Mostly photographs. A headsail meets the boat in few places, but getting the luff wrong means the sail will not go up at all, so those few places are worth doing properly.
| The furler | Make and model, and a photograph of the drum including any maker’s plate. Or, if the sail is hanked, one hank open and one closed with a ruler beside it |
| The sheet track | From beside and from above, with the car where it sits under the sail you have now |
| The three corners | The tack fitting, the halyard shackle, and the existing sail’s clew |
| The sail half rolled | So we can see which way the furler turns |
| Wear marks | Wherever the old sail rubs the rig |
| The old sail’s dimensions | If it sets well, this is the best information there is |
The rest of this page is what each of those is for.
The forestay
The furler’s make and model is the single most important thing about a furling headsail. The foil has a groove, the groove has a size, and the sail’s luff tape has to fit it — snugly enough not to pull out under load, loosely enough to run. If the model cannot be identified, photograph the groove opening end-on with a ruler beside it and we will work from that.
Whether the foil has one groove or two matters as well: a twin-groove foil lets a second sail go up before the first comes down, and it changes what the luff has to be.
If the sail is hanked, we need the hank type and the forestay diameter, because the hank has to close around it with the right clearance. A count off the existing sail is the reliable answer for how many and how spaced.
The sheet track
The forestay decides how long the sail is. The track decides how big it is allowed to be, and it is the item most often skipped.
The lead position that suits a sail depends on that sail’s own size and clew height, so a different sail wants a different lead. Tell us where the car sits with the sail you have now: mid-range means there is room to move, already at the aft end means a larger sail has nowhere to lead to.
If you are keeping the same size this is a check. If you are changing it, it is the first thing to settle.
Size and overlap
Most headsails are replaced at the size they already were, and that is usually right. If the old sail set well and the crew could tack it, its dimensions are better information than anything we could calculate.
A headsail can stop short of the mast, or carry on back past it and overlap the mainsail. The measurement is LP — the perpendicular distance from the clew to the luff — and against the width of the foretriangle it gives a percentage. A sail that stops short is a jib; one that reaches past is a genoa, and the further it reaches the bigger the genoa.
You never type LP in. It falls out of the sail’s dimensions and the configurator shows the overlap you are actually specifying.
The question is not how much sail the rig allows, but how much this crew will tack cheerfully in the wind this boat actually sails in. A genoa that spends most of its life partly rolled is telling you it is too big.
Clew height and sheet lead
A low clew puts area low down where it works well, but lays the foot along the deck, blocks the view underneath, and needs the lead well forward. A higher clew gives up a little area and gets that clearance back; it also behaves better partly rolled.
The lead position that suits a sail depends on its size and clew height. Photograph the sheet track and car with the sail you have now. Somewhere in the middle of the range means there is room to move; already at the aft end means a larger sail has nowhere to lead to.
The self-tacking jib
It sheets to a track across the foredeck, so nobody touches a sheet when the boat tacks. If your boat has that track it is a good option. If not, it is a rigging project before it is a sail.
The three corners
The tack — photograph what it attaches to and the space around it. On a furling boat that is the top of the drum, and there is usually less room than there appears. Note how the tack attaches: shackle, hook, soft shackle or webbing loop. They are not interchangeable.
The head — the halyard shackle, and the top swivel if there is one. What matters is that the sail can be hoisted high enough to tension the luff before the head reaches the swivel.
The clew — the existing sail’s, and where the sheets go.
Which way the furler rolls
Photograph the sail half rolled. That decides which face ends up outside, and so which side the UV strip and the foam luff go on. Getting it wrong puts the protection on the inside of the roll doing nothing.
Where the sail touches the rig
The spreader ends when the sail is eased, and the guardrails along the foot. Photograph the wear on the old sail and tell us when it happens — wear marks record what actually occurs rather than what should, and they are the best evidence there is.
Dimensions
Luff, leech and foot; LP is calculated from them.
If your boat is in our database they arrive filled in from the rig record. If the existing sail is sound and sets well, its dimensions are better. Note that the rig’s I and J describe the foretriangle rather than the sail — the sail is smaller, because the fittings at each corner and the halyard travel take up some of it.
If you cannot identify something
Do not pick the closest-looking option. Upload the photographs and we will identify it. Anything without a field goes in Design Note.
Configuring this at SurgeSails
Has Furling and Furler Model, or Luff Type and Hank Model; Luff Tape Size; Luff, Leech, Foot; calculated LP and LP%; Head Corner, Tack Corner and Tack Type, Clew Corner; Tack Cut Back and Luff Tape Tack Cut Up; Spreader Patches and Stanchion Patches.
Changing the overlap or clew height on purpose? Say so in Design Note and send a photograph of the track and car. Why the geometry works as it does is in Size, Overlap and Shape.