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

    Looking just at the 2D results for the two sections, the cambered one needs a smaller angle of attack by about 2.7 degrees to get the same CL as the 0012.

    But that would be much different when you consider the foil dihedral & other 3D effects, not to mention any differences in design between your boat & Broomstick. I don't think it would be worthwhile to try to quantify these differences very precisely.

    If you allow enough adjustment to the foil angles (at least between 0 & 8 degrees), you'll be able to settle on a suitable range of settings with just a small amount of trial & error.
     
  2. Doug Halsey
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    Doug Halsey Senior Member

    I don't think that's a good idea, both because of the smaller chord & the resulting shape.
     
  3. revintage
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    revintage Senior Member

    Precisely is of no concern . I am not asking for an exact figure. I know it is between 1 and 8. Just want to know where the the typical region is?
     
  4. Doug Halsey
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    Doug Halsey Senior Member

    On Broomstick, I used foil settings of 8 - 10 degrees to get it to take off most easily & settings of 4 - 6 degrees to reach its best top speed.

    I'm still digging through my notes to try determine if those angles are wrt to the horizontal, or to my deck line (there's a 2 degree difference). I'm hoping I don't have to go out & take actual measurements.

    The angles of attack also depend on the aft foil settings, which can change the pitch of the boat by several degrees either direction.
     
  5. Doug Halsey
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    Doug Halsey Senior Member

    I took your suggestion and plotted incipient cavitation for the basic NACA0012, the cambered 0012, and the H105 sections.

    I think the numbers seem reasonable. The video shows cavitation forming near the apex of the foil at about 20 knots. My VPP code says that the foil's CL should be about 0.3 in those conditions. My Vortex Lattice code says that the local Cl at the apex should be about 1.6 times the average value for the foil. This gives Cl =0.48 at the apex.

    The attached plot shows incipient cavitation at 21 knots for CL=0.48 for the NACA 0012, in good agreement with all of the above. It also shows that incipient cavitation would be at 29 1/2 knots for the cambered NACA 0012, and 33 1/2 knots for the H105.
    IncipientCavitation.jpg
     
  6. revintage
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    revintage Senior Member

    If I understand you right, CL should increase the closer you get to the apex at a given angle of attack. This could possibly be doable with the Nose Foil + G10, by making the camber decrease, going from the apex and up to the root.

    Do you have any ballpark figures of how deep below the water surface the apex was when going for max speed(4-6 degrees). Doesn’t look like much, but the projected surface needed could be approximately calculated knowing boat weight and speed.
     
    Last edited: Aug 18, 2018
  7. Doug Halsey
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    Doug Halsey Senior Member

    The CL varies along the foil from a maximum at the apex to zero at the top (the water surface). This happens naturally without your needing to do anything to make it happen, because of the 3D effects. (At least for untapered V foils with no camber or constant camber).

    If you were to vary the camber along the foil to give it the max camber at the apex, that would accentuate the CL variation & make the cavitation problems worse.

    You could possibly make a case for having the smaller camber at the apex, so there would be a smaller difference between the maximum & average CL, or equivalently for twisting the foil, but I haven't evaluated that option very thoroughly yet.
     
  8. Doug Halsey
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    Doug Halsey Senior Member

    Speaking of the Cl variation along the foil : here's a good reason not to give the foil more camber.

    I've added the foil with CLL=0.5 to the incipient cavitation curve that I posted earlier, so now there are 3 foils with NACA0012 thickness & varying amounts of camber. The blue curve has CLL=0; the purple curve has CLL=0.3; and the green curve has CLL=0.5.

    Notice that as camber is increased, the cavitation speed at CL=0 decreases. Physically, this is because of the velocity peak on the lower surface near the leading edge at small angles. Since CL=0 is the value at the water's surface, that's where the cavitation would occur (before the upper-surface cavitation at the apex).
    IncipientCavitation2.jpg
     
  9. revintage
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    revintage Senior Member

    OK, so I misinterpreted. The CLL=0,3 seems to be a good one. But what is it´s camber f/c and camber location xf/c. Seems close to 1,8% and 50%?

