Flat plate rudder vs naca drag

Discussion in 'Hydrodynamics and Aerodynamics' started by 23feet, Mar 20, 2016.

  1. Joakim
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    Joakim Senior Member

    Thin flat plate has slightly less drag than NACA 0015 at zero angle. Your rudder is not thin. The added drag is caused by disturbance at leading and trailing edges. Rounding the leading edge (at least as a half circle) and sharpening the trailing edge (say during the last 20% of the chord to below 1% thickness) makes it about equal at zero angle, but NACA 0015 will be better at high angles.

    Note that the values you found are for rather low Reynolds number (~700 000, about 3 knots for your rudder) and thus have higher drag coefficients than most data at higher Reynolds numbers (3-6 10e6).

    Note also that the aspect ratio is 1.0, which would be exceptionally low for a sailboat. Higher aspect ratio leads to higher lift coefficients (at the same angle) and much lower stall angle. At the same time the difference between NACA 0015 and a flat plate becomes much bigger (at higher angles). NACA 0015 is able to produce clearly more lift. Thick profiles are used for getting higher maximum lift while a thinner one would have less drag.
     
  2. Joakim
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    Joakim Senior Member

    CDo is plan area referenced CD for zero angle in this case. The unusual values are due to low Reynolds number. 0.085 would be very high for a NACA 0015. 0.0085 would be more typical at high Reynolds number.
     
  3. 23feet
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    23feet Junior Member

    I'm still not getting it (to Jehardiman). The numbers from Molland and Turnoch ARE for the same sized plan form rudders, one at 3.5% and one at 15% sections. The NACA at 15% has less drag even though it is the same plan form size rudder. Unless I'm missing something, either you or M and T are wrong.

    To look at your argument from a more intuitive perspective - you are effectively saying that the flat plate rudder is more efficient than the NACA. Why then do folks bother making NACA rudders at all?
     
  4. 23feet
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    23feet Junior Member

    OK, I'm trying to attach Fig 5.5, Pp 97 of Molland and Tirnock (2007).
     

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  5. 23feet
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    23feet Junior Member

    Hooray! So this should help. I'm looking at the 0.030 flat plate rudder (3%) vs the NACA 0015 (15%). These data show the NACA with less drag (Cd) from 0 degrees through 20 degrees of attack.
     
  6. Joakim
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    Joakim Senior Member

    The book you are referring to seems to be oriented to motor driven ships, which explains the odd aspect ratio, but not the odd Reynolds number. You should look into books and articles related to sailing boats or dig the relevant data from the experiments by naca during 30's and 40's.
    E.g. http://www.ericwsponberg.com/wp-content/uploads/keel-and-rudder-design.pdf
    and Principles of Yacht Design
     
  7. jehardiman
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    jehardiman Senior Member

    Actually that table has issues with it and the actual values are too low for the thicker shapes. I can say that because most of the data presented in that table (including the flat plate and NACA 0015 data) is lifted from Thieme's 1961 "Zur Formgebung von Schiffsrudern" (DESIGN OF SHIP RUDDERS, DTMB Translation 321, 1965). In the paper Thieme explicitly points out that the results are uncorrected for tunnel blockage (up to 20% using the Maskell method criteria) with such a low aspect ratio. This will tend to underpredict the drag of the thicker section due to the lower dynamic pressure, which also helps the IfS (Institute for Shipbuilding, Hamburg) shapes. Note that SNAME specifically left out the CDo factors when including the lift data in the 1967 and 1989 versions of PNA.
     
  8. tspeer
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    tspeer Senior Member

    None of the data on flat plate vs NACA sections are applicable to the Tiki 21 rudders. The rudders are really flaps on the trailing edges of the hulls, and much of their effectiveness is due to pressure changes on the hulls, in addition to the force that is generated directly on the rudders. The boundary layer characteristics on the rudders are dominated by the flow coming off the hulls and skegs, and are completely different from the boundary layers on spade rudders.

    So long as you retain the same planform as the Warram designed rudders, I doubt there is anything you can do with the section shape that will have an appreciable influence on the performance of the boat.
     
  9. 23feet
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    23feet Junior Member

    Thank you Tom and everyone else for all of the nuggets of wisdom.

    Sizing home made rudder blades for a Tiki 21, and going on the 2% of working sail area guideline for rudder area (found elsewhere in this forum) gives around 4 square feet of rudder area (208 square feet working sail). Is that area divided between the rudders for a cat (2 square feet each)?
     
