Large luff pocket sails with sharp leading edge

Discussion in 'Boat Design' started by Qmaran, Sep 1, 2021.

  1. Qmaran
    Joined: Oct 2019
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    Location: Thailand

    Qmaran Junior Member

    I am going to use heavy storm rated 11m tall aluminium flag pole as free standing mast. I was looking at creating wing sails but although I found some nice ways to make them they were not KISS friendly.

    While reading papers on wing sail research I read that one team was surprised that just the first 25% of the sail is most important. If only the first 25% is an actual airfoil and the rest just sail it is almost as efficient as a full wing. Other research shows the leading edge is the most important so luff sails reduce some drag but aren't particularly efficient airfoils with their blunt edge.

    So what if I make a sail with a large luff pocket and in the leading edge of the luff pocket I sew on the inside small pieces of pvc pipe of between 1/4 to 1/3 of the mast diameter at appropriate intervals. This should create a more pointy and thus efficient leading edge. As the sail rounds this edge is pushed to the opposite side of where the sail is.....exactly where it should be. And when the spacing between the PVC pipe sections is not too small it should still be reefable.

    Does this sound like a good idea?

    I still haven't figured out how to wrap the sail around the mast. Can a zipper be strong enough? Should I make the luff and sail separate parts?
     
    Last edited: Sep 1, 2021
  2. kerosene
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    kerosene Senior Member

    What is your sailing experience?
    What kind of boat?

    I recently asked a relevant question I will dig up a link to that thread. Conclusion was that the luff pocket is more successful on the paper than in the real world.
     
  3. kerosene
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    kerosene Senior Member

  4. Qmaran
    Joined: Oct 2019
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    Qmaran Junior Member

    Just a luff pocket only helps reduce drag a little, but a big fat round mast is a poor leading edge. I propose to put something inside the the luff pocket to create a shape closer to an actual wingsail airfoil shape. I sailed a lot in smaller boats. This is for a bigger cruising trimaran with the modest sail area divided between two masts.

    First I was looking at a junk rig but after some deliberation it seems more suitable for slower single hulled boats. Wing sails are more efficient, especially at higher speeds. Added benefit would be that I wouldn't have to mess too much with the round tapered aluminium mast. With a regular bermuda sail I would need to add a sail track.
     
    Last edited: Sep 1, 2021
  5. philSweet
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    philSweet Senior Member

    It amounts to a great deal of hassle for very little real gain. If you are trying to set some speed record, yes, otherwise, the difficulty with reefing and controlling sail twist will likely make for worse overall performance. People vastly overestimate the precision to which they trim sails in normal sailing. Give yourself the best chance at getting two factors correct - the right amount of area and the right twist distribution. Those are what are most important. You will already have some significant drag savings for loosing the shrouds. And using any luff pocket, parel system, or rotating mast lets you put the sail on the lee of the mast for additional performance.

    The only practical way to do what you describe is to use either a stayed rotating mast, or a wing that rotates on a fixed stub mast. The suction pressures at the leading edge are huge and you need a solid carbon or aluminum leading edge D section to get this to work. And you need an entire control system devoted to managing the leading edge - the pressures move around too much to rely on membrane tension at the back of the pocket to control the leading edge treatment.

    Yes, good zippers can work. I honestly don't know why there aren't more sails with zippers. They are often used to control draft at the bottom of a sail (zippered draft shelf). For a luff pocket, you should backup the zipper with Velcro, and also have a lacing system in case of a blowout. The top and bottom of the sail needs special treatment to handle outhaul and leach tension. Webbing works well. Run a thin stainless sheet over the aluminum where the webbing lands. Also use a lube such as PTFE.

    Narai Mk Iv Sail Data https://www.sailrite.com/Narai-Mk-Iv-Sail-Data
     
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  6. Qmaran
    Joined: Oct 2019
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    Qmaran Junior Member

    Thanks, good info.

    I need to explore this more and maybe do some tests. The limited amount of soft wing sails I have seen do without solid leading edge and some have huge spans of unsupported sail. But then again many of those are home brews and may indeed be no more effective than a regular sail plan. But there are commercial examples like Omer Wingsail https://www.buildboats.info/Omerwingsail.html

    I am not sure about this one. On my tiny little test sail in front of a large fan it seems to work. But would it work on a 30m2 sail in real world conditions?
     