    Checked 0012 against 0012 CLL=0,3 at same CL.


    CP03.png
     
  10. revintage
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    revintage Senior Member

    If using the NF the first 45mm has to be symmetrical.

    If absolutely necessary it might be possible to mill the bottom and add material on top to get it exact.

    But I suspect the it will anyway be hard to make the G10 follow the curve 100%.

    The G10 I have is 1,0mm, will try that first, but if it is flexing to much I will buy a 1,5mm sheet. 2mm is probably to stiff.

    Adding a few balsa stringers and thin plywood ribs might be necessary.

    Will try to manipulate the *xls to see what happens when comparing to the original.

    Some compromises will have to be accepted.

    NFCLL03.png
     
  11. Doug Halsey
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    Doug Halsey Senior Member

    I think your shape looks good & at most just needs some minor fairing at the joints. I have no doubt it will be better than Broomstick's. If this were for some AC contender, you would probably want to do more, but for an amateur project is very good.

    The shape of the camberline that you asked about is actually the same as the earlier one, just larger magnitude.
     
  12. revintage
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    revintage Senior Member

    If I am not mistaken, you suggested letting the outer foil go below the apex. Noted you had a vertical addition below apex on those you built for the Broomstick.

    Is this how you meant and is the length of the extra part critical? Question is also how you easiest end the cambered section. An vertical end plate?

    It would anyway make the apex joint easier to build.If going for 90 degree V´s instead of 60, would make the joint even easier to build,but would complicate the rest.


    prolong.png
     
    Last edited: Aug 19, 2018
  13. Doug Halsey
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    Doug Halsey Senior Member

    The diagram is what I was suggesting - sized more or less as you've drawn it. I'm not sure how I would try to optimize anything about it; I can think of arguments for both bigger & smaller lengths below the apex.

    Initially, I had vertical fins at the bottoms of Broomstick's foils, but the boat felt more lively & recorded faster speeds after I cut them off. It seems like the bottom end of your outer foils would be easier than mine for attaching various caps or fins as experiments.

    It's true that attaching the foils together would be easier with 90 degree angles, but I like the deeper V's better. Others might disagree...
     
  14. revintage
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    revintage Senior Member

    I am convinced the triangular section is what you should use for stability and ease of adjustment, if using material NOT being made of Unobtanium wrt to cost and labour.

    Going for a 90 degree V is impossible due to the width and also with the risk of riding to low.

    Got the idea of using a single 45 degree foil(red) supported by a FF13 strut(grey).

    In the sketch both alternatives has the same projected foil area and total wet area measured at bottom of center hull.

    Broomstick is 58*15,8cm projected foil area. Wet area is just above flat area 4*58*15,8cm.

    The 45 degree is 54,5*16,8cm CLL=0,3 projected foil area. Wet area is just above flat area 2*41*13,5cm+2*77*16,8cm.

    Weight will be the close to the same, although the strut Cd will be added but might be compensated by the cambered foil.

    On the downside is that the single foil will be loaded double so hard.

    I might have made some calculating errors so it might be worth to double check.

    Could this alternative work, the advantage being that only one hydrofoil a side needs to be built? Or is it to open a can of worms?

    prolong.png
     
    Last edited: Aug 20, 2018

  15. Doug Halsey
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    Doug Halsey Senior Member

    When I built Broomstick's deep V foils, I considered them a basic starting point for a whole series of variations. But I never had time for even a 2nd set. I'm hoping you will be able to experiment with different configurations & see how they perform.

    The variation shown in your last post might be worth trying since it would be easier to build, but I would worry that it would need a higher speed to get flying. And I wasn't especially happy with Broomstick's required take-off speed (10 - 13 knots, depending on foil settings).

    I agree with your reasons not to use shallower dihedral angles, but note that you could make them work by mounting the foils lower, either using longer vertical struts or through a well in the amas.
     
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