hydrofoil and prop ventilation questions to keep project afloat.

Discussion in 'Boat Design' started by fishnsurfn, Sep 19, 2008.

  1. fishnsurfn
    Joined: Sep 2008
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    Location: Santa Barbara Ca.

    fishnsurfn New Member

    I finally got started building a model of my dream boat. You know that one that everyone wants(23' fast and smooth in short period wind swell).
    I envisioned hydrofoils keeping the craft riding high over the 3.5' waves, a wave piercing design to cut though and/or launch me off the larger ones, with the foils and hull design softening the landings.

    In my research I came across the ventilation phenomenon. I'm praying it doesn't scrap my design.

    Does ventilation affect all foil types at any speed when they break or come close to the surface regardless of AoA, cord, camber, etc?
    Will a airborne re-entry into a oncoming swell make ventilation more or less severe?
    Can a vented foil recover its lift?
    Will the prop have the same issues?

    As a layman most of the articles I have found on the subject have been to technical for me to understand. If you could dumb down your responses I would appreciate it.

  2. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Ventilation can happen to any foil. There are four conditions necessary for ventilation to occur:
    - the local pressure has to be lower than atmospheric pressure
    - there needs to be a path for the air to get from the surface to the separated zone
    - there must be a region of flow separation
    - the cavity that forms needs to be stable

    By operating next to the surface, you've guaranteed the first condition is satisfied. By piercing the surface with the foil, you've met the second condition. Whether the third condition is met or not will depend on the angle of attack and the type of foil section you use. It's quite possible that when making the airborne re-entry, the angle of attack will be high and the flow will be separated. So, yes, ventilation is almost unavoidable for the kind of operation you envision.

    A ventilated foil can recover its lift. The most common way to do this is to reduce the angle of attack to eliminate the separated flow. The air in the cavity is swept off the foil as the flow reattaches. However, if the foil is supporting the boat, then when it ventilates the loss of lift will cause a downward motion that increases the angle of attack and can make the situation worse. So it may not recover until the hull has crashed to the surface of the water.

    Yes, the prop will have the same issues.
  3. Guest625101138

    Guest625101138 Previous Member

    This video shows the possibilities with a foil:
    You can see how effective the lift is. The foil will even plane if the AoA and speed are sufficient.

    Probably the bigger problem will be engine over revving if you are airborne for a period of time.

    Rick W
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