  7. Qmaran
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    Qmaran Junior Member

    I just tried with a larger sail model and you are right. There has to be a better kind of connection with battens and leading edge. Just sail cloth doesn't do the trick. So that will circle back to soft wing sails. Well, I'll keep trying to come up with an elegant KISS compliant solution. Thanks for your input, much appreciated!
     
  8. tlouth7
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    tlouth7 Senior Member

    Your idea is built on the premise that a sharp leading edge is more aerodynamically efficient. Do you have any reason to believe that to be true? Typically airfoils have fairly blunt leading edges to allow the stagnation streamline to move around as the lift regime changes.
     
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  9. Qmaran
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    Qmaran Junior Member

    I am just looking at applicable slow speed airfoils. Do keep in mind that I have a rather fat unstayed mast. A stayed mast might be appropriately sized to function as leading edge. And by sharp I do not mean sharp like a knife, more like the pointy side of an egg....
     
  10. Qmaran
    Joined: Oct 2019
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    Qmaran Junior Member

    I have another design for a sail that seem so work beautifully. I made a little model and tested it. It is a soft wing sail that reverses camber when tacking/gybing. Only limitation is that I have to choose a fixed camber and fixed amount of twist. For the rest it produces a near perfectly shaped double sheeted airfoil. But since I have no wind tunnel I wonder about the following:

    1. Is there a camber % and amount of twist that on average performs reasonably well in all conditions?
    2. Will a perfectly shaped airfoil with fixed camber and twist outperform a regular sail in some or most conditions and points of sail?

    Keep in mind it will be installed on a multihull so there will be limited heel and it has to compete with a regular sail on a relatively fat circular unstayed mast.
     
    Last edited: Sep 3, 2021
  11. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    1 absolutely not
    2 yes. But only in an extremely limited apparent wind speed and wave conditions.

    I adjust my sails if there is a greater than one knot wind velocity or five degrees of directional shift. Or any change of sea state.
     
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  12. Qmaran
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    Qmaran Junior Member

    Is this a from a racing perspective? If cruising and half a knot more or less speed is not that important are the answers still the same?
     
  13. Blueknarr
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    Blueknarr Senior Member

    My sails are kept trimmed even while cruising.

    A properly designed and built Bermuda rig is the most versatile and best performing in the broadest range of conditions.

    Other rigs can outperform it only in the narrow band they were optimized for.
     
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  14. Skyak
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    Skyak Senior Member

    I third the "don't bother" verdict for sharpening the leading edge and you can prove it by just looking at Xfoil simulations. The bluntness of your mast is not as detrimental as having the "sharp" mispositioned from the stagnation point. The big challenges are getting the sail tangent to the lee side of the mast and shape control. It's easy to draw a pocket luff sail making a nice shape but it is far from real world experience for a large rig. I don't see any chance of leach tension without creating too much pocket to mast friction. How do you support battens? I think you need to turn the mast to get the sail tangent and then you might as well use a sail track.
     

  15. Qmaran
    Joined: Oct 2019
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    Qmaran Junior Member

    OK, I fiddled around for hours and think I found a way to change camber in my soft wing sail. It seem doable to vary the camber from let's say 7 to 14%. But I see no way to change sail twist. If I use this setup I will have to pre-determine sail twist and live with it.

    I found this info but the site doesn't site a source. I have seen this float around forums before.......but for what it's worth:

    [​IMG]
    Code for
    Mast Type Ratio of max boat
    speed to wind speed**
    A 0.85
    B 1.00
    C 1.15
    D 1.25
    E 1.50
    F 2.0
    ** A rough comparative indication only,
    as many detail factors are involved.

    My current experimental setup mimics E without the need for a rotating mast. This table claims E is 76% more efficient than a regular sail plan. My guess would be that even if this is a little exaggerated and with the handicap of a fixed amount of sail twist it should do a little better than a regular sail.

    If I need to choose a fixed twist, how many degrees should I go for on a 1om tall sail?
     
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