fan boat

Discussion in 'Boat Design' started by lakelotman, Aug 7, 2015.

  1. lakelotman
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    lakelotman New Member

    I want to make a fan boat from mounting a horz shaft motor to my 12' johnboat and turn a 3 foot prop to push air instead of water, like mount motor in bottom of boat on springs for vibration and lift to allow a 12" pully to turn a 3" on the fan. will this work?
     
  2. SamSam
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    SamSam Senior Member

    A 12" pulley on the motor turning a 3" pulley on the fan will be a 4 time multiply, so an engine turning at 3600 RPM will turn a prop at 14,400 RPMs.

    At that speed, you'll probably put out your eye, kid, but you'd be going really, really fast when it happened.

    A 12' jonboat is probably too narrow to stay upright with sharp turns. It also has sharp chines which will catch and flip you in turns. Airboats usually have rounded chines so they can slip sideways in turns.
     
  3. lakelotman
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    lakelotman New Member

    ok redesign

    The motor would be only a 6.5 hp motor max 3800 rpm so i could double it to 7600 RPM at blades, which I would put in a cage of some sort.
    I have another old runabout boat which is a 14' vhull but thought it would be too heavy to start shuch a project.
     
  4. Dave T
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    Dave T Senior Member

    I would suggest you do some research on air boats. You will need at least 10 times this amount of horse power at the vary least even with a light aluminum boat.
     
  5. tom28571
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    tom28571 Senior Member

    Some more arithmetic is needed. 7600RPM on a 3' diameter blade works out to 814mph at the tips, which is beyond the speed of sound. There may be a small problem.
     
  6. Petros
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    Petros Senior Member

    Welcome to the forum,

    you need to limit your prop RPM to about 4500 max (less would be better) on a 3 ft prop to keep the tip speed well below the speed of sound. You can go higher rpm with a smaller prop, but you loose efficiency which means much less thrust for the same amount of hp. I would suggest switching your pulley size, put the small one on the engine side, and the larger one on the prop shaft, with the 12" and 4" that will keep your prop RPM to about 950, this would allow you to use a slightly larger prop. it will be much quieter at slower prop speeds too. Or if you have different pulleys available, use a ratio of 1 to 1, it will put your 3' prop rpm in the correct range.

    You also have to match the blade pitch to the rpm and engine hp, but if you are assembling this with salvaged stuff you have laying around or from the junk shop, than you just have to use what you can get. 6.5 hp is not much, but it will push you along at modest speed. 20 hp would be more useful, but of course that would mean a much stronger shaft, belts, pulleys, mounting rig, etc.

    BTW, you will want a rudder up in the air behind the prop to assist in turning, so it deflects the airstream to turn you. Using a water rudder only would be okay at very lower speeds, but likely would cause you lose control at higher speeds (which would mean you will be going for a swim).
     
  7. tom kane
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    tom kane Senior Member

    Don`t forget about the rotational torque (prop walk) making your boat less stable in sharp turns.
     
  8. lakelotman
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    lakelotman New Member

    airjohnboat

    Now this is the info I was looking for Petros. this sounds workable. the main thing I was wondering if it would push the johnboat enough to be worth the effort of the build. I hope to get started on a frame to support the prop & pully shortly. I was thinking of making a wire cage out of fence.

     
  9. Petros
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    Petros Senior Member

    you want to get as good a prop you can find. a small aircraft prop made of laminated wood would be best, but those are pricey. Or you can try your hand at making one yourself, you can find detailed instructions on the internet, I once knew a guy that made 10 or 12 wood props himself for his own home built aircraft (he was trying out different sizes and different blade pitches, he managed to double the amount of thrust out of the same engine over the one in the kit. So with careful design as home made props can be better than a bought one).

    you might be able to find one of those stamped sheet metal props they put in large industrial ventilators to work, not too efficient but it would like push you along and be cheap if you can find one in a scrap yard. One of those belt driven ventilator fans would give you the prop, shaft, pulleys, a sturdy mount, and a shroud.

    It will not be safe to be around without a guard, so be careful. Make sure the guard is sturdy enough so it does not get sucked or deflected into the blade, or it would become a hazard in itself.

    Good luck, post pictures if you get it to work.
     
  10. upchurchmr
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    upchurchmr Senior Member

    Another place to get information is on the hovercraft web sites.

    http://hovercraft.com/content/index.php?main_page=index&cPath=376 is one a friend is planning on making.

    I looked for "air boat plans" and Dave T looks to be right about typically using a lots more horsepower - but you probably don't have to for a modest speed.

    Show us pictures as you go.
     
  11. Village_Idiot
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    Village_Idiot Senior Member

    Check airboat forums and do a search for flounder-gigging boats. Lotsa guys using Honda 13hp motors with approx. 3' fan, probably lotsa used stuff for sale through fan boat forums. Nothing fast, but it'll blow you around in the water at several mph.

    Cage is a must. These things create quite a breeze and you'll suck in all kinds of trash that you're not aware of.

    I can't remember the exact number, but you want tip speed somewhere around .8-.9 Mach 1. You lose a lot of efficiency if you get too close to speed of sound (and way too much noise).
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I heard the Wright Bros homemade prop was scary efficient, but only at a certain RPM with a certain amount of forward motion through the air. IIRC they had to use other means to get up to speed, then start flying, which leads some to give others First Flight credit.
     
  13. whitepointer23

    whitepointer23 Previous Member

    Like the kiwi that built and flew a plane berore the wrights. Not recognized because he was not an american.
     
  14. upchurchmr
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    upchurchmr Senior Member

    Who is he?

    Lots of people did similar things in a short period of time.
    Others had more influence in their local area, no matter who was "first".

    The real point is the we have more movie makers, so we told our story. Right or wrong.
     

  15. Petros
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    Petros Senior Member

    There was no conspiracy about the Wright brothers. It depends how you define "first flight". Usually they refer to the Wright brothers accomplishment as first controlled powered flight, meaning that the human was operating controls to maneuver the aircraft. There have been many that have been recognized as having flown before the Wright brothers, but it was not controlled nor powered flight. Even the Wright brothers have recognized the German scientist Otto Lillianthath as the father of flight. He took hundreds of controlled flights in an unpowered hang glider in the 1800's. there were others that got in the air and rode the craft to a crash, not exactly controlled powered flight. The Wright brothers very well knew who was doing what at the time, who accomplished what before them (they acknowledge it in their book) and they knew who ever was able to demonstrate practical powered controlled flight first a big advanage.

    Two problems flight pioneers faced, that was no light weight engines, and good props. the Wrights designed and built their own engine (though Curtis had a better one, but he was their competition), and they had to develop the science of prop design. It was not that their prop was so efficient, it was just better than anything else out there and good enough to allowed them to get into the air on very little hp. It was a remarkable accomplishment, but certainly we can design and build better props now. You can even get some decent wood prop plans off the internet for free.
     
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