Rudder and daggerboard profiles.

Discussion in 'Sailboats' started by Guest, Sep 25, 2002.

  1. nico
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    nico Senior Member

  2. tspeer
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    tspeer Senior Member

  3. JWC
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    JWC New Member

    I am presently fairing a new NACA0010 daggerboard and I am curious about the trailing edge. I haven't been able to find much info on this topic, but most boards and rudders I have seen are squared off at the trailing edge. I assume this is more for practical purposes considering the lack of strength for a fine edge. I'd appreciate any insight on how the trailing edge affects drag.

  4. SailDesign
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    SailDesign Old Phart! Stay upwind..

    "as skinny as possible, but as fat as it needs to be" seem to be the words of wisdom concerning foils.
    However, If it is a high-speed boat, consider knocking the "square" edge into a 45-degree angle to stop the little trailing vortices from humming as you sail. They make a terrible noise sometimes, and the energy to make that is - you guessed it! - drag.
  5. Unregistered

    Unregistered Guest

    moulded pre preg carbon NACA foils

    Fastacraft manufacture two sizes of moulded dinghy foils, one is NACA0012 and the other NACA0011. see
  6. kenwstr
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    kenwstr Junior Member


    I have a small slow sailing tender, I did say slow (savage Sprat).

    It has what looks like a MJ rudder fitted which is totally ineffective
    with a somewhat sharp LE.

    I am making a somewhat larger deeper rudder and was considering
    using the Eppler 472 section. This is a low RN aeorbatic section
    large LE radius with max thickness at 18% of chord from the LE.
    I am slimming the section down so the rudder will be 20mm thick
    so it still fits existing fittings.
    I chose this section because the boat has a strong tendancy to swing
    upwind in a gybe, or gust even though in steady wind, the helm
    is slightly to the lee. I am hopeing to prevent separation that is currently making the boat uncontrolable.

    Any Thoughts?

  7. SailDesign
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    SailDesign Old Phart! Stay upwind..

    It sounds as though the problem is more with the rig than the rudder :)
    An Eppler section sounds like overkill for a boat like that - most are adequately controlled by a metal plate. Try using a good ol' NACA 00-series, maybe 12-15% max.
  8. ken wstr

    ken wstr Guest

    Ordinarily I would have thought so too.

    However I am a very experienced sailer and have never come across as severe a problem as this. I have tried to balance the rig and it is just not possible.

    I can rake the mast forward to the point of definite lee helm, 10 deg or more on the tiller in steady wind. However a gust will produce severe weather helm and in some cased completely overpower the rudder. I suspect the hull shape is the basic cause so my only option is to compensate with a more effective rudder.

    I was given this boat, and the rudder is obviously not designed for it and is way too small. I scaled a new rudder to the same proportion of the centre board as on a Europe class. Even so, I think these helm variations combined with the boats low speed might be too severe which is the reason for considering the E472 section.

    There are so many problems with this boat, I want to get rid of it but feel it would be irresponsible in its current state. It's a real handful to keep control.

    The other reason for suggesting the E472 is that I am very impressed with it though I have lost access to the online polars. If I remember correctly, the Cl/Cd and Cl crit are quite impressive and would allow smaller rudder and boards to be used on well designed boats. Alternatively a thinner version might be a better optimisation between profile and induced drag.

    D = 1/2 p V^2 S Cd is the general form for gas dynamics, I read the 1/2 is used because of the compressibility of gas. As water is incompressible, should this value = 1 in hydrodynamics?

  9. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Good luck with this one - sounds like you're going to need it.
    Two things -
    1, Keep the 1/2 in the formula. It is there by definition. :)
    2. Remember that we are not, in aerodynamicist's terms, speaking of particularly low Reynolds' Numbers here. What may be great on a slow plane is not goingot fare so well on a slow boat, given the relative "nu" values. (Yeah, I know there's a way to produce the little widgets, but I can never remember it)
  10. DavidG
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    DavidG Junior Member

    Angle Of Attack

    Having a new centreboard and looking for a good section (National 12 slightly slower than Merlin Rocket!)

    My constraints are, centreboard case 30mm thick,
    Maximum Chord 360mm
    Mean Chord 280mm
    Span 1070mm
    Maximum RN 6.6*10^5

    Does anyone have any thought of what angle of attack sailing dinghies really are operating at upwind, Marchaj quoted @ 7 degrees for International Canoes, which is out on the boundaries of most bucket sections?

    Is it realistic to think that a foilmaker can handcraft an Eppler or Wortman section well enough (sorry Andy P I'm sure you can) to make the effort worthwhile?

    What happens when my crew treads on the trailing edge of a laminar foil?

    If the answers to these questions are what I think they will be, I suspect that a trusty NACA 0008 fattening out to a NACA 0011 will be the reliable, but somewhat unimaginative solution.

    Any thoughts welcomed!

  11. brian eiland
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    brian eiland Senior Member

    HI-aspect ratio rudder!!

    Tha latest issue of Profession BoatBuilder had a very revealing photo on the cover. (#90 Aug/Sept 2004). Its a picture of the stern of the big cat Club Med flying a hull. But most amazing is the rudder profile.

    I have a very difficult time imagining that this rudder profle would not be SOOO sensitive to stalling, particularly on a hi speed run downwind and down the face of those waves in the southern ocean.

    And it doesn't leave a lot of margin for error in the balancing of the rig over the underwater foils....seems as though it could develop a nasty helm real easy.

