How can you tell how much draft to put into the mainsail? This is something I always have been experimenting with and have not been able to come up with any hard answers. I know that you should flatten the sail as the wind increases, and when the wind is very light, but how much? What is very light? How much is too much? What do you look for to tell? About the only thing I have been able to discover is that if I have too much draft down low, the bottom tell tales back wind when the top ones fly.
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Excellent question! Draft means power. For example, if you need to fight through a chop, a little extra draft will help. The down side of draft is that it comes at the price of increased drag. As with all sail controls, speed is king. The best way to adjust your draft is to watch the knot log. Remember to wait for up to 30 seconds for the speed to stabalize. If it goes up, try a little more adjustment. Of course, if it goes down you're adjusting the wrong way. With time you will learn to get close on the first shot, then use the kont log for fine adjustments.
13%
Maximum draft in your main should be 13% If you have a loose footed main measuring the draft at the foot is easy. If the foot is 10 feet long the maximum draft should be 13% or 1.3 feet (about 1 foot 4 inches). In Light Air (below 6 knots) the sail should be flatter and will need lots of twist. In Breeze (over 6 knots) the sail should have very little twist and full draft until the boat is fully powered up (all crew on weather rail and helm balanced). As the breeze builds the sail needs to be depowered to balance the helm. Flatten first, then open the top leech with the cunningham. Set the sail so the boat is fully powered in the LULLS, then dump the traveler or play the sheet to keep her on her feet in the gusts. When you are over powered in the lulls, reef.
Maximum draft in your main should be 13% If you have a loose footed main measuring the draft at the foot is easy. If the foot is 10 feet long the maximum draft should be 13% or 1.3 feet (about 1 foot 4 inches). In Light Air (below 6 knots) the sail should be flatter and will need lots of twist. In Breeze (over 6 knots) the sail should have very little twist and full draft until the boat is fully powered up (all crew on weather rail and helm balanced). As the breeze builds the sail needs to be depowered to balance the helm. Flatten first, then open the top leech with the cunningham. Set the sail so the boat is fully powered in the LULLS, then dump the traveler or play the sheet to keep her on her feet in the gusts. When you are over powered in the lulls, reef.
OK, Does the 13% work when close hauled?
When close hauled, do you still want the full draft? It lookes like too much of a bubble in my main when I do. the San Juan 21 is a 7/8 fractional rig with a small 110 jib.
When close hauled, do you still want the full draft? It lookes like too much of a bubble in my main when I do. the San Juan 21 is a 7/8 fractional rig with a small 110 jib.
It should
In any breeze up until you are fully powered up, the main should be as powerful as possible, particularly on a Frac with small headsail. About 13% is as deep as you can go and still have attached flow on the sail. On a Frac the max draft should be in the upper part of the sail (above the jibstay) and somewhat flatter below (about 8%) to avoid the backwind bubble. Less draft in flat water (less drag/power), more in waves/chop (because the boat has to accelerate through each wave). A 110 is the perfect sized sail for a Frac. There is a low pressure area just to leeward of the mast caused by the main. The leech of a small jib should be in this low pressure area. The jib then has lower pressure and higher airspeed at it's leading edge (opposite of normal), this works to create much higher airspeed over the entire sail plan than either sail would have by itself. If you are into sailing quickly, slug through Frank Bethwaite's book, "High Performance Sailing". If you can stay awake and get through it, you will change the way you sail.
In any breeze up until you are fully powered up, the main should be as powerful as possible, particularly on a Frac with small headsail. About 13% is as deep as you can go and still have attached flow on the sail. On a Frac the max draft should be in the upper part of the sail (above the jibstay) and somewhat flatter below (about 8%) to avoid the backwind bubble. Less draft in flat water (less drag/power), more in waves/chop (because the boat has to accelerate through each wave). A 110 is the perfect sized sail for a Frac. There is a low pressure area just to leeward of the mast caused by the main. The leech of a small jib should be in this low pressure area. The jib then has lower pressure and higher airspeed at it's leading edge (opposite of normal), this works to create much higher airspeed over the entire sail plan than either sail would have by itself. If you are into sailing quickly, slug through Frank Bethwaite's book, "High Performance Sailing". If you can stay awake and get through it, you will change the way you sail.
