Reefing a headsail??? - Page 3 - SailNet Community
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post #21 of 33 Old 01-09-2013
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Re: Reefing a headsail???

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The use of a (slab) reefable jib, etc. is a VERY good way ensure good sail SHAPE, especially needed for going 'upwind' in gnarly conditions. This is a strategic advantage over a roller furled jib that typically can only be 'roller reefed' to approx 30% sail area reduction before the shape -- goes all to hell and you get extreme bagginess in the important luff section and thus lose important 'pointing ability'.

I prefer the jib, etc. reef points to run along at an 'up angle' towards the clew for two reasons:
1. if carefully 'angled' you dont have to reset (much) the fore/aft position of the jib fairlead car.
3. the resultant higher (new) clew position will be 'far above the deck' for less trapping of 'green water'. Another way to state this is the my reef points are arranged so that I effectively change a relatively low clew 'normal' shape into a 'yankee' shape with 'high' clew when deep reefed.

Well said...I noticed this on my previous boat which really needed a jib reef when it got over 25 and would become very tame and able to beat upwind quite closely afterwards....Reasons I am thinking of removing my furling on the new boat. grrrr decisions decisions!
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post #22 of 33 Old 01-09-2013
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Re: Reefing a headsail???

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....WHile that's all good, the foot *is* what he sent me, dimension-wise; a measured 16'-3". Doing the math; it looks like I am the new owner of a 150% Genoa!! .
If you actually measured the length of the foot, that is not what you measure to determine the size of the jib, you need to measure the LP or luff perpendicular, the length perpendicular from the luff to the clew...so the sail should turn out to be a good deal less than a 150%.
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post #23 of 33 Old 01-09-2013 Thread Starter
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Re: Reefing a headsail???

sailingfool;
I'd appreciate a link to that info... on how ya calculate the percent overage.

Thanks,
Paul
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post #24 of 33 Old 01-10-2013
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Re: Reefing a headsail???

As an aside. When you furl a a genoa, the C of E goes higher and forward - exaxctly the wrong direction.

I used to use a boat with a slab reefing jib. It was great. I remember beating in 35 knots (with few waves, no fetch). No problem. Double reefed main.
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post #25 of 33 Old 01-10-2013
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Re: Reefing a headsail???

Here ya go on determining the LP (length perpendicular) ... see the second dwg. on the page.
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post #26 of 33 Old 01-10-2013 Thread Starter
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Re: Reefing a headsail???

Thanks Rich;

I'm having some trouble digesting the text stating; " The LP defines the sail size in terms of percentage increase beyond the J measurement as in the following examples." with the chart immediately below. It shows, under the "LP" column, a direct correlation to unit "J" past the "I" line.
It appears that roller furlers need the "LP" figure to relate the amount of sail exposed, as that's the direction of reduction; but regular (geometric term) foresail calcs rely on straight "J" numbers.
Am I missing something??
Doesn't the "IJ" fore-triangle calc describe a geometric area potentially filled by a headsail at 100% (not necessarily the actual measurements)? As I was led to believe, the increase in percentage was a function of the amount of the (base/"J") jib triangle passing the mast (height/"I") of the theoretical/geometric right triangle??

Last edited by deltaten; 01-10-2013 at 08:47 AM.
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post #27 of 33 Old 01-10-2013
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Re: Reefing a headsail???

Hmmm. Just lose the concept that the genoa size relates to the foretriangle area - it does not. It relates solely to the LP relationship to the J. If you do the geometry, a 100% genoa will have an area equal to the (square root of (I squared plus P squared)/I) times the foretriangle area. So if the measurements of a W27 are:

I J P E
34.75ft 11.00ft 29.00ft) 10.75ft

then the area of a 100% jib will be 1.05 times the foretriangle area of 1/2*IP or 529 sq. ft.

I think.

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post #28 of 33 Old 01-10-2013
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Re: Reefing a headsail???

I think SF has it... if your boat has a J measurement of 12 feet, say, and if you drew a line through the clew at 90 degrees to the luff (ie LP) and it measured 18 feet, that's a '150%' genoa

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post #29 of 33 Old 01-10-2013
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Re: Reefing a headsail???

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Originally Posted by sailingfool View Post
Hmmm. Just lose the concept that the genoa size relates to the foretriangle area - it does not. It relates solely to the LP relationship to the J. If you do the geometry, a 100% genoa will have an area equal to the (square root of (I squared plus P squared)/I) times the foretriangle area. So if the measurements of a W27 are:

I J P E
34.75ft 11.00ft 29.00ft) 10.75ft

then the area of a 100% jib will be 1.05 times the foretriangle area of 1/2*IP or 529 sq. ft.

I think.
529 ft² sounds big for a 100% jib on a 27-foot boat. Isn't P just for the mainsail?

I would think jib area is LP * Luff / 2. If Luff = forestay length, then for that boat it's

100% * 11.00 ft * sqrt((34.75 ft)² + (11 ft)²) / 2 = 200.5 ft².

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post #30 of 33 Old 01-10-2013
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Re: Reefing a headsail???

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Originally Posted by AdamLein View Post
529 ft² sounds big for a 100% jib on a 27-foot boat. Isn't P just for the mainsail?

I would think jib area is LP * Luff / 2. If Luff = forestay length, then for that boat it's

100% * 11.00 ft * sqrt((34.75 ft)² + (11 ft)²) / 2 = 200.5 ft².
So much for the back of the envelop. You are right, the correct the formula for adjusting the foretriangle area is stated as "(square root of (I squared plus J squared)/I) times the foretriangle area".

I used the correct formula in calculating the 1.05 adjustment for a 100% jib. If you calculate the foretriangle area of I*J/2 to
191, adjust by the 1.05 and you get the 200.5 that you calculated directly...

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