Some sail area math questions... - SailNet Community

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Old 06-14-2011
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Some sail area math questions...

I have been struggling with the math on this, and I am sure there is a simple equation for it, but I have yet to figure it out. I was hoping someone here could give me the answers I am looking for.

I am currently sailing a Redwing 30 with a 155% Genoa on a roller furling setup. I want to make some marks on the foot for roller reefing the Genoa down to a 135% & 100%

I need to know the distance from the tack or the clue to place the marks on the foot to equal these respective positions.

I am looking for something like "2.3 feet from the tack for 100%" etc.. if my question makes any sense?

Much thanks in advanced, sincerely confused mathematician Alstare...
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Being a new member I can't post links, but I have a link to the sail plan data fore the boat...
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3rd post should allow me to post links now...

Here is the sailplan info for the boat: Redwing 30
C&C REDWING 30 Sailboat SailPlan Data and Sail Quoting System
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Old 06-14-2011
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Lay the sail out on the floor and measure from the luff to the clew, perpendicular to the luff. That dimension will be your "LP" (should be nearly 18 feet [17.85]) which is 155% of your J measurement (ie forestay to mast along the deck - 11.5 ft)

So the 100% will be rolled up til there's still 11.5 feet of sail between the rolled portion and the clew (again, perpendicular to the luff). Just do the ratios for any other position you might want. Mark the position on that 'line' you measured, and extend it to the foot parallel to the luff.
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Last edited by Faster; 06-14-2011 at 11:14 PM.
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1. Measure the foot of the 155% sail = X

2. X times 135 divided by 155 = length of foot at 135%
2a. X times 100 divided by 155 = length of foot 100%

The 'typical' maximum reduction you can get on a reefing-furler and still have a 'reasonable' sail shape is 30% reduction. 155 X .7 = 108 or 110%
3. X times 110 divided by 155 = length of foot at 108 - 110%

:-)
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Wow... Thanks for the quick and extremely informative replies guys!

I will try out both solutions to verify my numbers. I know with the rolled furler I am not gonna get exact performance of the properly reduce hanked on jib sizes. But I think it will make for a good reference for setting the boat up quickly for different wind conditions as apposed to my "pull some in" method I am currently using!

I got the beers if you guys ever sail down this way!

-Cheers!
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Well, most of these suggestions are partly right. First determine the LP. Measure the length of the luff from the tack to the head ring. Mark the mid point of the luff. Then measure from the center of the clew ring to the midpoint you marked. That is the actual LP. The LP dimension is different from the foot dimension. You probably know your J dimension (look up in Mauripro rig database on their site if you don't). A 155% genoa should have an LP that is 155% x the J dimension. Calculate the the 135 and 110% dimensions. Measure from the midpoint of the luff to to a spot on the imaginary line from the clew to the tack which corresponds to135% and 110% dimension. If you measure to the foot it will be inaccurate because there usually a foot roach involved.
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Old 06-15-2011
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There is absolutely NO need for the complication of find a the "LP" of a genoa to determine the percentages you want for the foot dimensions for a roll-up to 135 or 100%, etc. LP is a simple direct ratio of foot length.

A genoa for this sized boat will NOT have roach of any significant measure ... its not a symmetrical spinnaker its a genoa. The 'roach', or as a sailmaker would call it an apron, will only extend several 'inches' and therefore will only add less than 1% of the foot length (as a tangent ratio).

Just measure the exiting foot length and ratio that value by what you want .... (X)135/155 or (X)100/155 and youll be within 1±% of the 'actual' or 'spherical' dimension.

... and the rest of the world wonders why the USA is in 40th place out of ~195 countries in Math education .....
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Quote:
Originally Posted by Sanduskysailor View Post
.... Measure the length of the luff from the tack to the head ring. Mark the mid point of the luff. Then measure from the center of the clew ring to the midpoint you marked. That is the actual LP. ....
Not sure you want to measure from mid luff.... LP is for Luff Perpendicular, ie a line at right angles to the luff through the clew.. It would only be mid luff on a very high clewed sail... (as I understand it, anyhow)
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LP (length perpendicular) is determined by establishing a line at exactly 90°, perpendicular, to the luff* running through the clew, ... has nothing to do with the head (ring).

What he's describing is to one segment of the 'centroid of area' .... finding the exact midpoint along an 'edge' (luff, leech or foot) and drawing a line from the opposite 'corner' TO the midpoint along the opposite edge ... the intersection of all three lines establishes the centroid of area - aka: center of effort or CE.

* in actuality this is an 'imaginary' luff, as with jibs/genoas there is usually some material that is cut away in a smooth curve to compensate for forestay, etc. sag ... and with mainsails some smooth curve added to match the expected mast 'pre-bend'. So to calculate LP, you construct an imaginary line running between the tack and the head (an imaginary luff), then establish a line at 90° to the 'imaginary line' that bisects the clew.

;-)
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