Experimental Split Sail Design

Discussion in 'Hydrodynamics and Aerodynamics' started by Philip, Jul 7, 2020.

  1. Philip
    Joined: Jul 2020
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    Philip Junior Member

    Hi all!

    I'm new to this forum and to sailing so please forgive me if this topic has been discussed or my terminology is incorrect.

    I have built a 10-foot sailing dinghy with a 48 sq. ft sprit sail on it. My primary goals were safety (staying upright) and simplicity of construction/operation. This design succeeded in these aspects, but just will not head into the wind very well at all. This often leaves me dragging my boat around the perimeter of the lake after the wind shifts.

    I have an idea for a rig that will combine (most of?) the upwind performance of a Bermuda rig and the simplicity of my spritsail, and I would like your learned opinions on whether on not it will work as planned:

    Hybrid Sail https://imgur.com/a/GpNjb49

    If my image does not show up, The idea is a triangular sail with the luff lashed to the mast, the foot free, and the clew attached to the end of the boom. The sail will have a vertical thin triangle of material removed from the center of the sail, effectively turning it into two tall, thin sails. The gap of removed material will be bridged at intervals by reinforced sail material so that both sail halves will remain attached to each other and function as one.

    My hope is that the fore portion will act as a jib and the aft a mainsail, giving improved upwind ability without any added complexity of operation. The image shows a sail 12' at the luff, 9' at the foot with a removed portion 1' at the center of the foot narrowing to a point at the head. I don't know what width of a gap is appropriate.

    Am I way off base here? I can't find any reference to a thing like this having been done before, but it seems like a nice compromise between simplicity and performance to me. Please let me know what you think.
     
    Last edited: Jul 8, 2020
  2. Zilver
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    Zilver Junior Member

    Hi Philip,

    FWI
    The holes you plan to cut in your sail will only result in a regular sail with holes (and wrinkles).
    The leech of a jib is not in line with the the luff of a mainsail (usually overlapping and allways more to the lee).
    The "squaretop" structure will be heavy and difficult to make strong enough if you construct it like the drawing.
    If you want to improve windward ability it's probably more effective to make a better/bigger daggerboard, and maybe a better conventional sail.
    Cheers, Hans
     
  3. Will Gilmore
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    Will Gilmore Senior Member

    Philip,
    Welcome to the forum. It is always great to see someone trying innovative ideas and designs.

    I'm not a naval architect, I just love to sail, so take my comments for what that is worth.

    I have little faith in your design as presented. You intend for a single set and trim sail to behave as two sails. Your design won't do that.

    In sailing, there is a concept called "angle of attack". The angle of attack is the chord angle of your foil to the wind. This determines how well your sail draws, whether you are luffing, stalling, or getting maximum lift.
    [​IMG]
    By cutting your sail in two, the way your design does, you have created two separate chord angles to two separate sails. If you trim to get a good angle of attack on the forward half, the chord angle of the aft half will be at a stall angle. If you trim for the aft half, the forward half will luff.

    To get some semblance of a main/jib combination, you would want the luff of the aft panel to be on a tightly tensioned stay, the way a jib is on a forestay. This would mean vanging the sail at the tack of the aft panel. You wouldn't want a boom for this because that would also vang down on the leach of the forward panel. I can't see that resulting in anything but a flatter overall foil.

    I suggest you read up on the sail theory of the "slot effect". The "slot" is the gap between a forward sail and an after sail. There is some controversy over how much, if any, the slot affects the performance of a sailboat, so keep that in mind. There is still a lot we don't know, but only theorize about sailing.

    One thing you will run into in every sail theory discussion, is the comparison of a sail to an airplane wing. That is all very reasonable, but I'd like to point out that there is a fundamental difference between the two. An airplane wing produces lift perpendicular to the vector of motion through the air. A motion produced by a force from an outside source, propeller or jet or gravity, in the case of a glider.
    For a sailboat, the lift needs a forward component to the lift because motion is produced directly from the vessel's relationship to the motion of the air and water together. The outside force is the lateral resistance from the water such that air moves over the sail at an angle to the chord. A wing's chord is angled almost directly into the wind. Never can a sail generate the needed drive by angling the chord into the wind.

    Pretty cool idea though. Keep thinking because I like the way you think.

