Controlling draft on flat cut sail?

Discussion in 'Boat Design' started by dustman, Aug 27, 2020.

  1. Will Gilmore
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    Will Gilmore Senior Member

    I am not a sailmaker, but it seems to me that with the gaff above and a boom at the foot, don't capture the foot and you can get a flat sail to take a good airfoil shape in the 2D cross section. Not having a deep batten roach nor a captured foot on a triangular sail, there is no need to cut for a 3D camber.

    Control the draft with outhauls. It should work well enough for what you are looking to do.

    Yes, you are kind of reinventing the wheel a bit, but sometimes we have to try out our own ideas, no matter the "inefficiency" ('inefficiency' is a term completely relative to purpose, by the way).

    -Will (Dragonfly)
     
    Last edited: Sep 1, 2020
  2. gonzo
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    gonzo Senior Member

    He is not reinventing the wheel. He is getting a perfectly round wheel, taking the air off the tire and thinking is going to roll better. Making a sail with some camber on it is not a secret science, but very easy. It takes no more work to sew a curved seam than a straight one.
     
  3. dustman
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    dustman Junior Member

    Ease and price. Of the many skills I have sowing is not one of them. And I don't want to pay thousands of dollars for sails. If I can somehow rig a flat cut sail to perform nearly as well as a complex shape sown into the sail itself then that is what I prefer to do.
     
  4. dustman
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    dustman Junior Member

    Ok, I don't think I have given you all enough information. Too many assumptions being made. I'll have to do some more sketching to show what is in my mind. The sketch I made is not adequate, and excludes important details, was meant just to show may proposed layout with the lines. I'll probably end up going with a little lower aspect sail. From all the reading I've done, it seems as though a low aspect sail is kind of a waste of sail area since the vortices at the ends basically negate the lift of the fat ends of the sail, and cause more overall drag.

    A 24' x 4" 6061 aluminum 1/8" walled mast tube would weigh 43 lbs. Not very heavy. 70ft2 of sail would have approx 300 lbs of wind pressure at 40mph. That tube, according to the calculator, should bear 2.5 times that load before starting to buckle at that wind speed. That should be strong enough to account for gusts. Not to mention I'll be reefing well before that.

    One of these days I'll be sailing right by you guys with my square wheel ;) Or not, then you can say you told me so.
     
  5. Blueknarr
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    Blueknarr Senior Member

    The sentiment of nearly every sailer. But not going to happen.

    Shape=performance

    How are you doing to add atchment points with out sewing?

    Used sails can be a great value.
     
  6. Rumars
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    Rumars Senior Member

    Well, I suppose the problem is not that you don't know how to plant seeds in the ground, and you are actually meaning sewing. Glue and the speed sticher were invented just for people like you.

    As for shaping a sail there are many ways, from simply adding round to the edges, to traditional broadseaming and modern shaped panels. Then there are the more exotic methods like wingsails.

    Anyway I wish you luck with your quest and please keep us updated. I am really interested how you will solve the mast bury with a centerline rig.
     
  7. dustman
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    dustman Junior Member

    Here is an illustration of the potential failure points in a stayed rig. This isn't even all inclusive. Screen Shot 2020-09-01 at 5.55.11 PM.png
     
  8. dustman
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    dustman Junior Member

    Unstayed. Screen Shot 2020-09-01 at 5.58.53 PM.png
     
  9. Blueknarr
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    Blueknarr Senior Member

    Now how to construct a sufficiently strong mast base for an un-stayed mast on a beach-cat?
     
  10. dustman
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    dustman Junior Member

    As soon as I have time I will make some drawings and post them. I have a long, exhausting stint of work coming up so it may be a bit.
     
  11. dustman
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    dustman Junior Member

    Thank you, Will.

    I am contemplating how to work this into my rig while maintaining it's simplicity.
     
  12. dustman
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    dustman Junior Member

    And while you're here, what's y'alls take on the biplane rig. I had considered it previously, but saw too much talk about interference on certain points of sail.
     
  13. clmanges
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    clmanges Senior Member

    Sewing is an easy skill to pick up and can probably save you a lot of money. If you want to make your own sails, though, the hardest part may be finding a space to lay the cloth out on to mark your patterns and cut it. This operation in itself might require a helper; I don't know.

    You can do a surprising amount with a regular home sewing machine. My father made a rig for a 17' Grumman canoe and did everything himself, including sewing the sail. I think he bought a set of plans and the sail pattern. Mom growled at him about potentially wrecking her machine, but it handled it okay. The biggest limitations of the machine will be the strength of its motor and the amount of material that can fit under the presser foot, but they don't make 'em like they used to; new machines are flimsily constructed. I just picked up a 30+ year-old machine for $25, and it weighs fifty pounds because it's as much solid steel as it can possibly be.

    If you do decide to try this, look for a walking-foot machine with a large capacity under the foot. You might find a used one in good shape if you're lucky; the new ones are expensive.

