I’ve noticed on this forum and also Windpower that there is lots of discussion about hull shape. Add a little rocker here- point the bow-flatten the stern-narrow the stern. That kind of stuff. But as to the engine of the boat-the thing that makes it go fast(or slow) there is little or no discussion about design. Why is that? All sorts of people build their own hulls which is a fairly complex procedure. Making sails is actually less complex from a construction point of view. The complexity in sail making is in the design and yet, unlike hull design, there is silence. Are sailmaker sworn to silence by their guild or is it a CIA plot to keep this info secret[:D].
It’s a beautiful day but the wind is blowing too hard to test the changes I made to my “A” rig so I’m bored! [:-grumpy]
Just like a master mechanic who can tune every last horsepower out of an engine but will not tell anybody else how he does it, some of us make a living by experimenting and perfecting sail designs. Giving it away does not put food on the table for our families.
We spend hundreds of hours cutting material and testing out sail shapes.
Just the drawings for our laser cut sails are several hours each.
Also, if I am racing against you, why would I give you my design secrets? [:D]
I am not a sail maker, so I can’t tell you their secrets. But I have talked to a few guys that have made their own sails so I can tell you some of the amature tricks…
There are two things you want to design into the sails - camber and twist. There are two ways to add camber to the sails. One is by adding some extra material mid-seam and the other is by adding luff round. Twist can only be added with extra material near the back of the seams.
As far as how much of each to add - well that is where the trial and error comes in.
One of the best ways I have seen to eliminate the trial and error is to build a mold. Then you can lay the pannels on the mold and the shape of the mold will force the panels to line up at the seams with the right amount of exra material added in the right places. Then it is a simple matter of carefully trimming the panels and maiking the seams. I think most sailmakers these days use glue to attach the seams, but a few still sew their seams.
As far as the shape you want to build into your mold. I would shoot for about 10% draft about 40% chord at the foot. Near the head, the camber can be a little less (say 6%-8%) and the max draft a bit further back (50%). I would aim for 15 degrees of twist from top to bottom. A bit less (say 10 degrees) with a full roached sail.
That’s about all I know… It’s not much and it is only my interpretation of the best sail shapes I have seen. Each sail maker has his own idea of what the optimal shape is. Some like more twist than others, some like more luff curvature than others and so on.
Don
Great post. Ive often thought the same thing. I think Peters mechanic analogy is a good on. I think its easy to see when a hull is not working properly, the wave pattern and trim etc are easy to see. But sails are harder to figure out (to me atleast!). Sometimes you can have two sets of sails which look almost the same and both set well but one is fast and one is slow. I guess its the subtle changes that make all the difference!
Troy
Peter
I realize there are speed secrets and I’m not asking anyone to reveal thiers. I’m just talking general building and trouble shooting. The average hobby sailmaker is probably not going to build top notch sails and if he lucks out and builds one good set he’s not going to be able to make another. The tolerances are just too fine on a 12" chord to be able to repeat them by hand.
Will
I’ve thought of the mold for a while. I was thinking of buying a sheet of 1" foam insulation and shaping it. I thought that the static cling that foam seems to have would help hold everything in place. What I wasn’t sure of is whether the mold should be slightly exagerated or slightly undersize or the actual shape of the sail. I am concerned that the loose sail might not take the exactly the same shape as the mold. With your mathematical skills you should be able to calculate the amount of curve to cut into the broadseams to arrive at a 10% or 12% chord. Don wants yet another formula! Speaking of broadseams. If a person has say-5 panels, should he cut curve into all 4 seams or just the top and bottom ones? I beleive that in “The Art and Science of Sails” he says that maybe there should be more draft at the top of the sail- Its been a while since I read it, I may be wrong.
Troy
It’s funny you say that hulls are easier to read. It never even occured to me that you could analyze a hull shape by looking at the waves. I can(sort of) read sails by looking at the tell tales. At least to the point of saying I need more or less twist. If I have too much twist and the vang is groaning from the strain I know that the next set of sails should have less twist. It took me a lot of thinking to realize how to cut less twist into the sail.(I should have just asked Will!) Whether a sail needs more or less camber is still a bit of a crap-shoot for me but it will come sooner or later. The subtle differences are very subtle. I have been cutting approximately 1mm. (3/64") curve in my middle length broadseams and that gets me close to 10 or 12% but if I get the joint less than exact(and I mean exact) what I get could be anything. As a last point there are a whole stack of plans for slightly out of date hulls on the USOM site but no where have I seen plans for slightly out of date sails.
After re-reading the original post, and my reply to it, I see you were asking more about sail design than why you do not hear much about it.
