Passive air cushion hull bottom

Discussion in 'Multihulls' started by Kaido Loor, Feb 4, 2023.

  1. Kaido Loor
    Joined: Feb 2023
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    Kaido Loor New Member

    Could this work?

    While monoholl can be at an extreme heel (angle towards sea level), catamaran hulls do not have big angles. This allows a passive air cushion below the hull. This requires (if viewed from aft or bow) a rectangular (H or A) shape of the hull, instead or a more common 'V' shape of a hull. Then large parts of the hull sides can be immersed, but hull floor can be raised, to leave a air cushion between the hull floor and water level. As the sides of the hull are well immersed (maybe 70 cm below water level), it creates a passive air cushion and air lift on the floor of the hull. The air cushion only ocassionally needs pumping in more air and is therefore mostly passive. This cannot be achieved in monohull due to sometimes extreme heel allowing air to escape. In cats, the heel is limted. The idea is picturised here. Could this work and give some effect?
    Air hull design.png
  2. Alan Cattelliot
    Joined: Jul 2021
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    Alan Cattelliot Senior Member

    Air cushion is used in different ways. On multihulls, the wet-deck is sometimes used to create the air bubble. In that case, the air is pressurized, and a lift effect can be observed, reducing the wetted surface area. Active or passive air injection in the boundary layer is another use, where the aim is at reducing the friction drag of the hulls. A third application is given in your diagram, where the boundary layer around the hull is "shaped", allowing some control surfaces to produce lift or reduce friction drag. Many of these applications have been de-commissionned over time, because the of its lack of efficiency. Multihulls are used to (i) reduce draft (ii) increase stability or righting moment (iii) increase habitable area. The hull shapes tends to be narrow. Because the air cushion technologie rely on having enough control surfaces for the effects to be high enough, the multihull shapes are not so much usable for this purpose, because you won't get enough acting area.

    But if you want your boat to be hypersonic (relative to the speed of sound in water), this is another story. Air cushion becomes aerospike, and the drag reduction obtained makes the principle very efficient, even with active pressure control. For a craft operating at moderate speeds, I don't see if there is much of an advantage in tricking the boundary layer. Building is more difficult because of the complicated hull shapes, the wetted surface area is increased, making the boat very inefficient at low and moderate speeds.

    Nevertheless, I find your diagram is very interesting, in order to revised this opinion. I observe that you use the fore third of the hull to produce some effects. In general, this part of the hull is "depressurized", on multihulls. The idea of combining one fore control surface and one aft control surface is interesting. There may be some applications of your idea where a benefit could be observed, within a particular range of displacement over hull lenght.
  3. baeckmo
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    baeckmo Hydrodynamics

    The short answer is "NO"!
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  4. Alan Cattelliot
    Joined: Jul 2021
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    Location: La Rochelle (Fr)

    Alan Cattelliot Senior Member


    In fact, G.Verdier did try this principle on a 60' racing catamaran for Y. Parlier... A step hull.

    2002 - Hydraplaneur - Yves Parlier - Plan Verdier

    I had the chance to get some "hot comments" from the skipper itself. Lots of hard work has been put in this project, including tank testing, before the short answer "No" was given, after a few races were sailed, on real conditions.


    Air cushion works well on motorcraft, so, to me, the "No" answer, on whether it works on a multihull, is far too short. Understanding "How" this principle works is very important, if you want to be more specific. And perhaps review applications that have not been efficient enough, in their time.
    Last edited: Feb 4, 2023
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  5. jehardiman
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    jehardiman Senior Member

    Let us talk meat, fruit, and dairy....just so everyone talks about the same thing.

    Passive air chambers: The principle is that an air/water interface is maintained to reduce skin friction. Short Answer: No. The dynamic drag of the edges of the pocket are greater than any skin friction benefit.

    Active air chambers: The principle that active or ram air pressure is fed into the chamber to reduce the displacement and wetted surface. Short Answer: Yes, typically. This is how SES's and some power cats achieve their performance. In this case there is always more air being fed into the chamber and it is constantly leaking out, often in a way that reduces other forms of drag.

    Dynamic air cavities: The principle that the shape of the hull forms an air pocket down from the waterline along the skin, reducing actual wetted surface and improving streamlines. Short Answer: Maybe. This, in several different forms, is used in stepped hydroplanes, high performance sailing vessels, and fast displacement hulls. There are several different methods that each address a particular drag reduction issue.

    Carry on.
    Last edited: Feb 8, 2023
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