Air Cavity rail beside spray rails?

Discussion in 'Boat Design' started by Titu, Jan 26, 2023.

  1. Titu
    Joined: Sep 2022
    Posts: 17
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    Location: Halifax, Nova Scotia

    Titu Junior Member

    Hello fellow boat designers,
    I am struggling with a hull study result. So far didn't found anything in google or maybe I am typing the wrong key words. I was wondering if I put cavity rails beside the spray rails below the hull, will the cavity rails work as an air tunnel thus improving hull speed? I have attached a diagram for easy understanding. So far as I have mentioned earlier did not find any answers or research on it. Thank you fellow designers for reading my post.

    Attached Files:

  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Most likely they will ventilate the propeller.
    Jimboat likes this.
  3. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Get over the notion that air will support all or a significant part of the hull. The lift potential is dependant on the density of the lifting medium. Compare the mass density of water to the mass density of air.

    Gonzo is right as usual...........You will probably ventilate the prop which is not what you will want to do. Back to the same premise. The prop needs to push on solid water, not air or a mixture of air and water.

    Examine the concept from a different point of view. ......The cavities will be exposed to water pressure, depending on the velocity of the boat and the incidence angle of planing. They will almost surely fill with water and only increase turbulence and wetted surface.
    baeckmo and Jimboat like this.
  4. BMcF
    Joined: Mar 2007
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    Location: Maryland

    BMcF Senior Member

    Boats that actually do create and ride on an air cavity are extremely fast, very high powered and very light. A 20' Eliminator that I drove once comes to mind. Sporting a 300HP outboard and surface-piercing prop on a hydraulic jack plate, that boat "lifted off" at around 80 mph or so. The effect of hull "mostly" separating from the water was very noticeable.

    But for more conventional craft, the only way to achieve an air cavity is through forced induction...high pressure ratio centrifugal fans or even Roots blowers have been used. Google "SES-X" if you have the time...they have a demonstration craft that is an air-cavity design. There have been numerous failed attempts at powered-air-cavity..the Purrseavearance air-cavity catamaran ferry being a highly visible example.
    Barry likes this.

  5. Jimboat
    Joined: Feb 2002
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    Location: Canada

    Jimboat Senior Member

    I would expect that your cavity areas would do more to disturb flow in region of lift strakes, resulting in reduced lift and increased drag. Additionally, any air/water mixture that might fill these cavities would reduce water density, similarly reducing hydrodynamic lift force available.
    Ad Hoc, Barry and baeckmo like this.
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