New Hovercraft design change ?

Discussion in 'Boat Design' started by DocScience, May 28, 2011.

  1. DocScience
    Joined: Apr 2010
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    DocScience Wishful builder

    My design is a Hovercraft SES (Surface Effect Ship) using a Hovercraft with rigid sidewalls.
    What I want is a vehicle for point to point traveling.
    I am NOT looking for a maneuverable design.
    If I can have a workable design that is mainly good for one speed instead of a range of speeds, that would be very acceptable.
    I would like the ideal speed to be above 40mph.


    The different addition I have made is a rigid rear skirt, on a hinge.
    A flexible skirt is attached to the rigid skirt as shown in the diagram.
    The purpose is to reduce wave resistance at the rear of the craft, as the craft is leaving the wave, and to reduce the amount of air that is expelled behind the craft.
    A small spring may be needed to keep the rigid skirt in better contact with the water.


    1. What somewhat similar arrangements have been previously used for this problem ?


    2. For the given weight and sizes, what speed is needed so that the air in front will provide the needed air pressure, (about 20 psf) without the need for more air to be pumped underneath ?
     

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  2. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    So a SES with a fan feed to get it started?

    Take a look at the Russian Bora class patrol boats (NATO designation Dergatch class).
     
  3. Poida
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    Poida Senior Member

    Totally not understanding the technicallities of hovercraft mechanics I would have to only assume that the benefits of a flexible skirt is to keep the gap between the water and the craft as small as possible.

    A rigid skirt would ride up with the highest wave crest allowing the air to escape at the trough, whereas a flexible skirt follows the contour of the wave (to a degree) holding more of the pressure inside which supports the craft.

    I was unaware that the front air pressure was used to provide lift, and I thought the opposite would have been true. The air produced by the forward motion would slide under the craft to give it a friction free bed to glide on.

    However in my ignorance I would not have thought this would reduce the amount of air required for lift.

    The driver is facing the wrong direction and he is going to crash.:(
     
  4. Yellowjacket
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    Yellowjacket Senior Member

    You need a lot more speed than you think.

    Here is a calculator that will let you calculate the dynamic pressure (stagnation pressure) that you would use to lift your craft.

    http://aero.stanford.edu/StdAtm.html

    Plug in sea level and the speed (in ft per second) that you want to go and the dynamic pressure is calculated for you.

    In order to create 20 lbs/square foot you need to be going about 130 ft/second (88 mph) to support your craft without a fan.

    This is why most hovercraft use a separate fan for lift.
     
  5. cthippo
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    cthippo Senior Member

    I thought you needed to get to that speed to activate the flux capacitor :confused:
     
  6. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    I must have done it wrong

    I got 2116.14 lb/ft^2 at 1 foot altitude, 20 mph, 20 ft length
     

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  7. masalai
    Joined: Oct 2007
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    masalai masalai

  8. Boston

    Boston Previous Member

    1 foot is a lot of space between the skirt and the water
    try 0.5 inches and you should get a far more reasonable number then triple it
    not that I've ever built an operational one, the guy you really want in on this thread is George
    he's the hovercraft guru around here
     
  9. DocScience
    Joined: Apr 2010
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    DocScience Wishful builder

    Yellowjacket
    I did a lot of searching in google for formulas for doing that, and all the formulas that I tried gave me results that did not logically seem correct.
    For example (pressure = .5 x density x speed x speed)
    Thankyou for this.
    http://aero.stanford.edu/StdAtm.html


    Poida
    The problem I have with the flexible skirt along the sides is that too much air loss goes through the sides.
    The rigid skinny sides cut through the water a bit deeper, increasing drag, but at least are not displacing much water creating waves.
    I do not want to loose so much energy by displacing water. Even if I am displacing just the water in waves, to cut through them, it would use a lot of energy.

    I am attempting to operate like a speed boat that gets a bit too much air underneath, and that lifts it up, but in the speedboat case, the air pressure makes it go airborn.

    I was just watching "Speed Boat on skis" and he acomplishes this same tecknique so very well, riding over the waves on skies.

    The one advantage over WIG (wing in ground effect) is that WIG uses more energy, to move the amount of mass of vehicle.

    .
     
  10. DCockey
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    DCockey Senior Member

    That's atmospheric pressure.

    Dynamic pressure is four lines below.

    Dynamic pressure has nothing to do with length. It is affected by altitude because density changes with altitude. But the difference between 1 inch and 1 foot is absolutely meaningless for this.
     
    Last edited: May 29, 2011
  11. DCockey
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    DCockey Senior Member

    That's the correct formula for dynamic pressure. The largest increase possible over atmospheric pressure due to ram air is the dynamic pressure. It does not depend on opening size.

    By definition dynamic pressure is the pressure differene between ambient air and air brought to a stop without losses.
     
  12. DocScience
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    DocScience Wishful builder

  13. DCockey
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    DCockey Senior Member

    Presumably the reference length is used to calculate the Reynold's Number, which would then be used to calculate the Laminar Cf and Turbulent Cf.
     
  14. whitepointer23

    whitepointer23 Previous Member

    have a look at the universal hovercraft site.
     

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  15. kach22i
    Joined: Feb 2005
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    kach22i Architect

    This definition means you will be using a lift fan, right? Right now half the people think you may be considering RAM AIR for lift, you need to clarify.

    I've monkeyed around with bow and stern skirts on my 10 foot long hovercraft. You soon learn to respect all of the early development of hovercraft which we enjoy today.

    The reason you might get way with the stern skirt the way it is shown is the air cushion pressure tends to flatten out the wave crest under the craft, but not as much as you or I would like, at least not on a smaller craft.

    The bow skirt has to be much more contour forgiving than the rear/stern skirt. I once tried wrapping a soft foam pool noddle inside the lower lip of my bow skirt just to see what would happen.

    From the shore my wife took photos. Even in very little chop on an inland lake the build-up of water at the contact line was ineffective to say the least. The skirt just could not lift over the water because parasitic drag and contact points adjacent kept pulling the bow skirt edge down into the water.

    The stern skirt design I tried once had a semi rigid flap like you propose, it too was sucked down into the water and forced the tail down. On land it was no better, the drag was just too much as the whole thing just seemed to scrape along.

    If you do a Google Patent search you will find several Patents similar to yours. I recall seeing French one from around 1965, I posted it on the HoverClub of America's website and the experts there said sure you can get a Patent for anything, but that means nothing.

    A simple drape of fabric across the bow like a Sevtec hovercraft will work.

    A simple bag skirt at the rear with sealed sides will work.

    I've experiment with a double bag at the stern, not as easy as the diagrams make it look.

    Go to the link in my signature to see my skirt experiments. Look over the post and e-mail or PM me if you like.

    Reading can be very misleading, things that don't work get copied over and over again by lazy authors. Models are great, but full sized hovercraft experiments will teach you, and teach you good.
     
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