Any one ever use a Flat bottom duck boat for a airboat Hull

Discussion in 'