formula 18 katamaran daggerboard

Discussion in 'Multihulls' started by cjs, Apr 8, 2006.

  1. cjs
    Joined: Apr 2006
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    cjs formula18

    Hello

    I'm designing new daggerboards for my formula 18 katamaran. I have build a VPP in MATLAB that I can use to studie different AR and section shapes etc.

    One section that seems to work out well is the naca 0009 section. The 0009 seems to be supperior to the 63-009. Is that possible? The Cd-Cl information that I have been using comes from Javafoil with (Re=900000).
    Can You recomend some other section shape and if so where can I find polar information?

    My VPP doesnt take in consider the increased drag that the hull is causing with different angles of attack. For example what is best

    7.33knots and 1.7679 degres of attack
    or
    7.25knots and 1.5184 degres of attack.

    brgds CJ
     
  2. tspeer
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    tspeer Senior Member

    It's not only possible, it's a virtual certainty. The NACA 63-009 has a very narrow drag bucket around 0 lift. When you get outside that narrow region, the profile drag doubles. A NACA 4-digit section like then 0009 has a somewhat higher minimum drag at 0 lift, but a much flatter drag characteristic away from zero lift, which is where the foil operates most of the time. So the NACA 0009 is much better for a daggerboard.

    The best approach is to use a program like XFOIL to design a section specifically for your requirements. Since you have a VPP, you can determine what the required lift is going upwind and downwind at different speeds, and you can optimize a section for those operating conditions.

    The problem is, with a symmetrical section you don't have a lot of flexibility, so a good all-round section like the NACA 0009 or NACA 0012 is going to be hard to improve upon by very much. You're much more likely to gain by increasing the aspect ratio of the board than you are by using a different section.
     
  3. cjs
    Joined: Apr 2006
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    cjs formula18

    Thank You for Your answer.

    Does the narrow drag bucket around 0 lift on the 63-009 getting even narrover with higher Reinholds number, (hydro foil compared wtih airfoil)?

    I have reed in "Priciples of Yacht design" that the 63-009 should have smaller drag then the 0009 in up to 2.5 degres of attack but that doesnt go hand in hand whith my VPP, and as I can understand with Your experience.

    Maybe the writer to "Prinsiples of Yacht design" (and others that is prefferin g the 63-0xx) series. is using data for airfoil common re number?

    How about the 65 series, I spoke to Göran Marström and he is currently using the 65-010 on all his cats.
     
  4. tspeer
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    tspeer Senior Member

    First of all, the profile drag makes a small contribution to the overall drag of the boat. So whether you have the minimum drag section or not may not be very apparent. The difference between two good section choices is going to be minimal. The difference between a good section choice and a bad one - like one that has a nasty leading edge stall, leading to lots of bad tacks - is where section design is going to matter.

    XFOIL would be a good tool to evaluate different sections. Transition seems to occur earlier (at a lower Reynolds number) in water than in air - but I can't tell you why. People have postulated everything from turbulence due to wave action to dissolved gasses to plankton and algae. You can simulate the earlier transition in XFOIL by using a value of 3 for Ncrit instead of the default value of 9.

    If you are operating in the drag bucket of the NACA 63-009 section, then you have a board that is way oversized. What's important is not the drag coefficient, but the drag itself. Or the drag area, Cd*S, if you divide out the dynamic pressure contribution.

    Say you had two boards, both with a NACA 63-009 section. One board operates at, say, 2 degrees angle of attack to stay in the drag bucket, and the other board is about half the size and operates outside the drag bucket at 4 degrees angle of attack (corrected for sidewash). Both boards have the same span, and for the moment let's ignore Reynolds number effects due to the different chord lengths. Since both boards produce the same lift (that's determined by the sail trim), which board has less drag?

    The answer is the smaller board, because while the drag coefficient goes up by 50% outside the bucket, the reduction in area more than makes up for it (150% * 50% = 75% of the drag area of the larger board). If you'd given the small board a NACA 0009 section, there would have been even less profile drag because the NACA 0009 has less drag just outside the drag bucket.