    And to go all the way around the world, in a VERY fast mode, with all of the obstacles out there in our oceans, with only two slivers of steering devices below you like this, Balls!
  12. tspeer
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    tspeer Senior Member

    Yes, that would be well outside the bucket of a NACA 6-series section.

    If the angle of attack is 7 degrees, it's because the board is well sized to the load. You could get the angle of attack down to less than 3 degrees by more than doubling the area. But then you're more than doubling the parasite drag, too.

    As long as the flow is attached, differences in section design are going to make fairly minor differences in the drag of the boat. The key is avoiding separation. A section that has a higher maximum lift can let you reduce the chord to make up for some increase in the minimum drag.

    Yes, especially these days with numerically controlled machining of plugs, cores and templates.

    That depends on how strong the trailing edge is! The trailing edge doesn't have much to do with maintaining laminar flow. The boundary layer will be turbulent by the time it gets there. Scratches and small dirt deposits will be buried in the boundary layer and not cause a lot of drag.

    A sharp-cornered square trailing edge of modest thickness is just as effective as a knife-edged trailing edge, and a whole lot more robust.

    At your Reynolds numbers, the problem isn't maintaining laminar flow, the problem is how to get rid of it gracefully! Laminar flow has less skin friction, but it also separates much more easily than a turbulent boundary layer. Below a Reynolds number of 250K or so, it's not possible to achieve transition naturally without separation within the chord length of the foil. So there will be a laminar separation bubble and the section needs to be designed to make the bubble short and to move smoothly from well back on the chord at low angles of attack to near the leading edge at high angles of attack.

    The NACA 4-digit sections have exactly these characteristics. So, yeah, I think I'd go with the tried and true.

    I'd look to reduce drag some other way, such as by making the board longer.

    Has anyone tried a Bieker-style pitch trim foil on a National 12 rudder?

    (p.s. I used to sail Merlins at Cookham Reach)
  13. DavidG
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    DavidG Junior Member

    Three people have tried varients of the theme, one with an adjustable tab on the bottom of a rudder daggerbox cassette, one with tilting pintles with a rudder with fixed tabs and one similar to the Beiker using push cables and windsurfer fins, in all cases there is little change in performance.

    I did a spreadsheet analysis of the reduction in hull wetted area using different foil areas and angles of attack and concluded that trim foils work in 14's because the V^2 function really starts working at planing speeds, and 14'ers ar not interested in lightwind sailing.

    Would be interesting to know of any CFD or Tank work that can analyse the apparent energy recovery from the upwash off the hull? What do you think Tom?

    I see Pat Blake around, in fact many years ago I owned Mythelated Spirit.

    Anyway I am off to Salcombe and then the Championships at Weymouth.
  14. Milton Thrasher
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    Milton Thrasher Amateur boat builder

    Daggerboards and rudders with scalloped edges proposed


    In the August 2004 issue of Scientific American there is an article that discussed the investigation of the hydrodynamic performance of humpback whale flippers. Dr. Frank E. Fish, a biology professor at Pennsylvania’s West Chester University and a specialist in the hydrodynamics of vertebrate swimming noticed that the humpback whale pectoral flippers had evenly spaced bumps along the leading edge.

    He worked with fluid dynamics engineer Laurens E. Howle of Duke University and David S. Miklosovic and Mark M. Murray of the US Naval Academy. Howle built 22 inch long plastic fins, one with the scallops and one with a smooth leading edge. In a wind tunnel at the Naval Academy testing the scalloped fins showed advantages, especially at higher angles of attack. The test results were reported in the May issue of Physics of Fluids, Leading-edge tubercles delay stall on humpback whale (Megaptera novaeangliae) flippers). The scalloped fins generated 8 percent more lift and as much as 32 percent less drag than the same size smooth fin, (probably at high attack angles). The scalloped fin resisted stall 40 percent steeper than the smooth fin.

    The reported reason for the performance improvement was that the flow coming off the bumps formed pairs of counter rotating swirls or eddies coming off each side of the bumps. The bumps act as vortex generators that keep the general flow attached to the suction side of the fin when there is an angle of attack that normally would cause the flow to separate from the leading edge.

    There may well be an application for r/c keels and rudders since they operate in the transient Reynolds number range between laminar and turbulent flow. The current trend for keels is to make them very thin with sharp leading edges, which makes them liable for flow separation at low angles of attack. The scalloped leading edge delays separation and thus improves turning ability without causing excessive drag. The drag of the scalloped fin was similar to the straight fin at zero angle of attack. A scalloped rudder would work the same going down wind, but when the rudder is kicked over for a tack the rudder would create a faster turn while losing less speed through the turn. It’s possible that a million years of whale evolution may have developed a better rudder for us.

    Dr. Frank Fish has patented this concept.

    Scalloped edges could be added to existing blades for
    testing against standard blades on matched pairs of one-design sailboats such as Lasers or Sunfish to prove or disprove the theory.

    I am considering trying it on my Irwin 23 with wing keel and high aspect
    rudder but without a trial horse it would be hard to judge the advantage.
    I would be interested in hearing the results if someone else tries it.

    I will post a graphic image showing a pair of plastic fins, one curved edged and one scalloped to be sure that the concept is conveyed correctly.

  15. tspeer
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    tspeer Senior Member

    I agree, but I don't know of any CFD code that would be capable of handling that kind of near-field free surface hydrodynamics and be affordable to the amateur designer.

    Sounds like it could be a good topic for a university student, if there was someone to fund the project.
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