The truth is that very few people know a lot about it. As stated in a previous post, it is not something that is easy to visualize. Subtle changes in sail set will change a great sail in to a poor one very quickly. Sails need to be seen to change their design. A lot of pictures are needed of good and bad sails from all angles to see what makes a sail perform well. Another factor is that a certain sail is not perfect in all conditions. Flat sails for strong wind, fat sails for the light stuff. In super light wind, a flat sail is often better.
There are no set rules for a set of sails so it is hard to make specific statements that are true for all models.
If you ask short, direct questions about sail design, I can probably help you out with some answers. I have designed some great sails, and also some real dogs. Both are a learning experience.
Thank you Peter. I started thinking of how to phrase my 1st question and realized how tough this will be without lots of pictures and arrows. I’ll think on it and post a little later.
If it helps I use double sided tape to stick together the sails that I make. Over here it is made by Sellotape. It can be brought in different widths. The one I use is 6mm. Personally I don’t think that a god set of sails is that hard to make.
I find that if you cut a convex curve into the top of each panel works best. The measurements I started with are:-
Firstly make a point about one third along the top of the panel from the luff.
Secondly measure down the luff from the top of the panel 4mm. Next measure down the leech 3mm. Join the three marks with an even curve and cut it out.
Repeat this on all panels.
Place the tape on the straight edge of the panel and stick the corresponding cut edge to it.
Repeat on all panels.
You should now have a rectangle with “shape”.
The next step relates to several points I.E. type of boat, type of mast, normal sailing conditions etc.
I usually measure from the head down the luff to around 1/3 height and put a mark at this point. I then measure along the foot from the luff about 15mm and then along the head about 25mm join these marks with an even curve and cut it out.
Now cut your leech from foot to head.
Attach the bolt rope, slides or however it fits to the mast, set up everything else, boom, vang, cunningham etc and see how it looks. If it seems to sit well go sailing and then see how it looks.
The method of making sails as descibed here works well for me on my mini40 but I understand that the basics of it are fine for any class.
If you want to learn more about sail shape, there are several great discussion out there published by North Sails. While the details are mostly related to full sized sails, many of the principles apply to all size sails:
Also, you might want to look into the sails shape measuring software put out by UK (free download) called AccuMeasure. I use this software quite a lot to ensure that my sail settings are producing optimal twist and draft.
Peter B.
I think your post will help me post my questions. How long is the chord that you are using 4mm and 3mm on? On my USOM the widest panel would be 30mm and I am using around 1 mm or both ends and it seems to be too much. Maybe I have too much luff curve? I should say here that I started with a sail made from SailCut so it already had some twist cut in it but I took the broadseam curve out. This was before I knew what caused twist so the panel still had twist in it. This brings me to my next question. Why do we cut twist into sails? It would seem to me that we can always add twist by loosening the vang but we can only remove twist to the amount we cut in. On my last sail I had too much twist and when I tried to tighten the vang to get rid of it I would get a big crease from the clew to about a third of the way up the luff. I fixed this (sort of)by re-cutting one of the top seams to get rid of some twist. You say in your post"You now have a rectangle with shape". Is that a typo? Shouldn’t that be “triangle”. I’ll leave the luff curve for now.
Will
I’ve read all those sites and more. I have also read all the books I can find. Maybe I already know all the stuff (I wish!!) and just have to get it straight in my head. I am to the point that I know what people are talking about in relation to sails and can (sort of) imagine what a change will do to the shape.
Well, the wind just came up and I have some sail changes to test. I cut some negative luff curve in my jib(I thought I had done that when I built them but somehow it turned out positve) and I re-cut the bottom seam on my main to put a in little more shape. I’ll post the results in a couple of hours.
Don
When I do my sails they are made as a rectangle then cut down to fit the size I want.
I’m working with panels that are 500mm wide.
A certain amount of twist needs to be in all sails for better control and shape, if I have to much twist, I re-cut luff.
Peter
I’m back. Boy,does it ever point better. At times it looked like it was going straight upwind (I exaggerate). The new luff curve on the jib works well. I noticed though, that the jib doesn’t seem to fill as well downwind. It has a few winkles. Is this normal or did I cut to much negative luff? A sort of related thing- I thought that with the jib working better(pointing higher) that I would have a little less weather helm. That didn’t seem to be the case, It almost seemed a little worse. Does pointing higher mean less power? Is that whats happening?
Peter B
Isn’t that an awful waste of sailcloth to cut a rectangle and then throw away a good chunk of it? I realize that sails need some twist but why does it have to be cut into the sail? Why not add it later with the vang? And last, How does recutting the luff affect the twist? I think I’m missing something here.
Yes, negative luff flattens the sail. I knew this before but I never really thought about it until I got Bob Sterne’s video a few days ago. He said that in order to compensate for jibstay sag you have to cut neg. curve in the jib or when the wind fills the jib it will be too full. You can’t see this on the stand because the wind isn’t filling the sails in the same many as when the boat is under way. Boy does it work. I highly recommend Bob’s video. It’s one of the best $50(Can) I’ve spent.
