Sail Area / Power

Discussion in 'Boat Design' started by dustman, Sep 2, 2020.

  1. sharpii2
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    sharpii2 Senior Member

    My understanding is that a flat sail will produce about 2/3rds the lift of a cambered one and about half the lift of one with a very exact camber.

    This lift is most important when sailing upwind. It is least important when sailing down wind.

    Another important fact to consider is that the shape of the camber is very important. It must at least approximate an air foil shape in section.

    If the camber has a fat curve at the trailing edge, it may be useless. The first sail I made had that problem. And the boat refused to go to windward.

    Since my knowledge of sail aerodynamics was, shall I say, very limited at the time, I merely took steps to successively flatten the sail.

    It worked. That boat taught four people how to sail, before it rotted away from neglect.

    A thing to remember about flat sails is that, though they may not be very efficient, they always work. Snarks(r), and Sunfish(r) sailboats both came with flat sails.

    Rounding a luff (the leading edge of a sail) is a time-honored technique for giving a sail camber. The beauty of this technique is that it produces a roughly airfoil sectional shape for this camber. It is the simplest way to do it. But it must be done right. And the material the sail is made of must be strong enough to not stretch too much. The rounded luff is attached to either a mast or a stay which is relatively straight. This forces the material in the fat part of the rounding aft. And this creates your camber.

    Taller sails have two advantages over shorter ones of the same sail area:

    1.) they catch more wind, because there is usually more wind higher up.

    2.) they are more efficient because they have a higher aspect ratio (this is why you see long, skinny wings on aircraft and not short, wide ones). You will never see a racing sailboat with short, wide sails, unless it belongs to a class which mandates such sails.

    This being said, there are three big advantages of short, wide sails:

    1.) the mast can be shorter, which may enable it to get under obstacles which the taller one may not be able to, such as power lines and bridges. The shorter mast is almost always easier to step, without a crane, than the taller one, even though it may weigh just as much.

    2.) they create less capsize moment, if caught aback in a squall.

    3.) they work better than tall ones, if cut flat.

    In short, what you lose in efficiency, you may very well gain back in convenience and, to some degree, safety.

    So, this is everything I have learned in the last fifty years about sails.
    Will Gilmore likes this.
  2. CT249
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    CT249 Senior Member

    So you ask a question; you get detailed answers; and then you tell us that we are wrong and you are right.... Have you ever actually sailed? How much? One what types? If you are not an experienced sailor on multiple types, on what factual basis can you decide that "the differences aren't going to be as radical as they are being made out to be"???

    Have you ever tried a storm jib? Have you ever used a floater? How much time have you spent on Code Zeros, on Moths, or watching a 12 Foot Skiff try to get through a light patch under its #3 rig? If you have not done such things, how do you know how big the differences are?

    No, those are NOT separate questions from what power a sail can deliver. Those are highly relevant points that actually determine what power a sail can deliver. Maybe you should stop thinking you know everything and rejecting the information you asked for.

    No one has said that you have to be a highly educated engineer to build a decently performing boat, because that is simply untrue. What you have to be is someone who does their research and knows how to learn and listen.
  3. sharpii2
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    sharpii2 Senior Member


    There is a third advantage for the taller rig I forgot to mention. That is:

    3.) the sails and booms can be kept inboard, at least when it comes to the length of the deck. This makes reefing them much easier. Shorter, wider sails usually overhang the ends of the boat, to get sufficient sail area.
  4. sharpii2
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    sharpii2 Senior Member

    Sheesh CT.

    I don't understand what your quarrel with this guy is.

    But to me, you seem to be beating him over the head with your expertise, in an "I'm the expert. And if you don't know what I know, you don't matter" kind of way.

    I'm not sure this is your intention, but it's how you're coming off to me.

    As a matter of record, I haven't done those things either. At least not with the boats you mentioned.

    My first two sailboats were home made. And one of them was to my own design. Both sailed successfully (the first with considerable tweeking).

    And this all happened without one minute of coaching from anyone. And dacron(r) sail cloth was beyond my means. So I made do with what I could afford: polyethylene drop cloths and duck tape.

    I think we all appreciate the vast knowledge of sailboat racing technology you bring with your posts. You have certainly been there and done that.

    But could you be a little less heavy handed?
  5. clmanges
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    clmanges Senior Member

    How fortunate are we? Not enough to escape the stupid, as for example, that $10M yacht that rolled over the minute it hit water. Not the designers' fault, oh, no, it did that because they used the wrong trolleys to launch it with. Riiight.

    An aside ... I think that if you dug deep enough you'd find that many of the thousands of tables and formulas used to design boats (and a lot of other things) were derived after the fact: Someone looked at successful and unsuccessful designs of a thing and analyzed the differences--this worked, that broke--why?--and eventually found independently provable math and physics to explain the difference shown in trial-and-error results. And some really big, embarrassing mistakes still get made, even with the dizzyingly advanced computational tools we now have available.

    These tools aren't really helping everyone who has access to them, either; anybody with a computer and some software can now draw boat hulls in pretty and impressive-looking 3-D renderings, but that doesn't mean they can design a boat. We've seen that happen here a couple times recently.

    It's good of you, and anyone else, to question conventional wisdom, but if you don't like the answers, you could wind up repeating established research just to arrive at the same place. That's expensive, but so is trial-and-error, and that is why we keep seeing the advice to look for a proven design and buying a copy of it, either in physical or plan form.
    Will Gilmore likes this.
  6. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    Nice and much needed chart but shouldn't there be much more info such as you described and shouldn't it have been mostly sorted out about 200 or more years ago by the Lords of the Admiralty or somebody, with about the top 20 or 50 most important factors and how they tend to affect each other?

