A short page explaining why the parameters that define a mainsail’s three-dimensional shape are not offered as choices, and what we need from you instead.
The mast bends, so the sail is cut for it
A mast is not straight under load, and how much it bends changes as the boat is sailed. A mainsail’s luff is cut with a curve that works with that bending: the extra cloth along the leading edge becomes the fullness in the front of the sail, and bending the mast takes it up.
This is why a backstay adjuster changes the shape of the sail, and it is why the same sail behaves differently on two boats with different rig tune.
What you would notice if it were wrong. A sail that stays full and closes its leech no matter what is pulled on, or a sail that goes flat and shows creases running back from the luff as soon as the backstay comes on.
Getting it right needs the rig, not a number from you. A cruising boat needs its rig type and section identified; a racing boat with a highly tuned rig may need more detail, and we will ask.
Roach, battens and cloth are one decision
These look like three independent upgrades and they are not.
Adding roach adds area high up, and it needs battens stiff enough to hold it. Those battens push compression into the luff hardware, which is why more roach eventually means cars on the mast. A lower-stretch cloth holds the designed shape more closely — and is less forgiving of a shape that was not right.
So “more roach”, “full battens” and “better cloth” are three faces of one decision, and they are decided together from your boat and how you sail.
What you see, and what we own
You see the outline and its girths, the batten arrangement, the cloth, the panel layout, the hardware and how the sail connects to your boat. All of that is checkable against the boat and against the sail when it arrives.
We own the luff curve, the assumed mast bend, how depth is distributed up the sail, and the production corrections behind them.
The division is not secrecy. These quantities only mean anything in relation to each other and to a particular rig — a single one of them handed over on its own would be a number nobody could check.
Configuring this at SurgeSails
Nothing on this page is a field, and that is the point of it.
What the configurator asks for instead is your boat and rig, your usage, and the visible choices — the outline, battens, cloth and hardware. The shape follows from those. If your rig has been modified or is tuned unusually, say so in Design Note.