Three men in a pub!

[Allan]

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

 

[chewi]

I understand it to be ration of LP(sail) to LP(foretriangle)

 

[Stork_III]

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

Does this help? sail dims

 

[Allan]

Does this help? sail dims

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

 

[Twister_Ken]

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

The percentage of the foretriangle that the sail represents.

 

[Allan]

The percentage of the foretriangle that the sail represents.

Many thanks, I've just realised that would mean two of us are correct!
Allan

 

[Plevier]

(LP/J)*100 = % overlap (that's what both Kemp and Crusader told me anyway!)

 

[neilf39]

As far as I know its LP divided by J so length of line line at right angles to the Luff from Luff through to the clew divided by the length between front of mast and forestay.

Regards
Neil F

 

[Stork_III]

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

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

 

[Allan]

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

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

 

[neilf39]

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.

 

[Stork_III]

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

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.

 

[Allan]

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.

Do you read "foretriangle length" as the length of the J measurement or the length of the foretriangle LP?
Allan

 

[westhinder]

Do you read "foretriangle length" as the length of the J measurement or the length of the foretriangle LP?
Allan

Logically that must be 150% of foretriangle LP

 

[chewi]

Logically that must be 150% of foretriangle LP

Which is what I said in post #2!

how can peeps find the maths too difficult?
Its primary arithmetic!

 

[Allan]

Which is what I said in post #2!

how can peeps find the maths too difficult?
Its primary arithmetic!

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

 

[davidaprice]

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.

 

[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.

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