Hydrofoil in front of a outboard

Discussion in 'Hydrodynamics and Aerodynamics' started by Tommifin, Jun 8, 2018.

  1. Tommifin
    Joined: Oct 2014
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    Location: Finland

    Tommifin Junior Member

    A question. Boat is a funny conversion from a saiboat and need more lift on the back.
    Hydrofoil assisted is something I would like to try and will try, make sense or not.

    the question is, will the foil in front of outboard be a possible option, or will it cause prop slip due to vortexes and perhaps cavitation? Big nono or something worth trying? Speed range in just under and over 20 knts.

    If it's feasible, how close foil could be installed to this new transom/planing extension on the boat ( dimension X) and how close / far from the outboard to avoid possible issues with cavitation, prop slip (dimension x).

    All I could find are hydrofoil assisted cats that have fins close to stream in front of prop.

    Mechanically this would be so much easier to try insteasd of split foils either side of the outboard.
    Boat is ready and foil is ready.
     

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  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    The leg of your outboard between the "torpedo" and the cav plate is like a foil, so I would not expect any problems, unless your foil has a crazy angle of attack and creates an eddy behind it, at times. A stainless, cupped prop would be mandatory, alloy props let go too easily when the incoming flow is disturbed or aerated. I can't see you are getting much advantage over trim tabs, though, and how do you retract this arrangement so it does not foul the bottom when the boat grounds ? There are probably reasons why this idea is not seen. I had a tunnel-hulled boat with a foil, with a central vertical strut as well, and it never gave any sign of upsetting the motor/prop, but it was slightly above the bottom of the boat, and not in the way when beached.
     
  3. Doug Halsey
    Joined: Feb 2007
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    Doug Halsey Senior Member

    The top of your foil and the bottom of the hull are so close together that there won't be much difference in pressure between them.

    So the foil won't provide much lift when the hull is in the water. And the foil is so close to the surface that, when the hull isn't in the water, the foil won't be either.
     
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Since you already have an add-on extension to the stern, it would be simpler to make it into a full stern that provides more lift.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Sounds right, especially if the intention is to motor at the 20 knots mentioned in the OP. I suppose if extended slow speed use was intended, a foil would have less drag than a buried transom. But transitioning from the slow speed to the 20 knots, without the submerged transom, seems problematic, the lift from the foil would be minimal much short of that speed.
     

  6. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Agreed.
     
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