Foil Cavitation at Lower Speeds Than Expected

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Halsey, Aug 11, 2015.

  1. Doug Halsey
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    Doug Halsey Senior Member

    I think things are a little more complicated when you're dealing with surface-piercing foils. By flying higher or lower, you can change the foil area (& hence CL values) over a fairly broad range, so you need to use sections that are more broadly suitable. Sections that are optimized at small values of CL may not fare as well at higher values.

    With that in mind, I used XFOIL to calculate the CP min values as functions of CL, for 3 foils (NACA 0009, NACA 0012, & NACA 63-012). As you can see, the NACA 63-012 is better at the CL value that you & Tom have been using (CL=0.15), but it loses its advantage over the NACA 0012 by about CL=0.45, and is considerably worse at higher CLs.

    3FoilsCpMins.png

    So, it's important to try to determine what CL values are going to be encountered. Of course, the answer is that it depends on weight, foil area, speed, etc. , but running a VPP code can give some idea.

    In order to try to approximate the conditions shown in the video at the start of this thread, I dug out some VPP runs for Broomstick on about a beam reach in a 16 knot wind. The calculated speed under those conditions was slightly over 20 knots, which is about where the cavitation started in the video. Apparently, when flying at the optimum height, the CL value for the leeward foil is about 0.3. But that's the average value; accounting for spanwise variations, the maximum local value would be higher. And if the boat is flying higher than the optimum height, then the CL value will be larger still.

    So the NACA 0012, or some other conventional section may not be so bad after all.
     
    Last edited: Aug 12, 2018
  2. revintage
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    revintage Senior Member

    Just discussed with Gareth, who makes the Windknife foil, and he had widened the 135x12mm 0009 section by inserting a tube to something like NACA 0011.

    We did the same with his larger section when I needed a 1500mm keel foil earlier.We then widened the section from 270x25mm to 270x30mm.

    Have anyway ordered a short sample to experiment with.

    IMG_20170108_142457083.jpg
     
  3. Doug Halsey
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    Doug Halsey Senior Member

    Last edited: Aug 13, 2018
  4. revintage
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    revintage Senior Member

    Haha, saw that post earlier today. Thanks for the added 0011.
     
  5. revintage
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    revintage Senior Member

    Another practical solution to build your desired hydrofoil sections, symmetrical or unsymmetrical would be to use Windknifes Foil Nose(2.FOIL NOSE - www.windknife.com http://www.windknife.com/full-foil/foil-nose) in combination with G10 sheets(aluminium is to heavy and carbon sheets to expensive).

    This way you can decide the profile and corda you like.

    This will end up with about 1,5kg/m(+epoxi) if building a 6" NACA0012 and using 1,5mm G10.

    Not knowing G10, maybe you could use 1mm sheets and a stabilizing piece of wood too to keep weight down? The inside of the sheets at the trailing end should be trimmed down on the inside to get as thin as possible though.

    I have both the FN and 1mm G10 at home, could be worth a try. The FF13 will anyway be candidate for the rudder foil.

    Skärmavbild 2018-08-13 kl. 15.29.16.png
     
  6. Doug Halsey
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    Doug Halsey Senior Member

    Combining the FN with G10 would also allow you to extend the chord beyond 135mm, which you might want to do.
     
  7. revintage
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    revintage Senior Member

    Just what I was thinking of ;-). Will pick up the stuff at our workshop tomorrow if I find it.

    How critical is rudder foil area? Yours is 6"x24", 144 square inch, the FF13 0009 sample I have ordered is 13% lower at 5,3"x23,6", 126 square inch.

    It would be nice if I could use it for something.

    ws.jpg
     
  8. Doug Halsey
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    Doug Halsey Senior Member

    The aft foil loading isn't as critical as the main foils. In the VPP run I referred to yesterday, the aft foil's CL was about -0.04.

    The value you quote for my aft-foil area was the original one. In 2011, I made a smaller one with about 115 square inches. Root chord = 5.25", tip chord = 4.0", span= 24.75".

    AftFoil_2011.jpg
     
  9. revintage
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    revintage Senior Member

    Thanks for helping Doug. Will soon have to start a new thread for my project.

    Nice build! Any info of the section profile, it doesn´t look extreme anyway?

    My idea is a 0012 rudderblade and the 0009 FF13 for the foil.

    Would you mind sharing some rudder head and tiller details?

    Have checked the Moth rudders but they a little to hitech, I like the idea with a fixed foil to the blade better.

    Sorry if I am draining you for info. This is far beyond the Nacras I sail ;-).
     