    -Will (Dragonfly)
     
  4. gonzo
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    gonzo Senior Member

    The drawing shows a wrong assumption. The flow will not be laminar, but turbulent as it goes through the gaps. That sail will have more drag than a traditional design.
     
  5. Philip
    Joined: Jul 2020
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    Philip Junior Member

    Thank you all for your replies!
    Zilver: if I made the joining portions between the sail halves follow radial lines from the clew do you thing this would reduce wrinkling?
    Gonzo: I understand that the gap in this sail design will introduce turbulent airflow, but perhaps replacing the joining strips with wire would reduce turbulence?

    Will Gilmore: You brought up the airplane analogy, which is exactly what I had in mind. When an airplane is landing with flaps down, its wing is at a high angle of attack relative to airflow (45 degrees?) yet it is still producing lift. Relative to the aircraft, this lift will vector forward at a 45 degree angle... very similar to what a sail is doing:

    https://www.researchgate.net/figure...flap-or-winglet-cross-sections_fig2_230924360

    As for angle of attack, I thought the fore portion of the sail would re-direct airflow such that the aft portion of the sail will have an effectively reduced angle of attack. I understand any sail to work this way... in the "attached flow" picture you posted, look at the angle of the downwind portion of the sail. If that angle were the angle of attack for the entire sail, we would definitely see a stall condition. My idea was simply to do what a flapped airplane wing does; create an overly aggressive foil shape, and bleed high pressure windward air to leeward at the point where flow wants to separate.

    If my assumption is accurate, and the slit sail can operate as one aggressive foil shape, I wonder if it's not necessary to tension the leading edge post-slit.

    Thanks guys.
     
  6. gonzo
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    gonzo Senior Member

    If you want simplicity and performance, a single foil is the answer. Referring to your analogy to airplanes, single foils proved superior to multiple foils in the early 1930's. That is the reason bi and tri-planes are a thing of the past.
     
  7. Blueknarr
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    Blueknarr Senior Member

    Philip
    Welcome to the club.
    I love to see innovative thinking.

    However your theory is is flawed. As Zliver and Gonzo point out you will achieve a lower lift and higher drag. To benefit from a slot the aft luff must be windward of the preceding leach, even if they are not overlapping.

    Will's airplane wing example is almost accurate. Lift is always purpendicular to cord. Flight manuals only show it as a counter force to gravity even though it is often not directly opposed to gravity. When an airplane banks while turning it also loses altitude because lift is out of alignment with gravity.

    When an aircraft comes in to land it needs to do lots of things. Two of them are slowdown while losing altitude as oppose to the usual acceleration in decent. Extending the flaps increase the arc thus increasing lift and drag. More lift at slower speed. Increasing the angle of attack increases drag and rotates lift AFT (not forward). This slowest the aircraft and causes it to lose altitude.
     
  8. Rumars
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    Rumars Senior Member

    The spritsail can set a flying jib without the need of a backstay, there is no need for reinventing the wheel.
    The true reason of your poor upwind performance may be the underwater part. A nice profiled deep board makes all the difference. Combine with a properly cut and trimmed spritsail (with or without jib) and enjoy.
     
  9. DCockey
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    DCockey Senior Member

    The general rule (there are always exceptions) is multi-element airfoils will have higher angle of attack before stalling and larger maximum lift - but - the drag will be higher at all angles of attack / lift. The seminal publication on the topic is High Lift Aerodynamics by A.M.O. Smith. AIAA Aerospace Research Central https://arc.aiaa.org/doi/abs/10.2514/3.59830

    The optimum tradeoff between maximizing lift and reducing aerodynamic drag for upwind performance of a boat depends on the overall characteristics of the vessels. High speed, low hydrodynamic vessels such as man performance catamarans and ice boats perform best with a single sail.
     
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  10. messabout
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    messabout Senior Member

    Philip, sailboats can go to windward if all the parts contribute to that ability. First and foremost, your sail has to be cut and finished such that it actually generates some lift. Next the lateral area of the boat and its appendages must be adequate to minimize the leeward drift when the boat is sailing to windward.
    Back to the sail.............the skill and /or knowledge of the skipper can often determine whether the boat goes to windward or not. You did say that you are new to sailing. Not being sufficiently knowledgeable about the ways of the sail, the wind, and the boat is not a character flaw. It just means that you may need some coaching or at least some more experience.

