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  1. #1
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    T foil or t foil

    This thread may be premature or it may be timely as foil assisted windsurf fins or semi-foils struggle to evolve....

    In the one corner we have FRP manufacturing inverted T foil fins:Click image for larger version. 

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    And at least 5 homemade successful attempts:
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    There's no ox so dumb as the orthodox.

  2. #2
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    In the other corner we have Virus Fins manufacturing t foil fins:

    Click image for larger version. 

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    And at least 5 homemade moderately successful versions:

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    There's no ox so dumb as the orthodox.

  3. #3
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    All of which fail to take into account the actual quality of the fin. I doubt any of them provide any real benefit over a high quality carbon fin of the right stiffness, right rake and right size to suit the board and sail. You can take 2 apparently very similar standard fins and obtain a very different level of performance. Unfortunately the higher performing fin is invariably carbon and expensive....................but still a fraction of the price for a foil!!

  4. #4
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    Well, two things:
    First, the high quality, high carbon, highly technical fins are developed for racing and neither these foil fins or actual foils are for high speed.

    Second, my normal gear planes at 6 knots with this fin while very specialized and technical formula boards and fins and huge sails start planing at 7 knots. There has to be a low wind market for this.....
    There's no ox so dumb as the orthodox.

  5. #5
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    I have read a dozen posters to forums who have made a foil fin and reported some positive results. So lets assume that they do work and use this thread to look at the positives and negatives of the 2 main placement areas for the wing on the fin - about 1/3 down from the top or at the bottom of the fin.... t or inverted T.

    Of course I would love to hear of any actual tests or theory that disproves the value of these foil fins, but maybe on the parallel thread of "foil, mini foil, with tails".....

    First off. 1 small advantage for the t:

    Several posters have made t foils with other symmetrical fins used for the wing and NO angle of attack and reported some lift of the back of the board and a looser feeling. How can this be with no foil shape for lift and no AoA for lift? Well, as the wing is pushed thru the water it displaces a volume of water equal to its own volume. This water is pushed up in a ridge in front of the wing which pushes up water at the surface. It also pushes water down but can't compress so moves the wing up. And it pushes water up above the wing so some water at the surface is pushed higher.

    Of course this amount of lift is not a lot, but at the same time with no AoA and no foil shape, the drag is the least possible. If the wing is placed at the bottom of the fin the forces are the same but the upward force has a lot more room to dissipate sideways except for the actual up force on the wing itself. So maybe half of this small displacement force will be better on the t design.
    Last edited by RazeR69; 22nd April 2018 at 09:28 PM.
    There's no ox so dumb as the orthodox.

  6. #6
    Senior Member Navegante's Avatar
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    This concept works in outboard engines and stern drives it has been used for decades. The difference is that in outboards and stern drives the driver can adjust the trim of the engine as needed.

    So providedvthe trim angle in these fins wing is the correct one for the conditions it has to work to an extent.

  7. #7
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    Maybe 4 small advantages to inverted T foil fin:

    - possibly less turbulence generated with T than t but my brain clamps up trying to figure this out. Anyone with visual evidence or applicable theory please respond....

    - no tip vortex to worry about

    - can maintain larger fin chord right to bottom so maybe less fin depth needed.

    - in the professional models at least, less drag at junction of fin and wing. In the homemade models so far, both good and bad examples of attachments on both types.


    On the other hand, the t foil fin allows the normal and beneficial flex and twist of the narrower chord fin bottom......
    Last edited by RazeR69; 23rd April 2018 at 06:06 PM.
    There's no ox so dumb as the orthodox.

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