Airboat with Horizontal Fan

Discussion in 'Projects & Proposals' started by tom kane, Nov 2, 2010.

  1. Yellowjacket
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    Yellowjacket Senior Member

    Stealth aircraft mount the engine inlet above the fuselage in order to shield the inlet from ground based radar. All of these inlets are basically forward facing, or are what is called a flush inlet, (do a search on flush inlets or NACA inlets and you will learn something about inlet recovery), none of them EVER turn the air 90 degrees from the direction of travel because that is basically a bad idea.

    Successful air cushion craft use a two different fan systems, one for lift and the other for propulsion. The reason for that is that you need higher pressure and lower flow for lift, and you want a lower pressure ratio and higher flow for propulsive efficiency. You are trying to do both with one fan and that is fundamentally inefficient because you are bleeding higher pressure air that you have put a lot of energy into and are basically trying to push that air out the back and that is an expensive way to propel your boat.

    If you are trying to pressurize an area under the hull, a horizontal fan can do that, but axial fans aren't as good at supplying the required amount of pressure at lower flow rates when compared to a centrifugal blower. The reason for that is that it is harder to get pressure ratio at the inner diameter of the fan where the blade speed is lower, so if you want high pressure ratio at lower flow a centrifugal blower is a better machine.

    For propulsion the higher flow rate and lower pressure ratio system suffers from a high percentage of losses in turning the air to create thrust, which is why a conventional fan is used on air cushion vehicles for propulsion.
     
  2. tom kane
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    tom kane Senior Member

    Airboat_Experiment

    Thank`s for you comments. I accept your explaination as being the conventionaly accepted theory, but I don`t think the airflow theory is correct. I want somthing different from the usual air boat design and I am finding designs closer to what I want but they are still too expensive and too noisy for my purpose. I think that many people do not understand even how a water propeller pushes a boat forward,it`s not a new idea but proven hundreds of years ago.

    A vaccum cleaner does not suck air but displaces air and atmospheric pressure rushes in to replace that air carrying dust with it. The inlet can be anywhere from verticle to horizontal without affecting the amount of air displaced.

    A baby sucking on a mother`s nipple displaces air and atmospheric pressure squirts milk into babys mouth. No air inlet involved here ???
     

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  3. Yellowjacket
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    Yellowjacket Senior Member

    I am sorry that you don't want to accept "conventional theory", but what I am explaining to you is not a theory, it is a simple FACT and that fact is that propulsive efficiency can be quantified and calculated. That means that it is no open interpretation or other "theories". The simple fact is that any means of propelling something through the air using air as the propulsion medium is based on the concept of accelerating the air through the system. You are thinking about the inlet side of the equation, and not the exhaust. However, to induce flow through the inlet it has to come out somewhere and if it comes out at a higher velocity the propulsive efficiency can be quantified.

    High speed aircraft do actually create thrust in the inlet, but that only works at high speeds, like above Mach 1, and we aren't talking about that here. The SR71 spy plane (at a speed of Mach 3.5) actually made more than 70% of its thrust in the inlet. But the engine behind it acted like a pump and moved the air away from the inlet, so there had to be an engine in there to do that work and the air leaving the system was at a higher velocity and the results were consistent with propulsive efficiency calculations.

    Propulsive efficiency can be determined based on the velocity of the air exiting the propulsion system (and providing the thrust) and the speed of the vehicle. If you have a very low differential between the velocity of the accelerated air and the vehicle (ie, you move a lot of air but the speed difference is low) you will have an efficient system. If you have a high velocity jet, and the speed of the vehicle is low, the efficiency will be poor. This is why turbofans are more efficient than old fashioned jet engines, they move a lot more air at lower pressure ratios and this gives them more low speed efficiency.

    Propellers (either in water or in air) are airfoils that are accelerating medium which is air (or water if the prop is in the water) and the larger the propeller, the better the low speed efficiency will be.

    You are trying to invert the frame of reference and think you can "pull" the craft forward and have the depression created in the inlet do that. But it really doesn't matter what your reference frame is, it is the acceleration of the air inside the system that provides you the momentum change to push you forward. I'm sorry if you don't want to believe it, but we all see airplanes flying around us and the performance of these aircraft is calculated and determined accurately, before the airplane is ever built, using these conventional analysis techniques.
     
  4. tom kane
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    tom kane Senior Member

    Thank you for your time and explanations. One thing which I have learned over the years is not to think what information I have aquired in the past is still correct now so I check for the most up-to-date information available. That is what I am doing now.
     
  5. tom kane
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    tom kane Senior Member

    Airboat_Experiment

    It look`s like you are right.. but I have not invented the hovercraft but an Air Boat With a Horizontal Fan..seeing that no one else appears to have done it before. I don`t think I will continue with my Patent Application I shall give it to the World for free'.

    As I have put the idea into the Public Domain no one else can Patent It.
     