Here's a good book for you...
"Getting Started in Sailboat Racing" by Adam Cort. It's only $14.95 and if you can't find it in your local nautical book store you can buy it on line right here. The chapters devoted to boat speed are very concise, easy to read and NOT BORING. Your bubble comment is specifically addressed in the aforementioned book. What you will find is that it is a combination of things, not just draft depth, that will give you more speed or power. Learn to look at the whole picture, the association between headsail and mainsail and the particulars on YOUR boat that only YOU can see or feel. Experiment with different settings and watch their effect on your speed log. TAKE NOTES! Then analyze. If they are consistant....put marks on your control lines so you can easily set them for prevailing conditions.
"Getting Started in Sailboat Racing" by Adam Cort. It's only $14.95 and if you can't find it in your local nautical book store you can buy it on line right here. The chapters devoted to boat speed are very concise, easy to read and NOT BORING. Your bubble comment is specifically addressed in the aforementioned book. What you will find is that it is a combination of things, not just draft depth, that will give you more speed or power. Learn to look at the whole picture, the association between headsail and mainsail and the particulars on YOUR boat that only YOU can see or feel. Experiment with different settings and watch their effect on your speed log. TAKE NOTES! Then analyze. If they are consistant....put marks on your control lines so you can easily set them for prevailing conditions.
So how do you measure the draft?
OK, 13% of my 9' sail is 14.04 inches. How do you measure that? Come up with a big straight edge and place it across the boom and mast and measure the depth? Or do we just guess?
OK, 13% of my 9' sail is 14.04 inches. How do you measure that? Come up with a big straight edge and place it across the boom and mast and measure the depth? Or do we just guess?
With a camera
Use a camera. Stick a piece of tape on the boom in the middle of the foot of the sail. Take a picture of the sail with the camera centred on the tape mark and aimed at the headboard. Use a wide angle shoot to get as much of the sail in the frame as possible. Print the picture. Take a ruler and draw a line from one end of each draft stripe to the other. If you don't have draft stripes ... have your sailmaker add them. Measure the length of the line. Measure how far off the line the draft stripe is. Divide distance off by length = draft % Better yet download a free sail analyzer from Carlson Designs and check draft, twist etc. from your digital photo.
Use a camera. Stick a piece of tape on the boom in the middle of the foot of the sail. Take a picture of the sail with the camera centred on the tape mark and aimed at the headboard. Use a wide angle shoot to get as much of the sail in the frame as possible. Print the picture. Take a ruler and draw a line from one end of each draft stripe to the other. If you don't have draft stripes ... have your sailmaker add them. Measure the length of the line. Measure how far off the line the draft stripe is. Divide distance off by length = draft % Better yet download a free sail analyzer from Carlson Designs and check draft, twist etc. from your digital photo.
How Much Draft
Stephen: It is almost an impossable question to answer in general terms because it depends on the boat and the condition of the sail just to name a few things. The first thing to consider is what are the controls used to adjust draft depth (belly) in the main and they are mast bend and the outhaul. For the jib the controls are the fairleads, halyard, mast bend and sheets. Going through the full spectrum of the wind from very light to heavy you want to start with very flat sails (because the wind does not have enough power to get around a sail with any depth) and as the wind start to increase you want to gradually induce belly into the sail. How much belly is the question -you have to play with it to find the best settings for your boat. Once you get at about the 12 to 15 knot range (this wind speed depends on the sailor - some may use high or lower speeds) you want to gradually take the belly out of the sail. When you do that you are reducing the power of the sails. Probably the max draft depth you would want to induce in the main or jib is about 30%. Everyone know how to calculate the % of draft depth or draft position - right? If you don't know how to calculate it just sing out and I'll explain it.
