Effect of air injection on lift and drag of submerged hydrofoil

Discussion in 'Hydrodynamics and Aerodynamics' started by hump101, Mar 20, 2015.

  1. Steve Clark
    Joined: Jul 2004
    Posts: 201
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    Location: Narragansett Bay RI

    Steve Clark Charged Particle

    Hump:
    Flaps, if you can pull it off, are the most efficient way to manage lift.
    Fouling is a concern not just for the mechanism but for the performance of the foils. Experience has shown that hydrofoils are extremely sensitive to surface imperfections and the loss due to a thin coat of slime may be enough to prevent the foil from doing any good at all.
    This is more critical, it seems than for normal centerboards and rudders, so I think you need to think about a trunk system that gets the foils out of the water. The A Class have taken to making the " Trunk" essentially two watertight bulkheads and the inside skin of the hull to provide maximum flexibility to get tightly curved foils out of the boat.
    Groupama C had trunks that were essentially Hypalon tubes sealed to the hull and deck.
    Perhaps some combination of filler blocks and space could be made to work.
    SHC
     
  2. hump101
    Joined: Oct 2004
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    Location: Brittany, France

    hump101 Senior Member

    Thanks Steve. The issue of fouling of the foil surface is not something I had previously given much thought to, on the basis that a hard antifouling (copper-filled epoxy) plus a regular scrub would be sufficient. However, I am finding that the boat has a slime layer forming very quickly (< 2 weeks in summer), even on antifouling. This comes off easily with a rub, but if not regularly removed seems to form the breeding ground for the next level of growth. My mooring platform is an excellent mussel breeding system!

    If this slime is going to be an issue for foil function, then this would require the foils to be scrubbed prior to every sail, which would be inconvenient, to say the least. I had previously devised a method of inserting the foil using a hinge system, but rejected it on the basis of simplicity, weight, reliability, etc., but it may be worth revisiting this. It constrains the longitudinal position of the foil, which is not ideal, but would permit the use of a flap system without much additional complication.

    I don't want to change my existing daggerboard casing, for several reasons, not least of which is it passes through the cabin and if it was made any wider the cabin would be unusable.

    I'm in the process of building a foil test rig, so I will include some slimed samples in the test programme to quantify the sensitivity.
     
  3. Steve Clark
    Joined: Jul 2004
    Posts: 201
    Likes: 14, Points: 18, Legacy Rep: 163
    Location: Narragansett Bay RI

    Steve Clark Charged Particle

    I'm basing my comments on reports that spray paint "orange peel" made Ben Ainslie's AC 45 "un-sailable" and being splashed down by a single strand of eel grass on my Trampofoil; wing. The Mothies have taken wet sanding of foils to a whole new level of obsession, which I assume isn't done for no reason.
    I go to the trouble of pulling Red Herring's canard board every time I leave her on the mooring because it seems to matter more than a bit of slime on the rest of the hull. My guess this all has to do with the foil loading and the harder t5he foil; has to work, the fussier it becomes. Kind of like the rest of us!
    SHC
     

  4. hump101
    Joined: Oct 2004
    Posts: 261
    Likes: 14, Points: 18, Legacy Rep: 58
    Location: Brittany, France

    hump101 Senior Member

    Foil load may be the reason for the sensitivity, but I would have thought rudders see similar, or higher, loading before stalling. I'm wondering whether the issue is not the ability to create lift, but the ability to create repeatable, controllable lift? If the dagger or rudder lift changes, you make more leeway or add more helm, but for a lifting foil it becomes a control issue? Presumably the slime is slowing the flow along the surface, maybe encouraging premature separation?

    It will be an interesting exercise to investigate further. I'm doing this in part as an educational project for my son to work on, so a few extra variables is all good!
     
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