Rudder Hydrofoils

Discussion in 'Sailboats' started by DavidG, Oct 11, 2002.

  1. DavidG
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    DavidG Junior Member

    I am looking at experimenting with hydrofoils (as is fashionable in I14's) on my 12ft sailing dinghy (National 12), I would be interested if anyone has any input on this?

    Using classical wing theory, on a pure lift drag basis, there is a penalty at low Froude numbers, due to wetted area, but benefits when sufficient lift is generated to reduce hull displacement and wetted area. On this basis the benefits are a close call.

    However I have not evaluated any pressure recovery from the wave immediatly aft of the transom, has anyone considered how this might be evaluated?

  2. giramonti
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    giramonti Junior Member

    Rudder Foils

    Be careful about playing around with foils. Without empirical testing on new sections it's very difficult to mathematically predict the lift and drag characteristics of a newly designed section. However, there is a very good program available for download called DesignFOIL. It's located at With the program, you can view a vast collection of airfoils collected from many sources, above and beyond the well know NACA foils. When you download the demo, you must download the collection of foils .zip file as well to access these numerous foil files.

    The program also has a build your own foil feature. It's quite good, but like I said previously, without empirical tests, it's hard to be certain that the sections will perform as the computer calculations guess.

    As far as interactions between a rudder and a stern wave, well you've asked a magic question. It probably depends on far too many factors to produce a reasonable answer. Good luck!
  3. DavidG
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    DavidG Junior Member


    Thanks for the tip, the DesignFoil looks very user friendly and much less thumbed than my copy of Abbott and Doenhoff!

    I agree with you about the dificulties of quantifiying the sternwave, a friend of mine investigated this for a masters using CFD and couldn't get an answer.

    Designfoil is a 2D package, my calculation is that induced drag is the killer.

    Using 0012 section wings looking purely at the effects of lift reducing hull wave drag and friction drag, there was a 2% penalty at low Fn, at @ Fn .4 the effect is neutral.

    So the question again is, how much lift is there in hooking up to the stern wave?

  4. DavidG
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    DavidG Junior Member

    Correction, I have found the 3D bit!
  5. tspeer
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    tspeer Senior Member

    See for a means of estimating the lift and drag of hydrofoils near the surface. The induced drag doubles at the surface compared to deeply submerged.

    However, you're asking about the interaction of the hydrofoil and the stern wave. This requires a much more sophisticated capability. Farfield hull drag codes, like Mitchell integral methods, won't do the job. You need a near-field capability to define the stern wave and the local environment for the hydrofoil.

    As I see it, you have three ways to go. One is a sophisticated computational fluid dynamics program - very expensive, but it will tell the story about all of the interactions. Tow tank testing would allow you to quantify things, but won't tell you why different configurations acted as they did. Finally, you have two-boat testing, in which you just experiment with different rudder configurations, sailing against another identical boat for comparison. This would seem to be the only practical approach.

    (BTW, I've never sailed a National 12, but I did own a Merlin Rocket for a while.)

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    Check this site for foils and carbon parts.
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  8. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Might it be possible to get an answer using Michlet? If not, then trial and error might be the best way. I think Russell Brown makes the boards and rudders for Paul Beiker's International 14 designs. You might beble to purchase a current state-of-the-art example from him.

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    Guest Guest

    Michlet won't do this kind of job, for several reasons. First, it doesn't handle any of the dynamic lift on a planing or semi-planing hull. Second, it's really better suited for long narrow hulls compared to wide shallow hulls - that's why you see Leo applying it to multihulls, kayaks and shells instead of dinghies or power boats. Third, it doesn't accurately predict the close-in wave patterns and pressures on the hull, so it's not going to tell you what's happening at the foil location.

    IMHO, XFOIL is a much better code than DreeseFoil. The user interface may be harder to learn at first, but is well worth the effort. A good section design code needs two things: a really good boundary layer calculation and an inverse design capability.

    XFOIL's boundary layer code is one of the most sophisticated in the aerospace industry - it's an inverse method able to handle modest amounts of separation like separation bubbles and it uses the e^n method of predicting transition that's based on boundary layer stability theory. Drela's method is well documented in the literature and has been extensively validated against test data.

    There's not much info on Dreese's SNACK boundry layer method It sounds like it's a very conventional direct integral method. He cites Buri - who wrote his paper in 1931. Direct boundary layer calculations are not capable of handling any separation at all, so aren't very good at low Reynolds numbers or at high angles of attack. The only validation data Dreese shows are drag polars for a few NACA sections at a Reynolds number of 3 million. There are no lift curves or pitching moments shown. In the comparisions he shows his code along with test data - probably from NACA reports - and an unnamed competing code (perhaps the Eppler code?). There are many more reports, published in peer-reviewed journals, validating XFOIL calculations.

    An inverse design capability allows you to specify the fluid dynamic characteristics (like the pressure distribution) and calculate the shape that produces them. XFOIL has two such methods; DreeseFoil has none. Without this capability, you're just stumbling in the dark, trying to improve sections by trial and error.

    Dreese has put a lot more effort into the user interface, and that's nothing to sneeze at, however. The difference is whether you want an expensive shiny toy or a free professional code designed for real work.

    Given the complexity of the interaction between the rudder foil and the stern wave, the most straightforward way to go is to just experiment. It'll be cheaper and less effort to build the real thing than to get at it computationally. Use XFOIL to design sections for your requirements. Transition happens earlier and at lower Reynolds numbers in water compared to air (due to microscopic critters, bubbles, etc. in the water), and this can be simulated by setting the exponent to 1 in the stability calculation from its normal value of 9. Then try to get as much span as possible in your foil planform, consistent with strength requirements. Constant chord is easy to build accurately and performs well. A taper ratio of 0.5 may produce marginally less induced drag and will make the root stronger, but is harder to build.

    Probably the biggest thing to guard against is flow separation. You can put tufts on your foil by wrapping dental floss in a spiral around it, dabbing the floss with CA glue at regular intervals, and slicing the floss just ahead of the glue dabs. If you hang a digital camera over the transom, you can probably get some movies that will let you see if the tufts are lying down or lifting in separated regions. Pay particular attention to the junction between rudder and foil.

    If the flow is fully attached, it's a matter of playing with foil position and incidence to optimize the installation. Two boat testing with a buddy is probably the most sensitive performance indicator for judging the effectiveness of the changes.
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