Experimental sail needs analysis

Discussion in 'Boat Design' started by lunatic, Nov 9, 2008.

  1. lunatic
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    lunatic Senior Member

    Sail design at http://www.boatdesign.net/forums/boat-design/sail-design-trim-camber-twist-leading-edge-23524.html has been built as in attached drawing and photos, and sailed on a Sunfish hull. Surprising stable, perhaps due to gaff piercing sleeve through Velcro closing slot flattening camber aloft, but elsewhere good variation in camber and twist. Vang and downhaul can pull max camber forward and flatten aft trailing section without battens, mainsheet sufficient in light air.
    Sail cobbled together from scrap with later sections glued in to increase lower to mid draught, creating hard spots and creases, but sails better than it looks. Sailed in 20-30 mph, enough wind to repeatedly knock down a Laser, could point as high tracking it but conditions not conducive for comparative sailing. On a beam reach, flattening and untwisting sail to telltales had best planning I have ever had on a Sunfish, right on the edge of rudder capacity. Never capsized in 3 hours of sailing, always felt in control. Rig is app 75 sq ft with center of area 9 ft above deck, highly cambered Sunfish is 80 at 5 ft and would have had high down time in similar conditions.
    Choice of wide sleeve over total double skin was for ease of fabrication and modifications, but the ubiquitous Tom Speer's "Double Skin Sails" at http://www.boatdesign.net/forums/showthreads.php?=603 argues for extended sleeve, which has benefit of less weight, but why less windward pressure for double?
    Tackable drooping D section built as in drawing. Upwash forward facing lee surface is highlighted by on edge sunlight in photo B, windward side in C. D shows rotation wheel, friction made lines impractical, unloading rig and tee hook work, but makes tacking an event. Seems close winded repeatedly tacking through a mooring field. Is the D section worth it? The power coefficient of a similar sail fabric bladed wind turbine rotor increases 5 fold from round to drooping D section leading edge and produced higher L/D for wind tunnel tested sail wings, Princeton Report NTIS PB 259898. Yet 'SoftWing' Sails at http://marine.bdg.com.au/spitfire_features.html section AA shows round leading edge?
    I modified rig with half wishbone gaff outside of sleeve and generous luff round at leading edge of sleeve resulting in wider range of camber. Still flattens for high winds though 2 windward mid level luff telltales tend to point up sailing upwind, might be separation bubble from excessive camber behind round mast? I also removed sheet metal D section due to damage and for ease of tacking. All this makes one appreciate incremental development over decades, but seems enough potential in this primitive prototype for a fresh cut sail, any suggestions for improvements? The rig will drop into a Laser for comparative sailing, but with falling temperatures, it may be awhile before I find a comparative sailor.
     

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  2. messabout
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    messabout Senior Member

    Nifty rig Lunatic. The mast rotation wheel, though effective, may be a PITA. An alternate mast rotation method consists of a piece of sheet metal wrapped around the mast at the gooseneck location. The ends of the sheet metal extend aft of the gooseneck a few inches. The sheet metal, or thin flatbar,is firmly secured to the mast. The boom will now rotate the mast by pressing against the sheet metal extensions. You can fiddle up the space between the extensions and the boom such that the camber can be fine tuned. This was a ploy used on Moth class boats way back in the olden days. I think there have been some cats that used a similar method.
     
  3. CT 249
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    CT 249 Senior Member

    It looks pretty much like a standard windsurfer sail, essentially. Great in strong winds, arguably not so great in light winds.

    Good to see you putting it to two-boat testing on comparable hulls.
     
  4. Petros
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    Petros Senior Member

    Great concept. I like the treatment at the mast/leading edge, it eliminates the dead area of flow separation behind the mast and gives an improved LE profile (the most critical part of the sail).

    The sharp peak is harming the efficiency at the top. You might consider altering the sail and gaff so the top edge of the sail is almost horizontal, eliminating the peak. You see this on all modern racing sails. It is to create a tapered "wing" like sail profile.

    You might also consider using simple levers to control the mast angle of attack instead of the wheel. The levers would allow about 150 degree of rotation of the mast.

    What you have is a vast improvement over conventional rigs, I suspect since the rig has lower drag you will not get as much heeling for the same amount of thrust, which is why this is more controllable in terms of heeling moment.

    Good luck.
     
  5. lunatic
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    lunatic Senior Member

    Self-regulating boom assissted mast rotation is so elegant. Thinking of local sail pressure on trailing sheet metal of D section rotating it on fixed mast, could have the nuance of twisting leading edge to flow. Maybe too elegant to be practical, cannot be adjusted but tempted by any self-regulating component.
     
  6. lunatic
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    lunatic Senior Member

     
  7. messabout
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    messabout Senior Member

    German dude named Manfred Curry did much research with sails patterned after bird wings. Curry was well placed in society and had the access to the Messerschmit ( or was it Junkers?) wind tunnels. This stuff was done pre WW2. I believe that the final conclusion was that birds have the unique ability to articulate their wing tips advantageously for whatever the conditions or demands might be at that instant. Not so with sails manipulated by mere humans.

    Mad tinkerer that I am/was, I once built experimetal dual surface sails that had all kinds of bell cranks and parallelogram linkages built inside the two surfaces. The mechanical gadgetry was to induce differential bends in the battens on the windward and leeward sides. A lot of tinkering was needed to get the separate cambers optimized. The down side of the whole madcap structure is that it was too heavy. I did manage to find some impressive bursts of speed along with too many swimming episodes. I found that the rig was super sensitive to trim. I am told that wingsails have that characteristic too.

    Keep up the good work, you may be onto something that we'd all like to learn about. I do believe that the Sunfish hull is not the best platform. Yes I have sailed and raced Sunfish. Neat boat but not exactly a hot rod that can most easily reveal the performance nuances of your sail.
     

  8. lunatic
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    lunatic Senior Member

    Hard to resist the beauty of bird flight, where I think much of the created form is shaped not by muscle, but by pressure on loose feathers, much is self conforming as in sails, though bird flight applied to sailing might best be done by tethers. My first prototype has no battens, sail trim is by traditional gaff and hollow leech with modern downhaul and vang to give wide range of shape. Soon to post second prototype design for comparative sailing trails on Lasers. Sunfish was on hand , easy to beach launch, and with prototypes, nothing is perfect. I am going for the perfect rig but looking for any improvement. Thanks for response.
     
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