12ft skiff dinghy centreboard discussion

Discussion in 'Hydrodynamics and Aerodynamics' started by Ben G, May 1, 2014.

  1. JRD
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    JRD Senior Member

    Suki, taking a ride on a 12'er can change one for ever. Highly recommended and totally unforgettable. Mining it at 20 knots and being thrown through the mainsail is even harder to forget, funny that.

    Ben G, Tank Girl is a huge achievement, well done. Designing and building your own skiff is no small feat. That photo of you on SA sending it down Wellington harbour was pretty impressive.
  2. Ben G
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    Ben G Junior Member

    Ha, thanks mr JRD!
    The trick to racing them is keeping your thinking cap on while on the edge of control, very entertaining!

    I've been going through some stuff on XFLR5, will post more when I have some results that are meaningful.

    Here's a comparison of some different foil sections at Re = 1e6. For reference, at 10 knots the reynolds number is 1 million, 3 knots = 300,000 etc.

    The operating point will be around Cl = 0.2 - 0.3 upwind or 2 - 3 degrees.
    One of the better options is an SD 8020 section that I scaled to 11%.. going up in thickness slightly increases the stall at low Re a fair bit without changing drag much. NACA 0012 still does pretty well all things considered. Haven't compared a 10 or 11% NACA section yet.


    Drag at low angles, Re = 1 x10^6 and 300,000:

    There's some low drag sections in there. 'HQ/Acro' sections look good but do stall earlier, though not as bad as some..

    It will be interesting to compare the drag of different aspect ratio foils. A comparison with Javafoil is below, at same foil load and different AR's. The trend is obvious, but the actual numbers probably aren't right. The curves represent each foil as its angle of attack increases to keep the sideforce constant, as the boat slows down.

    It's clear a lower aspect ratio board will be better on the runs (less skin friction) while a higher aspect board is better when loaded more heavily (upwind). Keen to get this working properly in XFLR5.

  3. SukiSolo
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    SukiSolo Senior Member

    That SD 8020 section looks pretty good. Nice to see some figures for it. Worth doing that size aspect (and AR) as I'm sure you will find the larger one better upwind in light air despite the extra area drag (minimal). Also real world turbulence means you need a little margin beyond theory ie a couple of percent bigger and you won't notice it. A couple to small and it crucifies you. Any Laminar flow say 63 series sections and/or derivatives? Check Tom Speers H105 section too, which is quite thick at 12.5%, but has a wider 'bucket', but may be better at the high end of he speed range.

    Is there any way you can get a sort of daggerboard box 'stuffer' in there? so you could have the foil at 100% depth or say 90% deep without suffering extra drag from the end plate (effect) of the hull underside?. Saves having to make an extra board and being caught out with the wrong one if the wind strength changes.
  4. Ben G
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    Ben G Junior Member

    The H105 section is cambered..
    iirc the 63 series sections stall fairly early, will do a comparison. The stall performance is needed at low Re. ie 2-300,000, which narrows down the choice.
  5. tspeer
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    tspeer Senior Member

    Try comparing the polars on the basis of Ncrit=1, or putting trips near the leading edge (say, @ 2.5% - 5%). This is because transition occurs earlier in water than in air. (I don't know why, but all the America's Cup guys I've talked to that have measured laminar flow on keel fins and bulbs have found it to be the case.) Then if you get laminar flow, it's a bonus.

    You should also add the induced drag to get the total drag polar. I think you'll find that span trumps section design by a long ways.
  6. Ben G
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    Ben G Junior Member

    Thanks, I did a little bit of reading and thought Ncrit=3 would do, but will compare having it at 1 and/or tripping early as you say...
    I'm planning to do polars for the full foil and so on (Javafoil gave me the idea, hopefully I can improve further)

  7. johnhazel
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    johnhazel Senior Member

    From the paper 'Bus is talking about.

    Selected airfoil is NACA 0009 ( as in my windsurfer fin experiment)

    Lift, and Drag calculated for each planform:

    Rectangular, 349.7N, 31.0N
    Trapezoidal, 349.6N, 29.5N
    Optimal, 349.5N, 28.8N

    7% difference in L/D of the fin between rectangle and optimized board.

    This may seem like a lot of difference, but when you look at it with regard to total drag of 250N, the 2.2N difference is tiny on 5o5 a class sailboat. And this is only for going to windward.

    So in a race there is roughly an overall half percent difference in drag for the optimal vs rectangular fin. All else being equal maybe a 5 to 15 second difference per hour of racing? (10 to 40 meter lead)

    But all else is not equal. The rectangular board will have better stall characteristics and it's acceleration after each tack will be less hampered by stall issues. For example you would be able to point high sooner after the tack because the rectangle fin has higher reynolds numbers.

    That being said there is a red flag in this paper: Lower induced drag is usually the result of optimized planforms. This implies that as angle of attack increases, the improvement should also increase. However fig. 9 shows the opposite trend. One way to explain this is that maybe the drag reduction is predominantly due to lower fin area rather than less induced drag.
  8. Mikko Brummer
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    Mikko Brummer Senior Member

    Good analysis - agree that the difference in the paper could be mostly due to optimal area for the 5 deg leeway and 5 kn speed. And the paper makes no allowance for the free surface, or the effect of the hull (board/hull interaction).

    Looking at the numbers, maybe interestingly my analysis for the rectangular 470 board at 5 deg leeway and 5,2 kn speed gives 354N of lift for 33,4N of drag. The pure skin friction drag of the board is 10,8N, and the average skin friction coeff is 0,0042. This is without the free surface, with a mirror image model, and assuming a 5% free stream turbulence.
  9. SukiSolo
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    SukiSolo Senior Member

    Interesting observation, maybe the greater the density of the fluid, the earlier it will trip. Any surface roughness would become super critical at triggering transition. Might also be the nature of leading edge separation is slightly different as the fluid becomes more viscous. Or is it more to do with the lack of compressibility of water and thicker fluids compared to air?

    Have they (AC techs) tried measuring it with a thicker (than water) fluid? A syrup type or maybe oil perhaps.

    Ben G, I am well aware the H105 is cambered, but the 'upper' section shape is sweet and if you made it symmetrical and around 10% thick or find an equivalent section, I doubt you would be far out from a decent foil. What do you estimate is your skiff's absolute top speed, 25? 28 Kn?
    Last edited: Jun 13, 2014

  10. Sergej725
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    Sergej725 New Member

    See: http://www.cerfacs.fr/4-26769-Aeroelasticity.php
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