12ft skiff dinghy centreboard discussion

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

  1. johnhazel
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    johnhazel Senior Member

    This should help. The square tip wing AR is 6:1 the raked tips, a bit more.
    You can use the L/D results as rough estimates basis.
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091205.pdf]

    Fancy but really hard to build...
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091648.pdf
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940012101.pdf

    other stuff
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091311.pdf

    reynold number effects on AR=6 wings: Note the drag shown for theroretically perfect elliptical wings with no skin friction compared to staright chord wing with rounded tips.....
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091301.pdf
     
  2. Mikko Brummer
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    Mikko Brummer Senior Member

    Interesting paper about elliptical/crescent/square wings, I had not seen it before. Would provide a good basis for CFD validation, too. The advantage of a square wing over an elliptical was common knowledge already when I studied in mid-70's. The Cessna wing.

    One more thing to consider when choosing the planform is the effect of the hull/CB interference with the hull. We are currently doing a study on the 470 centerboard, which is square(ish) with rather a simple profile stipulated by the rule. The attachment shows how the lift is transferred from the board into hull.

    Yet another thing is the effect of the free surface (not shown in the 470 sim attached) - while dinghies are sailed unheeled, the root chord still gets often exposed when planing upwind & jumping waves. Rumour is also that the bending & twisting of the CB is crucial to performance - that's what we are currently looking into.
     

    Attached Files:

  3. Ben G
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    Ben G Junior Member

    Thanks for the links John, I'll take a look. Might be a while!

    Mikko, it will be interesting to hear about what you find - if you're allowed to share! I've heard that 470 boards twist to increase AoA as they load up however this intuitively seems like a bad idea.. I would have thought you'd design a rectanular profile to twist away to unload the tip as the side force increases. 470's are one of those peculiar 'one design' cases!
     
  4. Mikko Brummer
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    Mikko Brummer Senior Member

    The conventional view has been twisting the LE to windward under loading, to emulate a jibing centerboard. Agree that doesn't seem like a brilliant idea. Twisting away the TE could be dynamically efficient, but difficult to achieve structurally, and complicated by the fact that sometimes you have forward rake (negative sweep), sometimes the board straight down and when windy plenty of aft rake (positive sweep).

    This caught my eye today, not much of lift transfer there - you have to allow some reserve area for situations like this.
     

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  5. Ben G
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    Ben G Junior Member

    [​IMG]

    Pressure plots compared for NACA 0012 and WortMann FX-76-120 at 0 deg and 1.5 deg aoa, 5 deg sweep. FX-76-120 has the higher -Cp values at x/c = 0.1

    More to come..!
     
  6. johnhazel
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    johnhazel Senior Member

    So build one or the other and see if it makes any difference. One might expect better tacking performance and upwind ability. Elswhere, no noticeable difference. Give it a try!
     
  7. BlunderBus
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    BlunderBus Junior Member

    Eppler 1161-24-0915

    Have you thought of using the Eppler 1161-24-0915.

    I get the impression that it may have been discounted as it is only 9.15% thick and may lack strength.

    I've recently built a rudder based on this profile for my Javelin (14' Skiff) as it has a good combination of low drag and wide stall angles. Unfortunately I can't quite tell you how it performs yet as the rudder gantry is still being glued into the boat. The construction is a laminated Pawonia wood core cut out on a CNC. The layup I went for was 3 layers of Uni, and one of twill (all carbon). I can't wait to get her on the water.

    The key I've noticed to strength in a board is making sure your core can cope with the compression loads. Recently I was out on a Shaw 650 which we broke the rudder on. I was so amazed to find it hardly had a core and you could see that it was the compression that gave way.

    I'm not sure if I've made the best choice but it was more to go through the process of design and build rather than going for perfection as the old rudder system was shot.

    Just a thought,
    Hamish
     

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  8. Cheesy
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    Cheesy Senior Member

    Slightly off topic, Im about to build new foils for a Sunburst I have aquired could you let me know where you got your wood from, or did you get it supplied as a blank from the CNC place?

    Thats interesting on the Shaw rudder, the one I used to sail on didnt have any core, hollow rudder and centre board.
     
  9. Ben G
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    Ben G Junior Member

    I don't think it's an Eppler, It looks like that section shape is by Bransford Eck as described here:
    https://docs.google.com/file/d/0B3Zekt2OTzO4SEVKc0VfQU5HMjA/edit?pli=1

    Yes I kind of overlooked it because it was thin. But if you read the document it sounds good.
    I'm currently somewhat confused by what pressure distrbutions are good (or what provides low drag). There is a reasonable difference between foils particularly at high speed low angle.. some have a Cd of 0.075 while others are 0.1.. 25% difference there..
     
  10. Ben G
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    Ben G Junior Member

    Polars Comparing the NACA 0012 and the Wortmann FX76-120. Would like to compare the 10% foils by Selig and Eck..
    Wortmann looks like the lower drag option, as well as higher stall angle, not sure if there's computer voodoo here, would like to compare actual experimental results.. not so easy..!

    [​IMG]

    Can force transition at a certain % of chord, not sure where transition is in this plot.
     
  11. Ben G
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    Ben G Junior Member

    So I decided that I could do better than Javafoil, so I've been playing around with Xflr5 which seems to be better.

    I have just found that the Eck 1161-24 foil shape (pressure distributions) is identical to the Selig 8020 foil shape. Seems like someone has made a copy?!
     
  12. Mikko Brummer
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    Mikko Brummer Senior Member

    There is good experimental data on the SD8020 in Summary of low speed airfoil data, up to Re 300.000. Still, I'm afraid you could be chasing shadows here, that the theoretical differences won't materialize in the real world. Maybe because the windtunnel tests are done in very low turbulence, and in the real world turbulence can be high. So I wouldn't trust a profile which relies too much on laminar flow. Or compare the profiles by tripping turbulent early.
     
  13. BlunderBus
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    BlunderBus Junior Member

    Paulownia Wood Supply

    Hi Cheesy,
    Try http://www.paulownia.co.nz/
    Rod will sort you out.
     
  14. BlunderBus
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    BlunderBus Junior Member

    Sounds like we should go sailing about now. i.e. Build, sail, and test.
     

  15. Ben G
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    Ben G Junior Member

    Yes good point.. it's becoming clear the differences between the decent sections are somewhat academic, depending on NCrit and Reynolds numbers.
    It is useful to get a feel for what is good though.

    I can then use the section drag coefficients to trade off aspect ratio. Earlier I had a look with Javafoil, which has some convenient features for this. Was showing an aspect ratio of around 12 (actual AR was 6.3 with say 1.9 multiplier; boat has a flat bottom at board exit).
    Hoping to get More exact info on section drag then will try to reiterate with XFLR5.

    Apologies to those who are impatient to build something.. it's winter here, and I can play on the computer while the new baby sleeps :rolleyes:
     
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