    Now with computer simulation maybe such spreadsheets could be created without undo effort and expense.

    Always bugs me was supposed experts refuse to offer "most likely" info. Reminds me a computer class and to every question the instructor would answer "that could depend on the system or particular application and install".
  7. dustman
    Joined: Jun 2019
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    dustman Senior Member

    Believe it or not I am taking in everything you guys are saying. Every night I sit on my porch with my beer and stare off into space thinking about all the implications about everything I have heard and learned and how that fits in with my goals, and my budget. And it is simply enjoyable and fascinating to think about.

    On the other hand, I don't feel like some of you are really taking in what I am saying. Or just how small my budget is. And even though I have designed and built a great many things successfully, I am no engineer or mathematician, so my design is unlikely to be optimal. As long long as it is safe and functional and performs reasonably well I'll be happy. I just want to get out there and experience the many wonders the ocean has to offer. This will be my first iteration, and based on what I learn, in the future I will be building a much larger version for a circumnavigation. That's assuming I enjoy it as much as I think I will.

    As far as my actual experience sailing, In my youth I sailed on a few times on a couple different boats my grandfather had, off the coast of California.
  8. dustman
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    dustman Senior Member

    I plan on sharing as many useful charts as I can on here at some point, and make my own based on them. I wonder if it is some kind of infringement to take someone elses work and post it on here though. I was hesitant to post Gerr's chart.

    That bugs me as well. I've had to dig very deep to find what I have. And even though there are a lot of qualifiers i think is it ok for something to stand alone. This is how a lot of science is done, break things down into components and piece them back together to attain an understanding.
  9. Rumars
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    Rumars Senior Member

    Well you are right, I don't understand you. If you have a small budget why on earth do you want to build everything yourself from new instead of using used material and paying pennies for the dollar? Learning by trial and error is expensive.

    So what are you trying to achieve? Maybe if we understand your goals we could steer you in the right direction. Make no mistake, right now you don't know what you don't know, and you can not even formulate the right questions to get the answers you want.
  10. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Dustman read and re-read the above dozens of times.

    I worked as a sail repair/maker. Your gaff sails will require dozens of hours of the sailmaker's time just to design by hand (because they aren't in the software data files) and therefore cost thousands of dollars. If you use a standardized mast and boom the computer will design it in minutes. And you save. If you have the mast and boom from XYZ production boat, a used sail can be had for less than the cost of the cloth to make it.

    Old beach cats can be found for less than $500. And another grand to prepare them for your trip. Of that's not in your budget then neither is your proposed voyage.
    clmanges likes this.
  11. CT249
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    CT249 Senior Member

    I'm not saying "I'm the expert and if you don't know what I know, you don't matter" at all.

    I'm saying that top designers (and I clearly said, I'm not one) are experts, and if we don't know what they know then we should respect them and learn from them.

    Yes, I admit I do have a problem with people who ask for advice and then repeatedly reject it, and with people who denigrate so many existing designs and by implication those who have created them.

    Maybe I could be less heavy handed, but also the OP could be less heavy handed in his way of rejecting the sort of advice he has asked for. But I'll zip away from this sort of thread again. It's not worth dealing with people who ask for advice and then reject it.
  12. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I'm taking what is useful from the advice you all are giving me and rejecting what is not useful. Is that wrong somehow? It's not that I disagree or think you are wrong. I just don't see what is so wrong with my approach that will be catastrophic. I could put up a sheet of plywood as a sail and do better than you are making it seem. I'm not even sure what you are telling me what is wrong with my ideas. My sail will have plenty good draft and a just fine aspect ratio and the rig will be plenty strong enough. Don't know what you're harping on. Anyway, I don't want to do this or have to defend my intelligence. You're on top of it, I got it.

    I am spending money in one place and saving it in another to get to my desired outcome. I am not concerned with maximizing every little thing for efficiency. Most of my efficiency will come from light weight, low windage and efficient hull design, along with that comes lower material costs. Aside from welding I'll be doing all the work myself. If we are talking about my flat cut sail question, it was a question to see if there is a way to make it work, and I don't know, that's why I asked. Through the discussion I got a number of ideas to pursue to see if there is a way. Just ideas, which I will iterate in my mind til I have something better than what
    I had before. Then I plan on running it by you guys again, if you will entertain it. I argue for it, others against it, hash out ideas, and we come out with something useful, that's what I'm looking for. And who knows, a useful, but imperfect, but economic option might come out of it.

    Is the point of this forum to tell people what's what? Or to bounce around ideas and information? I hope it's the latter. Much more interesting, useful and progressive that way.

    All that said, I VERY MUCH APPRECIATE all of you with experience and knowledge taking the time and providing an invaluable resource for discourse and learning on this subject. Many of us newbies would be at a loss without resources like this.
  13. gonzo
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    gonzo Senior Member

    The chart has a lot of information missing. For example, at what point of sail is that calculated power. Also, what type, shape and material are the sails.
  14. Will Gilmore
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    Will Gilmore Senior Member

    The secret to becoming a great boat designer is right here in this interview with Clark Mills.

    -Will (Dragonfly)

  15. ryanonthebeach
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    ryanonthebeach Junior Member

    Couldn't agree more with this. I moved from there to sailing anarchy form and only check back in here now and again because of the "don't even try modify or build a boat if you're not a seasons naval architect attitude" "it's way more complicated than you mere mortals could comprehend" condescending and unhelpful attitudes. I'm not going to name names but that handful of folks know who they are.

    Thanks for posting this info, very helpful summary keep at it.

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