    You can put a belly in a flat sail with as little as one dart, very easy to do; you just have to know where to put it and the dimensions it needs to be. Here's an example from the PDR pages:

    Sail Shaping - Why it is important http://pdracer.com/sail/sail-shaping/

    Another excellent resource is:

    https://www.sailrite.com/
     
  14. CT249
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    CT249 Senior Member

    Your last sentence indicates that you're certainly not someone who is actually asking for objective feedback. Do you really think that everyone else with far more experience is so ******* stupid that someone with no experience knows vastly more?

    A pair of 43lb masts in a small cat IS heavy. In small cats, you're not always reefing before you meet stronger winds than you expect, because issues like acceleration, nosediving and wave-induced wind strength have very considerable effects.

    What's going to keep the boat upright while you reef?

    The 2.5 inch (approx) T6061-T6 alloy mast of a Laser dinghy, which has something like 2mm wall thickness, can and does fail in strong winds. A Laser has dramatically less righting moment than a cat, and it has a "pinhead" sail that lowers the heeling moment. I'm no engineer but I would be surprised if such a thick section without stays would survive on a cat. As a top multi designer once pointed out to me, stays are incredibly efficient from an engineering viewpoint which is why they are used in bridges, radio masts, etc. The top cat designers and sailors are designing some of the world's most efficient racing boats, and they use wires.

    The drawings showing the failure points in stayed versus unstayed masts are wrong, as are many points in the article such as the utter rubbish claims that rating rules require wires. Anyone who writes an article that contains such false statements, and who has not corrected those statements when their falsity has been pointed out on this very forum as happened, is ignoring reality. I have sailed boats that have broken gear in the points indicated in the drawing, and yet the rigs have stayed up. I also own unstayed rigs and have had them snap, in ways the drawing does not allow for.

    Low aspect sails are not a waste of area. Even Marchaj shows that they work well at wider apparent wind angles in terms of lift, as well as having lower heeling moment. Designers are not ******, and the good ones use high aspect when they should and low aspect when they should, which in turn depends on righting moment and other factors that affect issues like apparent windspeed and angle. Actually, given the way that "real world" high aspect sails work compared to how they work in theory, I tend to suspect that the theory isn't allowing for what happens in the real world. I may have missed it, for example, but I can't recall seeing any theory to explain why the world's fastest classic A Class cat goes at its best downwind in light airs with 19% mid-chord camber and 20 degrees twist. Similarly, look at windsurfer rigs, where the fastest sails tend to be of fairly low aspect ratio for handling reasons, because high-aspect rigs don't perform on the water in the same way they do in theory.

    By the way, you can put draft in a sail without any seam taper. A surprising number of windsurfer sails are built that way, creating draft simply by using luff curve. However, to flatten sails by luff and leach pressure can dramatically increase compression on the mast, which can cause breakages in unstayed masts.

    The line in your sketch that leads to the aft corner of the head will cause the sail to develop enormous depth to develop, because forces will follow a straight line; in fact it's hard to see how the sail pictured will work with any efficiency at all.

    Top designers and sailmakers are NOT brain dead fools who have spent decades doing stupid stuff. They do what they do because it works. Top designers are often incredibly open and helpful to people who respect them enough to ask "why was a design done this way" and learn from the answer, instead of assuming that the top designers are wrong.
     
    Last edited: Sep 3, 2020
    Rumars likes this.

  15. dustman
    Joined: Jun 2019
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    dustman Junior Member

    Your last sentence indicates that you're certainly not someone who is actually asking for objective feedback. Do you really think that everyone else with far more experience is so ******* stupid that someone with no experience knows vastly more?

    ---My last sentence was said in jest, it is on you for taking it the way you did. I am here because I know there are many people on here with far more experience than me specifically related to sailboats. But you sell me short, I have been building and fixing things for my entire adult life(and before) as a profession and have developed a wide variety skills and knowledge, and have a pretty keen interest in physics, so I'm not starting from square one. And I never called anyone stupid, or even thought it. I AM here for objective feedback. Not all the feedback I get is objective, however. I am questioning everything in the pursuit of knowledge. And I don't just blindly accept someones interpretation because they have whatever credentials or experience, that would be a mistake.

    A pair of 43lb masts in a small cat IS heavy. In small cats, you're not always reefing before you meet stronger winds than you expect, because issues like acceleration, nosediving and wave-induced wind strength have very considerable effects.

    ---A 32'x16' base seems pretty sizeable to me for the weight and height of the masts and size of the sails. I still need to do the righting moment calculations, which I don't know how to do as yet. I feel it would be hard to save more than 10lbs above deck by going with stayed masts. And I dare say that trying to build in all those lower attachments would be comparable in weight and much more complex. When I decide to reef will be informed by worst possible scenario in whatever condition. I will be very careful until I gain plenty of experience.

    What's going to keep the boat upright while you reef?

    ---Righting moment? I don't really understand the question.