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    Last edited: Nov 12, 2010
  6. tom kane
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    tom kane Senior Member

    Airboat_Experiment

    One of the worlds newest inventions (world acclaimed) has horozontal fans and intakes and moves forward rather fast.
     

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  7. kach22i
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    kach22i Architect

    Tom Kane, have you seen the posts by BMcF in this thread?

    http://www.boatdesign.net/forums/boat-design/t-craft-16917-5.html
    ...and have you seen this one yet?
    http://www.industrialobject.com/
    [​IMG]
     
  8. kach22i
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    kach22i Architect

    Did a lawyer tell you this?:confused:

    RE: propulsive efficiency

    The formally educated engineers have all been pressed into a conventional mold which says propulsive efficiency is everything, nothing else matters. As I have not been pressed into this mold, but have been squeezed though an education system based on design, which is a decision making process, you must make some decisions. Sure, one decision could be to place propulsive efficiency above all else, but there are other choices.

    1. Choose to conceal the fan to limit noise.

    2. Choose to provide 90 degree bends before and after the fan to limit noise.

    3. Choose noise reduction and safety from spinning blades above all other considerations such as weight or propulsive efficiency.

    The very first hovercraft to cross the English Channel used a single fan with not only a 90 degree bend in it, but a series of exposed ducts for multi-directional thrust. To say it cannot be done ignores history, to say it was efficient ignores the laws of physics.

    SRN-1
    http://www.hovercraft-museum.org/srn1.html
    [​IMG]
     
  9. tom kane
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    tom kane Senior Member

    Thank`s for the replies and info but those craft are getting out of my size and budget
    and too big to be bullied by a small light weight 200 HP diesel. Personal transpot cheap to build, reasonably quiet about 15 feet x 6 foot 6 inches hull. I am sure a axial flow style fan (with special design blades) that is easy to balance and light weight will do what I want.
     
  10. Boston

    Boston Previous Member

    hey Gonzo I kinda liked your idea of a hovercraft sailboat, cat with hover components but instead of the motorized drive a mast and sails

    might end up being pretty fast assuming anyone is crazy enough to build it

    cheers
    B
     
  11. kach22i
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    kach22i Architect

    What?

    You will need less than half the engine for a hovercraft than for an equivalent boat and far less than an airboat.

    In the PDF below you can see a 9.5 meter craft (almost 30 FT) with only a 140 hp Diesel engine (Griffon1050TD).
    http://www.griffonhoverwork.com/images/pdfs/brochures/military.pdf?1259771992

    In general, hovercraft use half the fuel at speed than boats use (and airboats are worse). However the surface area must be about twice as large, so you will be loading it half as much which really upsets some people's mindset.
     
  12. Techecho
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    Techecho newbie to boat design

    kach22i's post a thread from BMcF showing the www.industrialobject.com showing the ASV-X1. My question would be if you consider a squirrel cage fan, is that consider as taking a 90 deg. turn from intake to exhaust.

    What I would look at is replacing your prop design and either using a back to back squirrel cage ( one motor connection, two side air draw ) or two vertical cages with top draw ( reverse motor connections ) in both cases you eliminate side torque.

    If all this sounds like I flew in from lala land It true. I be a newbie :)
     
  13. Techecho
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    Techecho newbie to boat design

    To follow on if you can balance the the boat for angle of attack of the air flow if the air stream is direct at an angle towards the water surface lets say down turn 20 to 30 deg you will get some thrust from the resistance of the the water ( back pressure ) most prop boats are looking to plain so the are looking for speed, the prop is directed at 90 deg to the water for maximum forward thrust at speed you would be looking for slow speed thrust less volume less horse power
     
  14. kach22i
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    kach22i Architect

    It would be nice to be able to direct it upward as well (pitch control). That way you could pop the bow up to plane once you get enough speed started. Otherwise with the thrust being set at 25 degree downward, the craft stern would start to lift up thereby sending the bow plowing in at speed.

    Like a see-saw.
    http://www.deltasigtu.com/philanthropy.htm
    [​IMG]

    http://www.langleyflyingschool.com/Pages/Attitudes and Movements.html
    [​IMG]
     

  15. Techecho
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    Techecho newbie to boat design

    I like the pitch idea at start up you can get good thrust ( stirs up the water ) then feather it out to not lose energy into disrupting the water.
    If it was the horizontal squirrel cage it could be swivelled and not require redirecting the air flow pattern.

    TOM in reading your posts you have made reference to 16' x 6'6" and 200 HP diesel are these items you already have.
    How much of the platform are you allowing for your propulsion, an outboard will only take up 1 or 2 feet but an air system will need at least a third of your length. 200 HP is a lot of power and weight, my 17' speed boat has 90 HP and can do 45 MPH. From pictures I have seen of air boats ( everglade style ) they are probably only using from 30 to 80 HP gas engines ( small and light ) I would think that should be the direction to go, maybe a 50-60 HP gas. Maybe a motor cycle engine 650 -750 cc they can be quiet enough with good mufflers.
     
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