  10. 23feet
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    23feet Junior Member

    I can see that the Tiki rudder is not literally a flat plate blade, but in searching the literature it is the closing thing to the Wharram flat plywood planform that I can find.

    When dealing with the skeg rudder combination, Molland and Turnock say that the skeg/rudder is less efficient than a blade rudder, but that the combination of the two can be thought of as an approximation of a single blade in terms of hydrodynamics. If this is correct, would there be in any utility in shaping the whole of the Tiki skeg/rudder planform into a NACA profile? Or, would it be a waste of time in that it is still in the wash coming off of the hulls?

    Thanks


     
  11. Joakim
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    Joakim Senior Member

    [​IMG]

    Here is a picture of Tiki 21 rudders. Are yours similar? It would have been helpful to show the design in the first place.

    Yes, it's a good idea to have the skeg and the rudder to have a combined form of a NACA profile. Since the aft section of a 00-series is almost straight, all you can do with the rudder is to taper it very slowly to a sharp trailing edge. If it is 5/8" now, you will get some benefit.
     
  12. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Might be worth mentioning, if it hasn't been already, that for improving performance to windward the rudder is particularly critical because with the right amount of windward helm they operate at a higher angle of attack, reducing leeway and induced drag of the hull. Of course you can do a similar thing with leeboards. In fact with leeboards you can go one step further and make them assymetric and cambered as well as offset. With centerboards, you can make them gybing, but not assymetrical and cambered.

    I love boats like the Tiki above, because they are so far out there from the America's Cup that you can find some very interesting solutions. I like those skeg rudders. I can see getting a lot of lift from those when sailing to windward. I am not sure what profile on the skeg and what profile on the rudder would work best there. I presume the widest part if at the joint. I think it might actually be better to have the widest part further back from there, a bit, so you can get some flow reattachment after the discontinuity of the joint, when going to windward.

    In fact, it might be looking at shaping the skeg, and the rudder, so that you get a smooth curve on the outside, when the rudder angle is set to say 5 degrees. That might give you the best lift and drag when going to windward, reducing leeway and drag on the hull, and it might not hurt too much on other points of sail. I can't remember what these boats have for leeboards. Cheers.

    p.s. No leeboards I see, so yes, you definitely want to induce a little extra windward helm and use those rudders to reduce leeway, and I think this is especially true with two hulls close together. So high lift is key, and that means low aspect ratio and some camber created with the skeg and rudder combo.

    I am not sure the rudder-skeg combinations need to be as thick as 15%. Maybe 10-12% is enough. The camber created when you angle the rudder is what gives you the lift. The extra thickness just gives you strength, room for the rudder post, and helps smooth the transition when the rudder is angled and the camber is induced.

    Also, it might be worth extending the rudder past the skeg, just a little, even just 4 inches. This should make it easier to have a good shape at the end of the rudder, which is critical. I think it can still be strong enough for beaching and hitting stuff.
     
  13. 23feet
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    23feet Junior Member

    Thanks Joakim and Jamie,

    Yes, that picture is of the standard Tiki 21 rudder setup. I've given the idea of profiling the rudder/skeg to a NACA profile a lot of thought, but am defeated by all of the different angles (rudder negative, skeg positive). It would be very difficult to do - probably more work than making new foils.

    Alternately, would it be worth while trying to roughly approximate a NACA-type foil by thickening and shaping the skeg and rudder, or would it just make it worse by adding drag? Because of the aforementioned angles in the planform, it would not be possible to keep the thickest section at 30% from the leading (skeg) edge. It would end up being more like a long teardrop, with the thickest point being the leading edge and a long taper to the trailing edge?
     
  14. PAR
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    PAR Yacht Designer/Builder

    Why not just install a set of kickups on the skegs. You can use parallel leading and trailing edged plan forms in the blade, making carving sections easier, you'll retain its shoal nature and the aspect ratio will increase dramatically. You might see some modest pointing gains, but don't get excited about big improvements with that rig and those hulls. If going this route, consider straightening out some of the rudder rake too.
     

  15. DennisRB
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    DennisRB Senior Member

    Can you beef the transom, rudder and attachments then get rid of the skeg? The rudder could then extend forward where the skeg was a little to provide balance and you could use a NACA shape. This would almost be a blade rudder then.

    Probably not a good idea but an idea nonetheless.
     
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