loads, increase friction when hoisting, and can chafe against rigging.

Many sailors prefer a hybrid approach: two full battens up high for shape and support, and two short battens below to allow easier sail trimming. It’s a good compromise between control and durability.

Furling mainsails whether in-mast or in-boom—are increasingly popular for their convenience. In-boom furling on a boom supplied by a modern recognised supplier nowadays, can be a reliable option, however there are considerable pitfalls with in-mast furlers, no matter which brand. You might get away with in mast furling in benign conditions, but for passage-making you are taking a risk. The sail can only have a roach if vertical battens are used. The vertical battens take valuable space in the mast cavity and when the sail begins to stretch a bit, these battens cause the sail to jam when partially furled. Due to the restricted space in the mast cavity, sailmakers need to use marginally under-spec cloth weight and reduce all reinforcing. Not ideal for longevity. Without battens, the leech is of necessity hollow. This results in high loads on a thin edge of cloth, like pulling on the one edge of a piece of paper instead of through the middle. The only outcome of this scenario is a sail that will stretch early in life and begin the issues of unreliable furling. In short, only the very brave will spec an in mast furling main for passage-making.

So, what’s the right mainsail for you? It all comes down to a balance of performance, durability, ease of use, and budget. Write down what you need from your mainsail, how you sail, and what you’re willing to spend. Then, cross-reference that with the options available. With a bit of clarity, the right choice becomes much easier—and your time on the water far more rewarding.