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Old 07-07-2008, 22:55   #1
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storm jib area calc

can someone confirm the orc formula for calculating the area of a storm jib is (I^2 * 0.5) and not (I * 0.5)^2

the way it is written out is confusing tho i suspect when shown as a formula it's clear enough

thanks
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Old 07-07-2008, 23:02   #2
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Here is a good primer...

Genoas and Other Jibs - UK-Halsey's Encyclopedia Of Sails
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Old 07-07-2008, 23:10   #3
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Rule one when dealing with formulas you don't understand try some numbers and see what makes sense. Based on that, both of the formulas you quote are very wrong. My 40 foot ketch has an "I" of 49 feet. So....

(49^2)*0.5 = 1200
(49*0.5)^2 = 600

Both of these numbers are way too big for a rational storm jib. My 100% jib is less than 300 sq feet!

The correct answer does not invovle the factor of 0.5 rather it is 5% of I^2. So for my boat it would be (49^2)*(5/100), or about 120 sq feet, something a lot more rational. Although if I was buying a new storm jib, I'd likely be looking for something a bit smaller than that.

Remember, this is a racing rule. If you are not racing the best size for your boat might not be (and is likely not to be) this size!
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Old 07-07-2008, 23:45   #4
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sorry that's a typo. should have been: can someone confirm the orc formula for calculating the area of a storm jib is (I^2 * 0.05) and not (I * 0.05)^2
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Old 08-07-2008, 00:46   #5
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I believe the former is correct (I^2 x 0.05), with the proviso that the luff length may be no more than 65% of the height of the foretriangle, but I'm surely no expert.
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