How to determine Genoa Size - SailNet Community
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How to determine Genoa Size

We all know that the jenny is expressed in percentage of size over the regular jib. i.e. 110%, 130% etc. (I think)
The question here is how do you determine what you jenny percentage is when you don't know the standard jib size and only have a storm jib (reduced size jib) and a couple of different size genoas.
Or is the size based on the passing point aft of the mast. How would I know if I have a 135%, 130%, 140% etc. without being able to lay the sail down and measure its sides to determine its area.
Any ideas?
what size is this
Thanks,
E-J
s/v Sailmates, 1973 Irwin 32 Classic.
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Old 09-16-2010
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The rating is how the sail's LP compares to the boat's J measurement.

The J is the horizontal distance from the forestay tang to the face of the mast.. on your boat probably around 12 feet.

The LP of the sail is measured by taking a line through the clew, intersecting the luff at 90 degrees (LP = Luff perpendicular).. so if that measurement on your sail is 12 feet, it's a 100%; 18 feet, it's a 150% etc....

The sail in your picture looks to be maybe a 120 or 130 at this angle.

Ron

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Old 09-16-2010
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A 100% genoa would extend back to the mast.

Measure the part that overlaps from the mast to the clew (O).
Get the sails "J" value

Percentage Genoa = (J + O)/J*100

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Dutchman

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Faster & Zanshin
Thank you gentlemen asked and answered. [The LP of the sail is measured by taking a line through the clew, intersecting the luff at 90 degrees (LP = Luff perpendicular).. so if that measurement on your sail is 12 feet, it's a 100%; 18 feet, it's a 150% etc....][(J + O)/J*100]
I have a 130, which is what I thought now only if I could afford a new 150.
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Old 09-16-2010
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Quote:
Originally Posted by Faster View Post
The rating is how the sail's LP compares to the boat's J measurement.

The J is the horizontal distance from the forestay tang to the face of the mast.. on your boat probably around 12 feet.

The LP of the sail is measured by taking a line through the clew, intersecting the luff at 90 degrees (LP = Luff perpendicular).. so if that measurement on your sail is 12 feet, it's a 100%; 18 feet, it's a 150% etc....

The sail in your picture looks to be maybe a 120 or 130 at this angle.
Exactly correct.

Area= (LP x J x Luff/2)

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Old 09-16-2010
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Area = LP x Luff / 2

Assuming the sails are full hoist, i.e. the luff length is the same, and the sails are triangles, then percent LP is the same thing as percent area.

LP x Luff x J would give you cubic feet, obviously that can't be right.
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Quote:
Originally Posted by tap View Post
Area = LP x Luff / 2

Assuming the sails are full hoist, i.e. the luff length is the same, and the sails are triangles, then percent LP is the same thing as percent area.

LP x Luff x J would give you cubic feet, obviously that can't be right.
You are correct, my earlier formula posting appeared incorrectly. The P in (my) LP shoiuld have been a subscript to indicate the percentage overlap of the sail. That x the J measurement to yield the actual LP in feet multiplied by the luff divided by 2 to yield area (1/2 of the height multiplied by the base of a triangle). I did not realize that subscripts and superscripts would be eliminated when one copies and pasts from a Word file. Dumb me, eh?

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Area will equal (luff x LP)/2. BUT< this will not always equal (IxLP)/2. As I is smaller than the luff! So just because you have a 130, does not mean it is 130% of the fore triangle! If you have a full luff pull, you may in reality be 132-133% of the fore triangle! My 155 genoa, turned out to me 157% of the foretriangle, which cost me some time, as the sail maker went by LP to size it, not area which my local PHRF uses!

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Re: How to determine Genoa Size

It is just convention or tradition in the sailing community to express a genoa's size as the percentage of the boat's J measurement represented by the genoa's LP measurement? Because it seems wrong mathematically. A 100% jib and a genoa share a common side: the luff. Let that be the base of each triangle in the area formula base*height/2, and the LP of each sail is the height in the area formula. So the ratio of the genoa's area to the jib's area is the same as the ratio of the genoa's LP to the jib's *LP*, not the boat's J measurement (which equals the jib's foot measurement). In other words Ag / Aj = LPg / LPj. But Ag / Aj <> LPg / J.

The math also works out if you let the boat's I dimension represent the height of each sail's triangle. Then Aj = J*I/2, and Ag = (J+O)*I/2, where O is the length of the genoa's foot aft of the mast. In other words the ratio of genoa to jib areas is the same as the ratio of genoa to jib foot measurements.

And yet it seems conventional to divide a genoa's LP by the boat's J to get the genoa's size as a percentage of the foretriangle, even though the math is wrong. Why is that?

Cheers,
Randy Stafford
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