    So it's vital that you optimize the area of the board for the lift range you need, and then pick a section to match. Minimum drag at zero lift is not really very relevant to an F18, because you proabably never sail dead downwind. I would guess that you go downwind at something like 90 degrees apparent wind, so the board is always loaded up to a greater or lesser degree. This makes a NACA 4-digit section a good choice compared to a symmetrical 6-series section.

    If you cambered the board to position the drag bucket over your operating point, then things would be different. You'd want a section that had a drag bucket wide enough to cover the difference in operating point between upwind and down, plus an allowance for waves and modest maneuvers. You would optimize the chord length at the same time. A larger board will shift the operating point to a lower lift range, making the percent change between the low-lift and high-lift sides of the drag bucket larger. The drag would go up with the larger area, but keeping inside the drag bucket would be worth it. And the larger area would make the boat more robust on the tacks. A smaller board would reduce wetted area, but make staying in the drag bucket more critical. So optimizing the section also depends on how you sail the boat - are you good at staying in a narrow groove, or do you need a design that's more forgiving?
     
  5. cjs
    Joined: Apr 2006
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    cjs formula18

    It´s really seems that the 0009 section is a good choice.

    Is a high aspect ratio daggerboard harder to tack with than a daggerboard whith lower aspect ratio, if both generates the same lift at for example 5 knots speed. The high AR board have smaller area.
     
  6. tspeer
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    tspeer Senior Member

    Under steady conditions, both boards will produce the same lift. But they will have different leeway angles. The smaller board will have a higher leeway angle and operate closer to stall. This gives it less reserve when the speed drops in a tack, requiring even more leeway to generate the lift required to oppose the loading from the sails.

    It's the depth that reduces lift-induced drag, not the aspect ratio, when the lift is kept constant. Aspect ratio is depth^2 / area, which reflects the influence of dept on low-speed drag and the effect of area on high-speed drag. Equal aspect ratios really say the tradeoff between low-speed induced drag and high-speed profile drag is the same.

    So the tradeoff is depth vs wetted area. If you keep the chord the same, a deeper board will have less induced drag and be more forgiving in a tack by virtue of its greater area. These both make it better at low speeds and when highly loaded. But it will have more drag at high speeds because of the extra wetted area. So in this case, the higher aspect ratio board will actually be more forgiving in a tack.

    You can reduce the chord to get back to the original board's profile drag at high speeds, and the same area for comparable tacking performance. Now you have the equal area, high aspect ratio choice. Better upwind performance, and no drag penalty at high speed compared to the baseline.

    But if you keep the depth the same and cut away the chord to reduce high speed, drag, then there's no reduction in induced drag and the board will stall easier in tacks because it's more highly loaded for its area. This time the high aspect ratio board is much more critical to sail.

    So when you hear, "High aspect ratio boards are harder to tack," you have to be very careful as to just what was changed to alter the aspect ratio. It's not so much the aspect ratio that matters, but the depth (squared) and area.
     
  7. cjs
    Joined: Apr 2006
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    cjs formula18

    Thank You very much.

    My previous daggerboards had the format 95cm x 26,3 cm.

    For the new one I am thinking of 130cm x 17cm. I think carbon fibre will make them stiff enough.

    My VPP is telling me that the cat with the old one is traveling 8.67knots and 1.82 degres angle of atack in 5m/s wind.

    With the new daggers the corresponding performance will be 8.85knots and 1.58 degres of atack. (not corrected for waves)

    The accuracy of the absolute value of these numbers is probebly not so good but the relationship between them and the real numbers is hopefully good.

    The reason why I am thinking of reducing the angle of atack is because the increased drag of the hull when it travels sideways. My VPP does not take in consideration the difference in hull resistace depending on the hulls angle trough the water.

    Do You think there is some difference of the hulls drag when it travels 1.58 resp 1.82 degres sideways or is it better to reduce the depth or the chord some more in order to reduce the wetted area.
     

  8. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    I would reduce the chord to cut the wetted area.

    The drag of a hull with leeway is something that is very hard to estimate - I don't have any tool to do it. Probably the fastest and cheapest way would be to make the both sizes of board and go racing!
     
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