Air boat

Discussion in 'Boat Design' started by Liighthead, Aug 26, 2012.

  1. Liighthead
    Joined: Jul 2012
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    Liighthead Junior Member

    Hey again :)

    Been looking around at small boats ect
    And was wondering how can you work out how much Power and prop size ?

    Would be something like 8 to 10 ft long and 3 to 4 ft wide
    Give or take

    I have 2 old lawn mowers ( both around 8 to 10 hp )
    But think theirs a 25 hp ride on out the back shed.

    Would I need to work out the boats weight ect then work on the power ?

    Those wondering why?
    I have a marshy land out back of my place, averages about 2 to 5ft deep haha
    So been using a pole to push myself around on my little plywood boat :)
     
  2. FAST FRED
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    FAST FRED Senior Member

    Air boats take HUGE power as the prop is so inefficient.

    A water prop can produce 20 lbs of push for each HP (some considerably more).

    The usual air prop will give 5lbs .

    So it takes at least 4X the hp, and fuel to go boating by pushing on air.

    FF
     
    Last edited: Aug 28, 2012
  3. Liighthead
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    Liighthead Junior Member

    Mmmmm thanks
    About 20lbs per hp is interesting,

    20 lbs is about 9kg so 25 hp motor push around 225 kg
    I'm about 80kg boat would be under 40kg motor something around 70kg ? Guess
    Which is like 190kg lol so pretty much on its limit

    Puts a spin on things haha thanks for the info btw :)
     
  4. Wavewacker
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    Wavewacker Senior Member

    Hmmm, do you like go-karts, little ATVs? I'd go with an old VW engine for a small airboat...
     
  5. Liighthead
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    Liighthead Junior Member

    Yeah was just thinking something like that, a small 3 or 4 cycilnder engine from a small hatch back even? Seam to be same price / close to a VW ?

    Only like 70hp ? Haha if that
     
  6. Submarine Tom

    Submarine Tom Previous Member

    Or a used Porsche motor.

    Air props can actually be more efficient than water props but not over the service range you're looking at, but that's a whole other kettle of fish.

    There is a nice little, very little, green, wooden air boat photo on this forum somewhere.
    Anybody have any recolection where that may be?
    Try "SEARCHING" air boats in the header above.
    Also, lots of good footage on Alaska State Troopers the other night if you want to Google that up.
    Great project but I'd look at a bigger power plant than the ones you mention you have "out back".
     
  7. Jeremy Harris
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    Jeremy Harris Senior Member

    Airboats can be pretty efficient, but to make them efficient means running a pretty large prop fairly slowly, which means a reduction drive from the engine a a cage around the prop that's pretty big and a bit cumbersome.

    Just as with water props the efficiency at low to medium speeds is better if the prop diameter is large and the rpm relatively slow, as it's generally more efficient at these speeds to move a large volume of fluid (air or water) slowly than a small volume more quickly.

    You can certainly get pretty reasonable performance from a small, light, air boat with just 6 to 10 hp, at least the equal of the performance you could get with the same size outboard, maybe better. The prop would need to be around 1.4 to 1.8m in diameter though, and with the prop guard would end up taking a lot of space. Two smaller props might be cheaper be easier to fit on the hull, and could be belt driven from the motor easily enough - take a look at some of the ways the cruising hovercraft people do it.
     
  8. tom kane
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    tom kane Senior Member

    An airboat with a horizontal prop is the way to go...more efficient, safer, because it has centrifugal balance, rotational torque will not roll over the boat and can be much quieter and easier to protect and easier to maneuver and steer. No need for air rudders. Much quieter fan noise and can make use of air induction ducting (like a Kort nozzle) for more efficiency from a smaller fan or prop.
     
  9. Liighthead
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    Liighthead Junior Member

    1. If I had the money for a 2nd hand porche motor I wouldn't put it on a 10ft airboat lol
    2. Yeah been looking around

    With a belt drive, could change the gear ratio between them Easyier and seams like the way togo lol
    Will have a look at some props and see doesn't have to go real fast lol :)
    Not sure between dual or a single prop?
    Could proably get a 2m prop ( with shroud ) on their have todo some designing with motor size prop ect
     
  10. whitepointer23

    whitepointer23 Previous Member

    lighthead, an old vn commodore v6 would be a great engine to use, you can pick up a whole car with a good engine for $ 300. very compact and light engines. 200 hp.
     
  11. whitepointer23

    whitepointer23 Previous Member

    one thing i don't understand about airboats is how they stay upright with all that weight up high, there is one on you tube doing 95 mph, i thought it would flip backwards at that speed.
     
  12. whitepointer23

    whitepointer23 Previous Member

    how can it be more efficient when you have to turn the air flow 90 deg. what about friction losses.
     
  13. Liighthead
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    Liighthead Junior Member

    Mmm nice idea , if boat is a decent size would be really nice, but was hoping for something for 1 person, small like 8ft type rig,
    Commadors engine gearbox ect would have to sit behind the motor lol


    Would make a nice motor for my 4.2m run about though :)
     
  14. Submarine Tom

    Submarine Tom Previous Member

    Ya, right.
    Probably one of those little motors out back will suite you just fine.
    I don't know what I was thinking.
    Oh, found that little boat.
     

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  15. Jeremy Harris
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    Jeremy Harris Senior Member

    Not sure I follow your logic here. Take your first point, "more efficient". This cannot be the case, for two principal reasons.

    Firstly, flow needs to be turned through 90 degrees with a horizontal prop or ducted fan to point the moving reaction mass in the right direction to provide thrust. Whenever you turn a fluid flow through an angle some of the energy gets lost in the process, from skin friction, making it inherently less efficient than an in-line arrangement.

    Secondly, the efficiency of a propeller or ducted fan can increase as the inflow velocity increases, particularly for a ducted fan where the acceptable range of blade loading can be greater. A vertical fan can exploit this to obtain an efficiency improvement with increasing inflow velocity (boat speed through the air) and so operate in a more efficient part of the prop/ducted fan operating curve for much of the time.

    I presume by "centrifugal balance" you're referring to gyroscopic precession effects and by "rotational torque" you're referring to P factor. Both are fairly minor in effect for small air boats, particularly if the rotating mass is kept to a minimum (as would normally be the case) and the power is modest (as is the case here. P factor is negligible at this power level and because a large prop turning slowly is most efficient, gyroscopic precession effects are also very small, because they are a proportional to the square of prop rpm.

    Noise reduction is primarily a function of tip vortex control, and can be as easily addressed with a horizontal axis system as with a vertical axis system. Adding a close fitting duct, ensuring that the tip velocity is kept well below about M 0.7 (preferably below M 0.5) and designing the blade planform to minimise pressure loading changes with span can all help reduce noise, as can other measures.

    Rudder losses are small, smaller than the losses you would get from turning the fluid flow through 90 degrees, and for a small air boat it might well be practicable to rotate the whole ducted fan/shrouded prop for steering.

    Sorry, but I don't buy the argument that fitting the prop/ducted fan on a vertical axis offers any advantage at all, other than reducing air draft at the expensive of consuming hull internal volume. It would be significantly less efficient and take up too much room in a small boat such as that which is the topic of this thread to be practical.
     
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