    All that said, there is the possibility that your boat is not rigged in such a way as to make windward progress. The center of effort of the sail (CE) and the center of lateral resistance of the hull and appendages (CLR) have to be arranged such that they are compatible.

    Tell us more about your boat. As suggested above.......don't re invent the wheel.....or the sail........do keep thinking, that is an admirable trait.
     
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  11. Philip
    Joined: Jul 2020
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    Philip Junior Member

    Thank you again for your replies.

    Messabout: my (10-foot) boat has a flat bottom that rounds up to the waterline at the stern and twice that high at the bow. Draft is 2 to 6 inches depending on occupancy, sides are straight, and the leeboard is 1 foot wide, 3/4 inch thick at the middle and goes 2'6" below the bottom of the boat. The leeboard profile is thin, coming to a knife-edge at both sides (so it functions the same on either side of the boat). The leeboard hangs on the edge of the boat and clamps in place, so I can put it anywhere I like. I tend to put it fore, because I like a sudden gust to rotate me into the wind and not rotate me into the water... I compensate with the rudder. The sail could certainly use some tuning, and I'll do that first before planning a new rig.

    The thing is, what really turns me on is the process of learning, coming up with ideas, making and testing. I also feel like if I'm going to make it, it might as well be unique and not something I could've just gone out and bought. This means I rarely (if ever) make something that's the best, but it's almost always satisfying. Even reinventing the wheel would give me a better appreciation for the wheel, if not a better wheel.

    So, it looks like my original concept won't work (thank you all for your input). But what about this revision:

    (file uploaded, let's see if it works)

    The image shows separate triangular jib and mainsails mounted to a boom. The jib luff is lashed to the mast, both heads are attached at the top of the mast, and the main luff is anchored to the boom by a steel cable. The jib foot is attached to a rail running horizontally perpendicular to the mast in front of the mainsail cable. The idea is that the jib and the line to its foot can slide over to the other side of the mainsail during tacking, and that the jib can be trimmed by location of the rail and length of line to it. I think this takes into account most of y'all's suggestions... Do you think it has the potential to be better into the wind than a (properly trimmed) spritsail?

    P.S.: The jib foot rail in the picture is a lousy design, and would be simplified.
     

    Attached Files:

  12. Will Gilmore
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    Will Gilmore Senior Member

    I like it.

    There is much better potential with that design.

    For better upwind performance, you will want to be able to sheet it pretty flat. I recommend mid-boom sheeting on a traveler.

    -Will (Dragonfly)
     
  13. gonzo
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    gonzo Senior Member

    There is so much wrong with that design that it is hard to figure out what to start with.
    1- The sail is shown as laced or hooped to the mast. That will have a huge amount of turbulence and very poor upwind performance.
    2- The traveler for the forward sail has a shape that will not allow the clew to tack.
    3-The traveler will apply a huge moment to the boom. If the gooseneck does not allow the boom to rotate,it will apply a moment to the mast increasing stress and will probably twist and break. If the gooseneck allows the boom to rotate, and the geometry of the traveller will not allow the clew to tack, regardless of its shape.
    4- The steel cable and traveler are applying a huge point load to a slender tube. It will get it out of column and collapse.
    There is more, but that should be enough to start.
     
    Last edited: Jul 9, 2020
  14. Rumars
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    Rumars Senior Member

    Philip, you need to google "balestron rig" or "AeroRig". What you have designed there makes no sense with both sails aft of the mast.
     

  15. Tiny Turnip
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    Tiny Turnip Senior Member

    Hi Phillip.
    As Gonzo points out, there are significant structural issues with your sketch. A conventional bermudan rig does not have these issues. Can I suggest that you look at a drawing of a bermudan rig again, and compare with your drawing? The bermudan really is no more complex in construction than what you are proposing. And I don't think you will find many sailors who complain about its complexity in operation, certainly in a small boat. The jib, mostly, will pretty much tack itself, and then can be trimmed at your leisure, post tack.

    If you really don't want to have to manage a jib as well, there's no reason why a cat rig shouldn't point adequately.

    It may be that the issues you have with the boat not pointing have another cause, perhaps to do with the size, performance or position of whatever is providing your lateral resistance. (keel, centre board, lee board?)

    If you are driven by exploring your ideas with sail, then models would be a low budget way of experimenting.
     
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