On another forum much opinion about how to calculate and/or approximate and measure size of Genoa sail. As I am a newbie I treat that style of discussion as if it were a multiple choose question on the sailor test. Usually within a dozen posts or sooner The expert will give the short answer then explain it and then quote the rule and where it can be found. Since I don't speak sailor yet (and there's another topic) I often don't get the answer right when I guess. this is one they haven't quite finished yet so I decided to ask You. Here is my guess and extra question. The Genoa size is determined by dividing the 'J' distance by the 'LP' distance and expressing it as a percentage. Where 'J' is the perpendicular distance from the mast to the headstay at the deck and 'LP' is the perpendicular distance from the luff of the Genoa to the clew. sail size as a proportion using a right triangle should be useful to determine need. What's the difference between a Genoa and a Jib or are they the same? Is a headstay and a forstay the same? if the sailboat has a bowsprit is the 'J ' measured from the tip of the bowsprit?
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no trig involved...
'J' is measured from the base of the mast to the point of attachment of the jib...wherever that actual point may be. No right triangles involved in figuring genoa size. Take a right angle square(carpenters works well), place one leg parallele against the luff near the bottom(tack), slide it up until the inward pointing side of the square points directly at the clew. Mark the spot on the luff. The distance from that mark to the clew is the Luff-Perpendicular(LP). Divide LP by J multiply that by 100 and you have genoa size as a percentage. LP/J * 100 = genoa% ex: LP =15' J=10' then genoa is a 150% easy huh!
'J' is measured from the base of the mast to the point of attachment of the jib...wherever that actual point may be. No right triangles involved in figuring genoa size. Take a right angle square(carpenters works well), place one leg parallele against the luff near the bottom(tack), slide it up until the inward pointing side of the square points directly at the clew. Mark the spot on the luff. The distance from that mark to the clew is the Luff-Perpendicular(LP). Divide LP by J multiply that by 100 and you have genoa size as a percentage. LP/J * 100 = genoa% ex: LP =15' J=10' then genoa is a 150% easy huh!
Simple math again David
1) The longest dimension will be the luff. So usinf a scale i.e 1 foot = 1/4 inch, Trace a line representing the luff on a piece a paper 2) Second longest dimension should be the leech. Using same scale as for the luff, draw a circle whose radius will be equal to the leech lenght scale from one end of the luff line . 3) Now, using again the same scale, draw another circle from the other end of the luff line, using the foot scale lenght as a radius. 4) Your clew will be at the intersection of the 2 circles. 5) Draw a line from that clew to a point crossing the luff line at exactly 90%. That will be your LP line 6) Measure your LP line, then calculate actual real lenght of LP using same scale of 1/4" = 1 foot 7) Then do as Didereaux suggests: LP/J X 100 and voila, you have the % for your boat. Good luck
1) The longest dimension will be the luff. So usinf a scale i.e 1 foot = 1/4 inch, Trace a line representing the luff on a piece a paper 2) Second longest dimension should be the leech. Using same scale as for the luff, draw a circle whose radius will be equal to the leech lenght scale from one end of the luff line . 3) Now, using again the same scale, draw another circle from the other end of the luff line, using the foot scale lenght as a radius. 4) Your clew will be at the intersection of the 2 circles. 5) Draw a line from that clew to a point crossing the luff line at exactly 90%. That will be your LP line 6) Measure your LP line, then calculate actual real lenght of LP using same scale of 1/4" = 1 foot 7) Then do as Didereaux suggests: LP/J X 100 and voila, you have the % for your boat. Good luck
Moving on to High School Math
David, buying a used sail is going to involve a little geometry and trigonometry. Besides the LP measurement, you need to calculate where that clew is going to be in relationship to your boat. For a given LP, the clew can be placed relatively high (a.k.a “Yankee Cut”) or low (a la “deck sweeper”). The lower clew will give you more drive and power at the expense of more weather helm, the sail getting hung up on life lines when you tack, and prone to being overpowered in gusts. The higher clew can carry higher winds but by sacrificing power. A clew placed too high or too low might mess up the geometry of the fairleads. You might not have a track long enough for proper fairlead placement. You will need to do some trig to convert the perimeter measurements to LP. You will also want to take down your current sail and measure the edges and LP so you will have something to compare against. A 150% genoa is pretty much a light air specialty sail and you will find that most clews will be rather low – perhaps lower than you want for your (general) purposes. You could consider getting a smaller sail (like, a 135%) with a low foot which will give you almost the same drive as the 150, but you will be able to carry it in higher winds. If it is really light conditions, you can use your a-kite in a reaching mode. A thought on foam luffs. Reefing a roller furling headsail doesn’t work very well IMHO. When the wind picks up the first thing you want to do is flatten sails which is done by tensioning the halyard for the headsail, thus stretching the luff. Once you begin to roll in the sail, this tension is transferred from the sail to the furling gear. Furthermore, the remaining tension is transferred to the head and the tack with little support in the middle part of the luff. This part then sags out and you loose the flat shape you were trying to achieve in the first place. Sail makers will try to compensate for this by sewing in foam strips into the sail so that when you furl, the added material will bind up, giving you more tension in the luff. Does it work? The fast guys say no, the guy selling you the luff will say “pretty good” and practically no one will emphatically say, “yes, if works just as good as a sail with a smaller LP”.