  10. Doug Halsey
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    Doug Halsey Senior Member

    The aft foil was something like a NACA 0012 section & the rudder was something like 0013. I made the rudder thicker because it was quite a bit deeper than my previous rudder & I wanted to make sure it was stiff enough.

    Total length of the rudder was 48", which puts the aft foil 33" deep when floating level. It's important to have the aft foil deeper than the tips of the main foils to allow for nose-down pitch of the hull.

    The rudder head is pretty agricultural-looking, but it works fairly well. The blade pivots around a bolt, giving it a few degrees of adjustment (but preventing it from being raised). There's a control rod under the tiller that pushes & pulls the blade, attaching to a fitting at the top/front of the rudder head. That really needs to use a much coarser thread to give faster adjustment. (I really like the mechanism on my Mach2 better.)

    Because of the pivot bolt, it's not easy to raise & lower the blade to launch from the beach as you would on most cats. Instead, I would anchor the boat out in chest-deep water, then mount the entire aft foil to the transom via a rudder pin. I had to bolt the main foils in position in deep water anyway, so the aft foil wasn't much more trouble.

    P8310210.JPG P8310211.JPG

    At the front 0f the tiller, there's an even more crude fitting to allow the 2 tiller extensions to turn the control bar. (note- the flexible linkages aren't shown). This was still a work in progress, so I'm sure a much cleaner alternative could be made.
    P8130891.JPG
     
    Last edited: Aug 13, 2018
  11. Doug Halsey
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    Doug Halsey Senior Member

    Getting back to the subject of cavitation, I've modified the graph in post #63 to show the values of the critical CP's at boat speeds of 20, 25 & 30 knots.

    The faster you go, the more important it is to be using a smaller CL or a different section.
     
  12. revintage
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    revintage Senior Member

    Thanks Doug, great info, I really have a lot to set my teeth into now.

    At the front of the tiller, the design doesn´t need much refinement, maybe adapters for todays flexible tiller joints.

    At the rear I think a stainless 8mm lead screw with delrin block would be perfect. 8mm Metric Acme Lead Screw https://openbuildspartstore.com/8mm-metric-acme-lead-screw/

    For adjusting the angle of the Vfoils, lead screws would probably work as they can not work themselves loose.

    It would also be great to find a way of lowering/raising both the V foils and the rudder sitting on the boat.

    HA5098_L_3.jpg lead.png
     
  13. revintage
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    revintage Senior Member

    Tom and Doug,

    Trying to learn and imported H105 into JAVA-foil to compare against 0012 and succeeded. First made the polar and then did the Cp where both generated the same lift.

    0 degrees for H105 and 3 degrees for 0012.

    The same for 2 and 5 degrees.

    The same for 4 and 7 degrees

    Could you please comment on these as it would probably improve my and maybe a few others learning curves.
     

    Attached Files:

  14. Doug Halsey
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    Doug Halsey Senior Member

    When I look at the Cl & Cd values on the pressure distributions in the 2nd-4th figures, I don't get points that fall on the polars in the 1st figure, so something might not be right. Figures 2-4 are also more confusing than they should be, because the symbols for the 2 foils aren't consistent in the 3 plots, but they seem to be showing the smaller peaks in the - Cp values that you would expect for the H105.

    The lift curves in fig. 1 show typical differences you might expect when comparing cambered versus symmetrical sections. The major effect of the camber is to shift the lift curve upward. This means you need to extend the H105 calculations down to negative angles to be able to compare to the NACA 0012 at Cl values smaller than about 0.5

    It looks like the H105 has a more abrupt stall than the NACA 0012.

    The polar plots really don't need to show the values much beyond the stall. You'll get much better resolution of the differences in relevant conditions if you only plot Cd over the range from 0 to 0.02.

    One interesting aspect of comparing the H105 & the NACA 0012 will be to see how much of the differences are due to camber alone, and how much additional benefit the H105 gains from designing for a "flat rooftop" style pressure distribution. I can easily add various amounts of camber to the NACA 0012 (& also the NACA 63-012 shown earlier) to get some idea about this.
     

  15. revintage
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    revintage Senior Member

    Began with editing the polar to 0-5 degrees for H105(red), together with 3-8 degrees for 0012(green). Also changed the symbols in one of the Cp plots to make them consistent. H105 is dotted.

    Have also tried to find info, to see if it would be possible to use an over rotated round tube as mast, as buying a carbon wing mast for the old A-cat sail I have, is beyond my reach.
     
    Last edited: Aug 14, 2018
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