Rudder and daggerboard profiles.

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

  1. Guest

    Guest Guest

    Hi,

    Actually designing appendages for an international 14 dinghy, i wonder what kind of profiles i could use out of the classic 0012. Maybe some kind of laminar section. I heard about "DAG" profiles designed specially for sailboats but could find no documentation on those.
    Could anyone help?
     
  2. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    The best approach is to download XFOIL (http://raphael.mit.edu/xfoil/) and design a section for your specific requirements. The NACA 6-series laminar flow sections are not especially well suited for sailboats because they are prone to leading edge stall at low Reynolds numbers. The same leading edge separation problem would lead to ventilation when used as a rudder at high speeds. But you could use XFOIL to tailor the leading edge profile and improve them considerably.

    Other sections to consider would be the Eppler E520, a 15% thick symmetrical section originally designed for sailplane vertical tails; and the E836, E837, and E838 series, which were specifically designed as hydrofoils and range in thickness from 12.7% to 18.4%. Personally, I think Eppler placed too much emphasis on cavitation for these hydrofoil sections, and I suspect they will also have difficulties at low speeds. But they could be good starting points for a new design. There are also some Wortmann sections that would be good candidates.
     
  3. DavidG
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    DavidG Junior Member

    Four digit NACA sections in my experiece are pretty good for sailing dinghies. Bucket sections such as 63 or 64 series may look attractive, because of the reduced drag in the bucket, however Marchaj identifies the angles of attack that dinghies operate upwind (Int Canoe) which is on the borders of the bucket, offwind the board is up (or partly up) so the reduced drag is less relevant.

    I tried discussing sections with Bethwaite, but he couldn't support his sections with hard data.

    I would give more consideration to using a gybing board, the effect of this is that you effectively sail 2.5 degrees freer (therefore faster) than those around you, for the same pointing!

    All this is meaningless, if you mangle the LE or TE. THe board is also useful to stand on.

    DG ex GBR1301,1242,1208
     
  4. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    Yes, they are. I was surprised to see that at low Reynolds numbers and low angles of attack, the NACA 0012 (basis for all the NACA 4-digit sections) has laminar flow over 70% of the chord! The transition point moves forward smoothly with angle of attack, which is exactly what you want for a robust low-Reynolds number section. It goes a long way toward expaining the well-deserved repuation of the NACA 0012 as a good all-round section.

    I agree that the NACA 6-series sections are not a good choice for sailboats. Many of their vices have been associated with laminar flow sections in general, but that need not be the case. However, modern tools, like XFOIL, allow you to make the bucket much wider so that it does encompass the normal range of angles of attack. The 6-series used a constant roof-top pressure distribution right to the leading edge, and a straight-line pressure recovery to the trailing edge at their design condition. So they were pretty much single point-design solutions.

    You can make a more robust section by designing each segment for a successively higher angle of attack as you get to the leading edge. This effectively rounds off the pressure distribution compared to the NACA 6-series. It widens the bucket and makes the section much more robust at higher angles of attack, eliminating the leading edge separation of the 6-series sections. This addresses the nasty ventilation characteristics of a 6-series rudder at high speeds, and makes a 6-series board easier to accelerate out of a tack.

    You can also make a more rounded transition between the rooftop and the recovery region, and make the recovery itself somewhat concave for better performance. The rounded transition promotes a short laminar separation bubble to control boundary layer transition from laminar to turbulent. Making the pressure recovery somewhat concave allows a greater amount of recovery in a shorter distance. So you can extend the laminar roof-top for a lower minimum drag. Or you can raise the rooftop for the same length of laminar flow, which increases the design angle of attack and makes the bucket wider. But don't make it too concave, or you will lose the progressive trailing edge stall.

    Past investigators have tended to use the NACA sections without these adaptations to make them suitable for use in boards and rudders. Or they compared ad hoc changes (sharper or blunter) to leading edge shape without calculating what the flow would really do. As a result, the whole concept of low-drag laminar sections has fallen into disrepute among sailors.

    If you don't want to design your own sections, the Eppler and Wortmann sections would be better choices than the NACA 6-series. There are a number of databases where you can get the coordinates for more modern sections. Two good ones are
    http://www.aae.uiuc.edu/m-selig/ads/coord_database.html
    http://www.nasg.com/afdb/index-e.phtml
    If polar data are not available, you can easily generate them using XFOIL.

    He's definitely an experimentalist, not a theoretician. Plus, the main airfoil design tool available when he was doing his research was the Eppler code, and it's very hard to use as well as not being able to handle things like Bethwaites wingmast sections. However, his data are not to be sneezed at - he's done a great job of basing yacht design on rational observation rather than "just-so-stories".

    Bethwaite has sent me some tracings of Taser mast sections for me to compare with tear-drop wingmast sections. Unfortunately the tools I have now can't handle the backward-facing steps between mast and sail. So I need to work on getting a Navier-Stokes code installed so I can generate some CFD data to match his experiments.

    It seems to me that a jibing board will operate at the same angle of attack as a conventional board. The reason is that the leeway will increase until the board produces enough lift to match the loads applied by the rig, and the contribution of the hull at low leeway angles is probably small compared to the board and rudder. The board is going through the water with essentially the same orientation whether it is jibing or not, because it still needs the same angle of attack to produce a similar force.

    So what you're really doing is rotating the hull relative to the board. This has implications above and below the waterline.

    If the hull is generating a significant amount of lift, this load will be shifted to the board, which should be able to carry it with less drag than the hull, due to the much greater span of the board. However, if all the hull lift is eliminated, this reduces the effective span of the board to being the same as its physical span. So there's a benefit to having the hull carry some of the load, as long as it doesn't cause flow separation or shift the lateral balance and increase the rudder trim drag. From a lift-induced drag point of view, the optimum hull load would result in the wake coming off the hull being deflected sideways at the same angle as the wake coming off the board. You want the whole vortex wake shed into the water to behave as if it were a rigid sheet along the whole span from waterline to board tip.