    The 2.5 inch (approx) T6061-T6 alloy mast of a Laser dinghy, which has something like 2mm wall thickness, can and does fail in strong winds. A Laser has dramatically less righting moment than a cat, and it has a "pinhead" sail that lowers the heeling moment. I'm no engineer but I would be surprised if such a thick section without stays would survive on a cat. As a top multi designer once pointed out to me, stays are incredibly efficient from an engineering viewpoint which is why they are used in bridges, radio masts, etc. The top cat designers and sailors are designing some of the world's most efficient racing boats, and they use wires.

    ---The laser has 76ft2 of sail and a 20' mast. Assuming 2.5", 2mm wall, T6061-T6 mast, it would take 230 lbs of force to bend permanently, that would be a little over 30 knots of wind. To do the same to a 4" mast of the same height would take 4 times that force, or double the wind speed. I'm not going with an unstayed mast to save weight, I'm doing it for numerous other reasons.

    The drawings showing the failure points in stayed versus unstayed masts are wrong, as are many points in the article such as the utter rubbish claims that rating rules require wires. Anyone who writes an article that contains such false statements, and who has not corrected those statements when their falsity has been pointed out on this very forum as happened, is ignoring reality. I have sailed boats that have broken gear in the points indicated in the drawing, and yet the rigs have stayed up. I also own unstayed rigs and have had them snap, in ways the drawing does not allow for.

    ---I had no comment on the article itself, was just an illustration of how many individual parts and attachments a stayed mast can have, all potential failure points. An unstayed mast obviously has WAY fewer potential failure points and you have to engineer all those points. It is relatively easy to make the base strong, not as easy to string up a web of lines and build in attachments for all of them.

    Low aspect sails are not a waste of area. Even Marchaj shows that they work well at wider apparent wind angles in terms of lift, as well as having lower heeling moment. Designers are not ******, and the good ones use high aspect when they should and low aspect when they should, which in turn depends on righting moment and other factors that affect issues like apparent windspeed and angle. Actually, given the way that "real world" high aspect sails work compared to how they work in theory, I tend to suspect that the theory isn't allowing for what happens in the real world. I may have missed it, for example, but I can't recall seeing any theory to explain why the world's fastest classic A Class cat goes at its best downwind in light airs with 19% mid-chord camber and 20 degrees twist. Similarly, look at windsurfer rigs, where the fastest sails tend to be of fairly low aspect ratio for handling reasons, because high-aspect rigs don't perform on the water in the same way they do in theory.

    ---I guess some my reasoning for using higher aspect sails was to make up for some of the purported upwind issues of catamarans, cat ketch rigs, and gaff rigs. I don't know what to think really, there are so many different opinions on these topics. I think a lot of the trash talk on catamarans is a bunch of hooey, and doesn't make sense when you really look into it. From my research on aerodynamics its pretty clear that high aspect wings are generally more efficient. High aspect sails with higher lift/drag ratio seem to have an advantage going upwind. Thank you for bringing up the point that a lower aspect sail can maintain lift over a wider range of angle of attack. How much does this come into play while sailing? I imagine it could save you from having to mess with your sails all the time in changing wind conditions, and when your boat is wandering in its course. I looked at those graphs, and from them it seems that a 3:1 aspect ratio is a good all around performer. Have you had experience with different aspect ratios on similar boats? What was your experience? I'd rather go with something closer to 2:1.

    By the way, you can put draft in a sail without any seam taper. A surprising number of windsurfer sails are built that way, creating draft simply by using luff curve. However, to flatten sails by luff and leach pressure can dramatically increase compression on the mast, which can cause breakages in unstayed masts.

    ---My thought was to use a low modulus cloth and rely on evenly tensioning the entirety of the fabric reduce draft and vice versa, as well as leech tension. However, I'm concerned that even a low modulus material wouldn't have enough stretch to shape well at low wind speed. Due to Will's input I am looking into another method.

    The line in your sketch that leads to the aft corner of the head will cause the sail to develop enormous depth to develop, because forces will follow a straight line; in fact it's hard to see how the sail pictured will work with any efficiency at all.

    ---The top has a solid gaff, and the gaff itself is preventing the sail from being pulled to the mast(same with the boom), so the aft end of the sail will only see vertical tension. That illustration was only to show the layout of the lines. There are a great many details omitted.

    Top designers and sailmakers are NOT brain dead fools who have spent decades doing stupid stuff. They do what they do because it works. Top designers are often incredibly open and helpful to people who respect them enough to ask "why was a design done this way" and learn from the answer, instead of assuming that the top designers are wrong.

    ---The reason we have everything we have now is precisely because not everyone clung to tradition. These top designers and sailmakers owe their current knowledge to those people who didn't take the word of those in the past as gospel. I don't think that they are fools, I just think that I want to take a different approach that is more likely to have a good outcome in my specific case. Part of that is keeping everything simple so I can actually build the thing in a reasonable amount of time on a reasonable budget. And honestly I think things are made way too complicated now, to a point where it no longer serves its intended purpose and is unreliable.
     
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