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  1. #43
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    Kitchener, Ont.
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    Now I'm going off on another tangent and if it makes your eyes glaze over, I'm sorry. My wife says I spend too much time in left field, but I'm happy there....

    I think there must be some "righting moment" of an angle of attack foil whereby the force of the swiftly moving heavy water tries to "flatten out" that foil as well as lift it up. I've been unable to find any info on this so, in honour of FRP who, if there is a semi-foil revolution, will certainly have helped it; I'd like to name it frp, that is forward rotational propensity!

    My fly-fin wing has an angle of attack of 5 degrees. The wing has a chord of only 2.5 inches and at 2.5 inches from the rear of the wing to the front the vertical distance between 0 aOa and 5 aOa is only 1/4 inch or 6 mm. BUT, the wing, fin, and board are solidly fixed, so at a distance of 250 cm length of board, even the small angle of 5 degrees causes a large change in tilt; that is about 8 inches or 200 mm.

    Consider 2 extreme cases: If the wing was located under the very front of the board, the front would dip 1/4 inch and the back end would rise up 8 inches. If the wing was located below the very back of the board, the back end would dip 1/4 inch and the nose would dip down 8 inches. Now these 2 cases might seem the same as the board angle is the same, BUT, in the first case only about 160 lbs. of frp lift is needed to lift me and some of the board up. Helping to lift is the regular fin lift, the angle of attack lift and the foil wing lift! In the second case ALL of the board in contact with the water has to be pushed down to displace water. Helping to prevent this or making it harder is the rider's weight on the rear teeter-tottering the nose up as well as air under the nose pushing it up.

    Where am I going with this? Well when I re-worked the FRP foil-fin and fastened it to a workable length and rake fin, I expected the resulting move of the wing 7 cm. to the rear would cause big problems. Instead, it solved the nose pearling problem! And moving it a further 3 cm. to the rear was also good!

    So Virus semi-foil mounts their wing high to keep it forward and mounts it on a jutting out front to keep it more forward. FRP uses a no rake fin to keep the wing forward and on some models juts forward on the bottom to keep the wing more forward. And on some models overlaps the front of the tuttle box to keep the wing more forward. Etc.

    FRP claims their fly-fin tilts the board horizontal so it planes better all along at just the surface of the water. (I've used it. Trust me, it nose pearls!) I would love to see video of Virus fin sailing.... My board planes normally with nose up in the air.

    One of us is wrong. Jus say'n.
    Last edited by RazeR69; 20th April 2018 at 07:25 PM.

  2. #44
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    Mar 2018
    Location
    Kitchener, Ont.
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    47

    pocket foil

    O.K. I've just spent a few hours reading about the evolution of the "pocket foil" on a French windsurf forum here:

    https://translate.google.com/transla...4%26start%3D90

    Here is a picture of the original: Note the swept back fin with the mini wing below the tail of the board. It underwent quite a few revisions until the final version had a 3 times larger wing, a foil shaped wing for more lift, an optimal 2 degree angle of attack. Pretty much what I've found out works for me. Except the originator of the pocket foil was NOT interested in low wind planing from 6 to 10 knots or even smaller size sails. His goals were 10 to 30 knot wind
    -gain in navigation flexibility
    -keeping a more level board
    -and especially, flying over chop without hitting it.

    So his wing ended up about half the size of mine....

    Click image for larger version. 

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    And in that thread at least 2 other windsurfers repeated his success with different but similar fins, boards, and sails........

    Here is an example:Click image for larger version. 

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    Last edited by RazeR69; 18th April 2018 at 01:52 AM.

  3. #45
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    Kitchener, Ont.
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    Are you getting this, FRP? Your fins need some length and backward sweep so they can do a normal fin's job with no spin-out. Also it is not necessary to contort the fin to keep the wing further forward. And furthermore maybe only 2 to 3 degrees of angle of attack is needed. The flexible trailing edge is amazing, of course, but unnecessary to prevent spin-out, and of dubious benefit until tested by an experienced windsurfer.....

    The size of the wing will have to be determined based on weight of rider and desire for low wind planing or high wind performance.

    I'm happy to help since right now you are the only person capable of making a custom fly-fin such as this.

    I still like the idea of the wing on the bottom, unless Virus fins or someone else can show as good results with the wing high up.

  4. #46
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    Mar 2018
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    Kitchener, Ont.
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    further tests

    Because of where I sail, I`m a big fan of low wind capabilities in windsurfing. If I could test several more prototypes, here`s what I`d be interested in:

    One. Lowest possible wind speed for planing
    . When using larger and larger sails for earlier planing, one soon runs into the limiting factor that larger sails, masts, and booms are heavier and have more drag so the benefits rapidly decrease to zero. When using larger and larger fins for lift, the drag and weight also increases until further benefits cannot be found. And using lighter, wider, thinner boards also reaches a limit. The best example of all three is formula boards with 100 cm. of width, 70 cm. fins, and 11 to 12 m. sails able to plane in 7 knots. But that comes with a high price tag, a difficult transportation problem, and for me, an impossible strength to lift such large sails.

    With my modified fly-fin I was able to plane in 6 knots with a normal size board and sail! Could I do better? Probably. It is unlikely that the wing I had available just happened to be the optimum size. Both angle of attack and foil shape cause drag - I'm not sure which causes more, but reducing AoA was the only option I had and lowering it to 1 or 2 degrees promoted earlier planing. A thicker foil shape causes more drag and a wider chord causes more drag so I'm guessing the best way to more lift with least drag would be to increase the span of the wing. How much? Maybe 3-4 inches greater than the 13 inch 2017 model span.....

    Two. Optimum rearward position of wing. I was only able to change rearward position in conjunction with AoA and not by itself. But getting more rearward allowed the board to plane on its intended planing surface at the tail. If even further rearward was beneficial or at least not harmful, then proper fin shape of all swept back or at least tip swept back could be used. And in light of the difficulty in beach-starting such low winds with a 50 cm. deep fin, more weed-like fins could keep the total depth less....A starting point is from the rear of the back footstrap to the rear of the board.

  5. #47
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    Mar 2018
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    And still, nobody even asks....how is speed affected? Obviously at low wind the speed increases a lot by getting on the plane. But what about medium wind and high wind? Does the fin keep you going faster and faster or does more drag slow down potential top speed?
    There's no ox so dumb as the orthodox.

  6. #48
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    Mar 2018
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    Kitchener, Ont.
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    difference in jibing

    Click image for larger version. 

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    Well this picture might be a clue as to the difficulties with jibing at speed with a wing on the bottom of the fin....

    The foil shape of the wing will give lift towards the board no matter what angle it is travelling forward. In this case, a good shove to tip the board over....
    There's no ox so dumb as the orthodox.

  7. #49
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    Mar 2018
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    Kitchener, Ont.
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    I had some success in keeping the rear foot more rearward on the turning rail and pivoting the board around rather than carving around, but then unloading the rear of the board before sinking was an issue. I need more practice when the warm weather comes....
    There's no ox so dumb as the orthodox.

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