Stephen: It is almost an impossable question to answer in general terms because it depends on the boat and the condition of the sail just to name a few things. The first thing to consider is what are the controls used to adjust draft depth (belly) in the main and they are mast bend and the outhaul. For the jib the controls are the fairleads, halyard, mast bend and sheets. Going through the full spectrum of the wind from very light to heavy you want to start with very flat sails (because the wind does not have enough power to get around a sail with any depth) and as the wind start to increase you want to gradually induce belly into the sail. How much belly is the question -you have to play with it to find the best settings for your boat. Once you get at about the 12 to 15 knot range (this wind speed depends on the sailor - some may use high or lower speeds) you want to gradually take the belly out of the sail. When you do that you are reducing the power of the sails. Probably the max draft depth you would want to induce in the main or jib is about 30%. Everyone know how to calculate the % of draft depth or draft position - right? If you don't know how to calculate it just sing out and I'll explain it.
Exactly how do you calculate draft?
I have your book and chart and I am not sure - I think you take the J dimension and multiply by percent to get feet. Is this correct? Thanks
I have your book and chart and I am not sure - I think you take the J dimension and multiply by percent to get feet. Is this correct? Thanks
Calculate draft
Jerry: The following is not the scientific method and I won't bore you with the actual definition. The first thing you want to do is measure the foot of your sail. Lets say it is 10'. Now say you want to induce 30% of draft depth (belly). That means you have to put in a 3' belly. Suppose you only want 20% - that would be 2'and on and on. That takes care of draft depth. Draft position is a bit more difficult because it is hard to see until you get the hang of it. In its simpliest terms draft position is an imaginary line that runs from the top of the sail to the bottom and through the deepest part of the belly. In my book I say to mark the 50% position (5' in our example) on the boom but I'm not sure that is a good idea anymore. Anyway, lets say you want the draft position to be set at 50%. I didn't want to discuss the "chord of the sail" but you want the imaginary line to intersect the chord line on the foot of the sail, somewhere around the 5' mark. If you wanted to set the draft position at 45% the imaginary line would fall on the 4'1/2" mark. 55% would be 5'1/2". Draft position is very important because you want this line to fall in the 45% to 55% range because as the wind pipes up it want to push the DP aft and the farther aft it goes the less efficient the sail becomes. I wish I could come up with a better system than marking the boom at the 50% location because beginners need a reference pont to see DP. You can tell a begginer all day what DP is and they will never get it. They have to see it. I know some pretty good seat of the pants sailors and when I tried to show them DP they could not see it either. Eventually they did but it took a little time. Once you know what to look fo you can actually move your DP sail trim controls and see it move forward and aft.
Jerry: The following is not the scientific method and I won't bore you with the actual definition. The first thing you want to do is measure the foot of your sail. Lets say it is 10'. Now say you want to induce 30% of draft depth (belly). That means you have to put in a 3' belly. Suppose you only want 20% - that would be 2'and on and on. That takes care of draft depth. Draft position is a bit more difficult because it is hard to see until you get the hang of it. In its simpliest terms draft position is an imaginary line that runs from the top of the sail to the bottom and through the deepest part of the belly. In my book I say to mark the 50% position (5' in our example) on the boom but I'm not sure that is a good idea anymore. Anyway, lets say you want the draft position to be set at 50%. I didn't want to discuss the "chord of the sail" but you want the imaginary line to intersect the chord line on the foot of the sail, somewhere around the 5' mark. If you wanted to set the draft position at 45% the imaginary line would fall on the 4'1/2" mark. 55% would be 5'1/2". Draft position is very important because you want this line to fall in the 45% to 55% range because as the wind pipes up it want to push the DP aft and the farther aft it goes the less efficient the sail becomes. I wish I could come up with a better system than marking the boom at the 50% location because beginners need a reference pont to see DP. You can tell a begginer all day what DP is and they will never get it. They have to see it. I know some pretty good seat of the pants sailors and when I tried to show them DP they could not see it either. Eventually they did but it took a little time. Once you know what to look fo you can actually move your DP sail trim controls and see it move forward and aft.
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