How to determine Genoa size?

Davidasailor26 · Sep 10, 2023

You’ll need to take one more measurement for that - the Luff Perpendicular (LP). That’s the distance from the clew to the nearest point on the luff. (It’s along a line perpendicular to the luff, thus the name). The sail percentage is the percentage of that LP relative to the boat’s J dimension. J is the distance from the front of the mast to a point in space directly above the base of the forestay, and you can generally look that up for the boat model.

shemandr · Sep 10, 2023

I wasn't the best geometry student but isn't a jib basically a triangle? I don't think the hypotenuse (Leech) can be shorter than either of the legs (luff or foot).
If the luff were the 41' then by the formula, A2 + B2 = C2 , the leech would be 43.6' or likely the 44' length. Are those dimensions reversed?
I wouldn't sell any sail until I had a season of sailing the boat under my belt.

Davidasailor26 · Sep 10, 2023

shemandr said:
I wasn't the best geometry student but isn't a jib basically a triangle? I don't think the hypotenuse (Leech) can be shorter than either of the legs (luff or foot).
If the luff were the 41' then by the formula, A2 + B2 = C2 , the leech would be 43.6' or likely the 44' length. Are those dimensions reversed?

It’s a triangle, but not a right triangle, so the Pythagorean theorem does not apply. You could split it into two right triangles, along the LP line.

jssailem · Sep 10, 2023

I’ve understood that the percentage label is based on the relationship between the boat’s fore triangle formed by the mast, foredeck, and forestay. And the area measurement of the fore sail.

A sail that is smaller than the area is called a jib. A sail larger than the area is referred to as a genoa. The percentage label is shorthand for the relative size of the genoa or jib based on the specific boat.

jssailem · Sep 10, 2023

But I could be wrong.
The folks at MIT describe it thus.

Foresail - MITNA

sailing.mit.edu

dlochner · Sep 10, 2023

Davidasailor26 said:
It’s a triangle, but not a right triangle, so the Pythagorean theorem does not apply. You could split it into two right triangles, along the LP line.

Which is what the Luff Perpendicular does.

sailme88 · Sep 10, 2023

The distance from the tack to mast on boat A can and will be different from Boats B,C,D etc... thus the % is moot unless the you are dealing with same made boats. All this is aside from the mast height..

Davidasailor26 · Sep 10, 2023

sailme88 said:
The distance from the tack to mast on boat A can and will be different from Boats B,C,D etc... thus the % is moot unless the you are dealing with same made boats. All this is aside from the mast height..

For the purpose of selling the sail the foot, luff, and leech that you’ve already measured are really the most important parts. A buyer will know what dimensions they’re looking for based on their boat. Even if another owner of your same model is buying they’ll know to look for the dimensions regardless of the original overlap percentage.

Tally Ho · Sep 10, 2023

The % will vary on the boat you intend to use it on…

If you intend to sell it for YOUR boat, the % could be relevant. But if you give the measurements and someone es to to but it for some other boat make/model, the % will be different for them.

Greg

dlochner · Sep 10, 2023

Advertise the sail by its dimensions, condition, sailmaker and the size sail it was for your boat.

Given the other jibs you have, is this one larger or smaller? Is there a number on the bag? Often sail makers will label a bag #1 Genoa C&C 37. The smaller the number the larger the sail. A #1 is around 150%, #2 around 135%, # 3 around 100-110%, #4 under 90%.

Stu Jackson · Sep 10, 2023

Jimongee said:
Is there a formula to determine sail %?

Of course there is:

Richard19068 · Sep 10, 2023

Here is a handy calculator for the LP:

Jib Overlap Calculator - Precision Sails

Use our handy calculator to find the right size jib for you.

www.precisionsailloft.com

ttps://sailingfortuitous.com/jib-calc/