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Go Back   SailNet Community > Skills and Seamanship > Learning to Sail > Reefing a headsail???
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Thread: Reefing a headsail??? Reply to Thread
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Topic Review (Newest First)
01-11-2013 02:34 PM
AdamLein
Re: Reefing a headsail???

Quote:
Originally Posted by RichH View Post
Once you arrive/calculate (or do this 'graphically') all further dimensions will be directly proportional (ratio'd). Eg. a 120% LP will have is 'foot dimension' at ~120% of the 'J' dimension.
Hm... this is essentially saying, "If x is an altitude of a triangle, then an adjacent side is also x." That's only true if that side is perpendicular to the side that the altitude is perpendicular to. I think the angle at the tack of a jib is usually less than perpendicular.

There is a proportionality, but only in the component of the measurement perpendicular to the luff.

Okay, to anybody still confused about LP, it's just the width of the sail. Just imagine stretching the sail out with the luff along the ground and the clew held up off the ground so the sail is flat. The LP is now the height of the clew off the ground.
01-10-2013 10:45 PM
deltaten
Re: Reefing a headsail???

Ummmm... yessssss.. 592 ft/2 is more than entire sailplan PLUS the wetted surface of the hull !
So then, the discussion is the manner in which I calculate the jib ....OK; but the OP was about reefing. Does it matter if I wanna reef a 150% genoa or a 145% genoa?. Any way ya slice it, it's still too damm'd big ta suit me... being a novice and all

Tastes great....less filling!
For all intents and porpoises... same-o, same-o. I don't feature being knocked down; whether it's a 150 or a 140. JUst hafta wait and see what the learning curve is... *THEN* get knocked down! LOL

thanx guise
01-10-2013 08:35 PM
RichH
Re: Reefing a headsail???

Quote:
Originally Posted by deltaten View Post
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??
Its really VERY SIMPLE once you define 100%LP dimension that equates to the "J" dimension. Once you arrive/calculate (or do this 'graphically') all further dimensions will be directly proportional (ratio'd). Eg. a 120% LP will have is 'foot dimension' at ~120% of the 'J' dimension. .... you just have to first establish the 100% LP dimension in relation to the J dimension, .... first.
01-10-2013 04:07 PM
sailingfool
Re: Reefing a headsail???

Quote:
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...
01-10-2013 02:48 PM
AdamLein
Re: Reefing a headsail???

Quote:
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.
01-10-2013 02:17 PM
Faster
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
01-10-2013 11:55 AM
sailingfool
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.
01-10-2013 08:41 AM
deltaten
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??
01-10-2013 02:05 AM
RichH
Re: Reefing a headsail???

Here ya go on determining the LP (length perpendicular) ... see the second dwg. on the page.
Sail Measurement Assistance
01-10-2013 12:03 AM
jackdale
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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