How to determine Genoa Size - SailNet Community
 
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post #1 of 9 Old 09-16-2010 Thread Starter
EJO
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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
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E-J
s/v Sailmates, 1973 Irwin 32 Classic.
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post #2 of 9 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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post #3 of 9 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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post #4 of 9 Old 09-16-2010 Thread Starter
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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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post #5 of 9 Old 09-16-2010
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post #6 of 9 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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post #7 of 9 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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post #8 of 9 Old 09-16-2010
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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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post #9 of 9 Old 09-16-2010
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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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I drives me dinghy!
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