Peter B
I figured out the relationship between luff curve and twist. I had my sail tensioned up on the bench and noticed that when I put positive bend in the mast(more luff curve) that the twist increased. So more luff curve equals more twist and less curve equals less twist. Right? The funny thing is that I was doing this three days ago. Sometimes stuff just has to rattle around in this empty head for a while til it drops into place.
Well, I don?t know that you can say that luff curve creates sail twist really. It?s mast bend in relation to luff curve that is one of the factors in twist. From a straight luff to a 1? luff curve, you can create <s>twist</s><font color=“red”>draft</font id=“red”>. It?s just that the 1? luff curve sail will have a larger tuning range.
<s>Luff</s> twist is adjusted by mast bend, cunningham and boom vang. I?m sure I?m not telling anyone here anything new…
[:-boring]
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by greg V
Well, I don?t know that you can say that luff curve creates sail twist really. It?s mast bend in relation to luff curve that is one of the factors in twist.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Greg
Bear with me a sec. I can be thick. If I were to make two sails identical except for luff curve and hang them both on straight masts would not the one with more luff curve hang with more twist?
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>From a straight luff to a 1? luff curve, you can create twist. It?s just that the 1? luff curve sail will have a larger tuning range.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Can you explain this so even I can understand it? Do you mean that a straight luff sail is already as flat as it gets and that a curved luff sail can be flattened by bending the mast?
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”> Luff twist is adjusted by mast bend, cunningham and boom vang. I?m sure I?m not telling anyone here anything new…
[:-boring]
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
I don’t think I’ve heard the term luff twist, is this the same twist we always talk about? I don’t consider this boring by any stretch. I think you are telling me something new and I imagine there are many other newbys are hanging on your every word.
Thanks
Don
Vancouver Island
Greg
I just went and looked. This thread has been read 195 times in 2 days-I think someone (besides me) is interested.[:D]
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Don
Greg
Bear with me a sec. I can be thick. If I were to make two sails identical except for luff curve and hang them both on straight masts would not the one with more luff curve hang with more twist?<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Yes, you are right about this. The answer is in your next question
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>From a straight luff to a 1? luff curve, you can create <s>twist</s><font color=“red”>draft</font id=“red”> . It?s just that the 1? luff curve sail will have a larger tuning range.
Can you explain this so even I can understand it? Do you mean that a straight luff sail is already as flat as it gets and that a curved luff sail can be flattened by bending the mast?<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>Yes, along with cunningham you can flatten out a sail with a good size luff curve. <s>Twist</s><font color=“red”>draft</font id=“red”> is also created by paneling a sail. The combination of paneling and luff curve define your sails temperament. So, even a straight luff sail will have <s>twist</s><font color=“red”>draft</font id=“red”> if it’s a paneled sail. Actually I think I?m using the wrong term when I say ?twist? Twist is what you get when you bend the mast, etc. I think the word I should be using is <font color=“red”>draft.</font id=“red”>
I don’t think I’ve heard the term luff twist, is this the same twist we always talk about? I don’t consider this boring by any stretch. I think you are telling me something new and I imagine there are many other newbys are hanging on your every word.
Thanks
Don
Vancouver Island<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
That was a typo! I meant twist, not luff twist[:I]
Just to make sure. I define draft as the airfoil shape of the sail.Like the curve in the top an airplane wing. I define twist as the amount the leach curves when looked at from the stern. Like the washout I unintentionally put in the airplane wings I used to build.
Don - I’m not familiar with the other software program mentioned, but search and download SAILCUT (free) software. It starts with a full size sail, but you can enter your r/c dimensions (metric) and it will provide plot dimensions for anyone with access to a XY-plotter/cutter, and it will also provide full size sail sections so you can print on paper and then tape up for general shape. Until wind actually hits the fabric, everything is still estimates.
Within the program you can play with sail draft, chords, twist, luff curve, foot curve, and leech curves, etc. I used it to layout the panels for my test sail on my MultiONE - which was then translated to a fabric sail. Still waiting for ice to leave before I can try them. (8 inches of snow this morning!- Uggh!)
Paper paste up of sail panels: [ smSailPanelPasteUp.jpg](http://www.rcsailing.net/forum1/data/dick lemke/200435195939_smSailPanelPasteUp.jpg)
24.25KB
Easy to see the pieces of paper that are taped up to create each sail panel. Only problem is paper has less ability to take on a compound curve along broad seams than fabric. Still gives a good idea of general sail shape. Fabric Panels under construction: [ smSailSet1.jpg](http://www.rcsailing.net/forum1/data/dick lemke/20043519557_smSailSet1.jpg)
30.05KB