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Old 11-16-2013
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Genoa measuring, help please.

Hi!

I am in the market for new/slightly used Genoa for my US Yacht 25. I took my trusty 30' tape measure and took measurements.

I'm getting confused with regard to the foot of the sail, and the J, and the LP.
Some websites with sails listed show only the sail's Luff and Foot. I guess I'm getting confused when it comes to the foot. Other sites show the Luff and L/P. What should I be looking for when a seller only lists the luff and foot.

Here's my measurements:
Headsail Luff: 26' 3" max.
LP: 11' 3"
J: 9' 7".

Goal is a 133% Genoa.

Thanks for any help!

Chris
Richland, Wa.
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Old 11-16-2013
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Re: Genoa measuring, help please.

There are two jib calculators that I find helpful:
Jib Overlap Calculator
http://l-36.com/LP_calc.php

Both will compute LP for you given sail size. The L-36 one will also compute foot height relative to tack if you know your forestay angle.
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Re: Genoa measuring, help please.

You really should look at the headsail area. You can calculate the area using Luff x LP/2 = Area. If they give you luff, leach and foot, you'll have to do a some geometry/trig calcs to get there. If they only give you the luff and foot, you will have to guess. The LP will be less than the foot, but it depends on how high the sail is cut. Looks like for your boat I=30.27' and J=9.5' so 100%=30.27x9.5/2=143.8 so 133% would be 191.2 square feet.
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Re: Genoa measuring, help please.

This might help.

US 25 sailboat specifications and details on sailboatdata.com
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Re: Genoa measuring, help please.

I'm looking for a smaller headsail now (leaning towards having a new one made) and find it useful to look at both sail area and LP. You can have 133% LP sails made to almost any sail area by varying the luff length of the sail. On a 133% it is unlikely that you'd make the luff length short, but on smaller sails (<100%) they are often cut that way.

I also find that doing a search on sailritesails.com will give you target dimensions for common sail sizes on your boat. Here is a link to your boat (the second genoa listed is about a 135%):
http://sailritesails.com/ShowAd.aspx?id=6486&sourceid=2

The sail for my boat that will be about 200sqft. A 200sqft sail for my boat (39' forestay, 37.5 I, 11.25 J) could be made as a sail with a roughly 32' luff and 14' foot (around 105%, called a working jib), or with a 36' luff and 11' foot (around 95%, called a blade jib). Both have the same sail area, the aspect ratio and LP are just different. The blade seems easier to tack.
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Re: Genoa measuring, help please.

Thanks to everyone for the great help! Much appreciated.

Chris
Richland, Wa.
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Old 11-18-2013
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Genoa measuring, help please.

BTW. For virtually all masthead rigs, when you do the trig, foot and LP are virtually the same.
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Re: Genoa measuring, help please.

If virtually means within 20%, then yes, you are correct. It depends on the angle of the foot to the luff and the angle of the forestay.

I check filter as a first gauge, but still do the actual LP calc to check percentage overlap. I also check square feet if the sail that I'm looking at doesn't use the full luff.
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Re: Genoa measuring, help please.

Quote:
Originally Posted by Sabreman View Post
BTW. For virtually all masthead rigs, when you do the trig, foot and LP are virtually the same.
I must be using different trig then you. it would make the sail makers job a lot easier . all sails would be square. the LP and foot can never be the same on a triangle sail.
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Old 11-18-2013
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Re: Genoa measuring, help please.

run a tape up your forestay attached to the halyard. That gives you a good idea of the luff length you can use. LP is easily determined by measuring shortest distance from the clew to the luff. Divide this by your boat's J measurement and you have LP.

Mike
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