    Obviously, it's hard to determine just what amount of lift this is without a very sophisticated calculation or test program that takes into account the board, hull, and free surface.

    I suspect the greater effect of a jibing board comes from the rig rather than hydrodynamic effects. Rotating the hull makes it act like a ballestron boom. This opens up the angle between foretriangle and the apparent wind, effectively moving the forestay to leeward. The main should not be affected as much because it can be sheeted to the same angle of attack in either case.
     
  5. JWC
    Joined: Oct 2003
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    JWC New Member

    Why not make a daggerboard that has a compound foil?

    What I'm thinking about is a NACA 0010 or 0012 for the upper 2/3 of the board and a 63-010 or 63-012 for the lower 1/3. This way you get mostly the characteristics of the 00 series when the board is fully down sailing upwind, and the lower drag 63 series section when the board is raised sailing offwind.

    Perhaps there is not much point to going with the 63 section for the lower 1/3 since the area is reduced so much downwind that it doesn't matter much?

    I have an NS14 which is a box rule boat similar to an I14, but with a lot less sail area. I'm planning to make a new board soon and am searching for ideas.

    Thanks for the info on the earlier postings. Lots to consider.

    :confused:
     
  6. ggggGuest

    ggggGuest Guest

    Foil sections

    0063 is pretty conventional for high performance dinghy sections. I understand that increasing the chord has the effect widening (and shallowing) the drag bucket, the twelve foot skiffs for instance carry relatively fat foils. Better a fatter foil operating in the bucket than a thinner one operating out.

    Rudders are another game entirely and need to be very good at high angles of attack with excellent stall properties. I don't know of a conventional section.
     
  7. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    JWC,

    A very good idea indeed, the problem is, though, that most of us (LARK Sailors) sail downwind with no board at all, just relying on the hull area if we possibly can. However, your idea may come into it's own using a dagger-board, where it is very difficult to remove enough board without fouling the vang. Remember, though, that as a centerboard is lifted, it rotates, and therefore, the effective chord (parallel to the flow) increases. Therefore, if you sail with a 12% foil fully down, you sail with a 6% section when the board is raised 60 degrees. Tspeer mentioned X-Foil, it is one of the best freely available codes, it does not consider cavitation, but it is pretty good for the rest of the time.

    Cheers,

    Tim B.
     
  8. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    Rudder foils are strange - they do not actually operate at high angles of attack in lighter, easier-to-turn boats, as the stern starts swinging quickly, altering the vector to a much smaller angle than it would at first appear. The rudder on American Express (Steve Black's tri) was a 10" chord 0020 foil, and worked extremely well. When you are likely to be moving fast, reduced area is the key. you don't need area, but you do need strength, hence fatter foils with smaller chords. This is also good for spade rudders that are simpler to build if the stock can be enclosed within the blades at least at the root.
    Steve
     
  9. Guest

    Guest Guest

    I have a 14' dinghy designed by John Spencer which is similar in many ways to an NS 14. I have built a rudder using a foil designed by Martin Hepperle. It seemed to me that the reynolds numbers would be similar to racing type model aircraft wings, so I decided to give it a go. I can't really be sure yet because of limited testing against other boats but it hasn't slowed me down any.
    Does anyone have an opinion on this reynold number similarity. Also another possible foil series worth looking at are those designed by Harry Riblett for light aircraft. They specifically address some of the the leading edge problems of NACA sections and are optimised for RN 1000000.
    Both are laminar flow sections and Martin Hepperles web site is at: http://www.mh-aerotools.de/airfoils/index.htm
     
  10. nico
    Joined: Jan 2003
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    nico Senior Member

    Xfoil - Ncrit

    What ncrit should be used for analysis of underwater sections?

    Nico
     
  11. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    I've heard ncrit = 1 because transition happens sooner in water than air - all those microscopic critters, bubbles, and particles. I haven't made any runs to compare ncrit = 1 with the default (ncrit = 9, IIRC), though. Look through some of Drela's posts in the XFOIL mailing list archives.
     
  12. Matt Lingley
    Joined: Dec 2003
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    Matt Lingley Junior Member

    I'm playing with a NACA 63-009 section for rudders on a fast Cat, is this a good idea? I've been told it may be prone to stalling?
     
  13. Andy P
    Joined: Nov 2003
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    Andy P Junior Member

    I make lots of foils for many classes, and normally use the NACA 00 series, but use a NACA 63 variant for faster boats that make less leeway eg int Moth, Cherub.
    The disadvantage of the 63 is after a bad tack, when it takes a while to get going again.
    I sometimes also use the 63 section for rudders - they appear to ventilate more easliy than the 00, but by quickly moving the tiller and unloading the foil, the flow reattaches much faster than for 00, where you need to slow down as well ( if you can before spinning out)

    Gybing boards:

    These work best in flatter bottom hulls without pointy stems. ie they don't work well in boats that use the narrow bow sections to help with lift.
    So not used in cats, moths, UK National 12, Merlin rocket, but work OK in 5o5, Fireball.
     
  14. Unregistered

    Unregistered Guest

    hi;
    my name is emad and i am m.s in naval architect and work in arvandan ship building co. in iran.
    i want naca series for rudders , if you have please give me copy.
    thank you,
    best regards.
    bye.
     

  15. Unregistered

    Unregistered Guest

    hi;
    my name is emad and i am m.s in naval architect and work in arvandan ship building co. in iran.
    i want naca series for rudders , if you have please give me copy.
    thank you,
    best regards.
    bye.
    my email address is :"emad_rabeaee@yahoo.com"
     
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