David, buying a used sail is going to involve a little geometry and trigonometry. Besides the LP measurement, you need to calculate where that clew is going to be in relationship to your boat. For a given LP, the clew can be placed relatively high (a.k.a “Yankee Cut”) or low (a la “deck sweeper”). The lower clew will give you more drive and power at the expense of more weather helm, the sail getting hung up on life lines when you tack, and prone to being overpowered in gusts. The higher clew can carry higher winds but by sacrificing power. A clew placed too high or too low might mess up the geometry of the fairleads. You might not have a track long enough for proper fairlead placement. You will need to do some trig to convert the perimeter measurements to LP. You will also want to take down your current sail and measure the edges and LP so you will have something to compare against. A 150% genoa is pretty much a light air specialty sail and you will find that most clews will be rather low – perhaps lower than you want for your (general) purposes. You could consider getting a smaller sail (like, a 135%) with a low foot which will give you almost the same drive as the 150, but you will be able to carry it in higher winds. If it is really light conditions, you can use your a-kite in a reaching mode. A thought on foam luffs. Reefing a roller furling headsail doesn’t work very well IMHO. When the wind picks up the first thing you want to do is flatten sails which is done by tensioning the halyard for the headsail, thus stretching the luff. Once you begin to roll in the sail, this tension is transferred from the sail to the furling gear. Furthermore, the remaining tension is transferred to the head and the tack with little support in the middle part of the luff. This part then sags out and you loose the flat shape you were trying to achieve in the first place. Sail makers will try to compensate for this by sewing in foam strips into the sail so that when you furl, the added material will bind up, giving you more tension in the luff. Does it work? The fast guys say no, the guy selling you the luff will say “pretty good” and practically no one will emphatically say, “yes, if works just as good as a sail with a smaller LP”.
Lloyd and David W
Lloyd: To answer some of your other questions... 1)A Genoa is a jib that overlaps your mast or main. So, when your headsail dont overlap, it is a jib. 2)Yes, a forestay and headstay are the same thing. David W One thing you might do before you make up your mind on whether a 150 or a 135 is to check with a lot of your local sailors with similar size/weight boats. Talk to several, not just one or 2. When I was living on the northern gulf coast almost everyone had a 150 because the average winds were fairly light. Now I'm on the south Texas coast and most i have met use a 135 due to the average winds being much stronger. As for how much can be furled in, that is a matter of how your boat will respond to it. Not all boats fall into the norm of i think approx 25-30% rolled in. I have a Catalina 30 and i have a 150 on my roller furling. I have had it furled down to about 100% on many many ocasions and i still maintained control and exceeded hull speed. I know many will find a million reasons why it wont work, but i will tell you that it does for me. Once your sail is on your roller furling, you wont be able to change it during a blow. Is it better to be safe than sorry or sorry you cant sail 95% of the time because your sail is too small? If it gets that bad, roll in your jib all the way, reef your main amd motorsail back to the dock. If you have the money in your budget, get the 135, a storm jib, a spinnaker and a cruising spinnaker. Unfortunately i dont have the budget or the room aboard for all the sails i "need" so i'll just stay with my 150 for now.
Lloyd: To answer some of your other questions... 1)A Genoa is a jib that overlaps your mast or main. So, when your headsail dont overlap, it is a jib. 2)Yes, a forestay and headstay are the same thing. David W One thing you might do before you make up your mind on whether a 150 or a 135 is to check with a lot of your local sailors with similar size/weight boats. Talk to several, not just one or 2. When I was living on the northern gulf coast almost everyone had a 150 because the average winds were fairly light. Now I'm on the south Texas coast and most i have met use a 135 due to the average winds being much stronger. As for how much can be furled in, that is a matter of how your boat will respond to it. Not all boats fall into the norm of i think approx 25-30% rolled in. I have a Catalina 30 and i have a 150 on my roller furling. I have had it furled down to about 100% on many many ocasions and i still maintained control and exceeded hull speed. I know many will find a million reasons why it wont work, but i will tell you that it does for me. Once your sail is on your roller furling, you wont be able to change it during a blow. Is it better to be safe than sorry or sorry you cant sail 95% of the time because your sail is too small? If it gets that bad, roll in your jib all the way, reef your main amd motorsail back to the dock. If you have the money in your budget, get the 135, a storm jib, a spinnaker and a cruising spinnaker. Unfortunately i dont have the budget or the room aboard for all the sails i "need" so i'll just stay with my 150 for now.
sail size
The formulas given previously are probably more accurate, but a sail maker gave me this formula for rough dimensions. (J x % x I)/2 Insert 1 for a 100, 1.35 for a 135, 1.5 for a 150 etc.
The formulas given previously are probably more accurate, but a sail maker gave me this formula for rough dimensions. (J x % x I)/2 Insert 1 for a 100, 1.35 for a 135, 1.5 for a 150 etc.
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