Three men in a pub!

Allan

Allan

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Last night we were in the pub talking about sails. The subject we could not agree on was how to measure genoa percentages. One said it was foot length compared to J length. one said sail area compared to full jib size. I said LP length compared to LP length of standard jib. Who was right?
Allan
 
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Allan said:
Last night we were in the pub talking about sails. The subject we could not agree on was how to measure genoa percentages. One said it was foot length compared to J length. one said sail area compared to full jib size. I said LP length compared to LP length of standard jib. Who was right?
Allan
Click to expand...
Does this help? sail dims
 
Allan said:
It is interesting but I don't think it will answer our question. The question is really, what is it 130% 150% etc. of?
There is beer resting on this.
Allan
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What about :-

155% Genoa = (( J x I ) / 2) x 1.65

135% Genoa = (( J x I ) / 2) x 1.44

100% Jib = ( I x J ) / 2

Taken from the link
 
Stork_III said:
What about :-

155% Genoa = (( J x I ) / 2) x 1.65

135% Genoa = (( J x I ) / 2) x 1.44

100% Jib = ( I x J ) / 2

Taken from the link
Click to expand...
Yes I saw that, my point was that saying 135% is 144% of the foretriangle doesn't show what it is 135% of. If that makes sense?
Twister Kens' explanation seems easier and may give the same answer as above. My maths is not good enough to work it out!
Many thanks for your input.
Allan
 
If say J = 1, LP=1.35 then 1.35/1 as a %age gives 135%. it's just the ratio of two lengths as a percentage. As both lines are offset at an angle which varies as a geometry of the rig its not really 'of' anything other than as described... Gives a relative overlap compared to a normal jib which is 100% as LP would equal J so small clearance of clew past mast.
 
Allan said:
Yes I saw that, my point was that saying 135% is 144% of the foretriangle doesn't show what it is 135% of. If that makes sense?
Twister Kens' explanation seems easier and may give the same answer as above. My maths is not good enough to work it out!
Many thanks for your input.
Allan
Click to expand...
Ok, Wiki says:-
Genoas are categorized by the percentage of overlap. This is calculated by looking at the distance along a perpendicular line from the luff of the genoa to the clew, called the LP (for "luff perpendicular"). A 150% genoa would have an LP 50% larger than the foretriangle length.
 
Stork_III said:
Ok, Wiki says:-
Genoas are categorized by the percentage of overlap. This is calculated by looking at the distance along a perpendicular line from the luff of the genoa to the clew, called the LP (for "luff perpendicular"). A 150% genoa would have an LP 50% larger than the foretriangle length.
Click to expand...
Do you read "foretriangle length" as the length of the J measurement or the length of the foretriangle LP?
Allan
 
chewi said:
Which is what I said in post #2!

how can peeps find the maths too difficult?
Its primary arithmetic!
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Sorry I missed your post #2.
It seems that there are two schools of thought.
1. Ratio of LP to J
2. Ratio of area or sail LP to foretriangle LP.
The only maths I find difficult are trying to work out if the factors in the link from post #3 give the same results as 2. above.
Allan
 
Here's another link which thinks that percentage is LP/J:
http://www.jasperandbailey.com/tech/headsails.html
and another:
http://www.secondwindsails.com/measurement.php
and another:
http://dan.pfeiffer.net/boat/ratios.htm
and another:
http://www.usedsails.com/index.htm?measuring.htm~right

So a jib which fills the fore triangle is not a 100% jib. It depends on the ratio of I (height of top of forestay above deck) to J (distance between mast base and forestay base), but e.g. where this is 3:1 then a jib which fills the fore triangle is an approx. 95% jib.

The area of a jib (ignoring roach) is LP*sqrt(I*I+J*J)/2. For a 100% jib that's J*sqrt(I*I+J*J)/2.
 
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davidaprice said:
Here's another link which thinks that percentage is LP/J:
http://www.jasperandbailey.com/tech/headsails.html
and another:
http://www.secondwindsails.com/measurement.php
and another:
http://dan.pfeiffer.net/boat/ratios.htm
and another:
http://www.usedsails.com/index.htm?measuring.htm~right

So a jib which fills the fore triangle is not a 100% jib. It depends on the ratio of I (height of top of forestay above deck) to J (distance between mast base and forestay base), but e.g. where this is 3:1 then a jib which fills the fore triangle is an approx. 95% jib.

The area of a jib (ignoring roach) is LP*sqrt(I*I+J*J)/2. For a 100% jib that's J*sqrt(I*I+J*J)/2.
Click to expand...
Many thanks. That means all three of us were wrong, as were the other people on here that agreed with 2. in my previous post.
At least the three of us from the pub can be assured that we are not the only ones that are wrong!
Wiki must mean the length along the boat ie. the J measurement.
I really thought that I was up for some free beer as I thought my definition came from Andy Lees at Crusader but I must have misunderstood what he said.
Allan
 
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