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01-11-2013 03:34 PM | |
AdamLein |
Re: Reefing a headsail??? Quote:
Originally Posted by RichH
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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.
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 11:45 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 09:35 PM | |
RichH |
Re: Reefing a headsail??? Quote:
Originally Posted by deltaten
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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 05:07 PM | |
sailingfool |
Re: Reefing a headsail??? Quote:
Originally Posted by AdamLein
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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. 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 03:48 PM | |
AdamLein |
Re: Reefing a headsail??? Quote:
Originally Posted by sailingfool
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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. 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 03:17 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 12:55 PM | |
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 09:41 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 03:05 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 01:03 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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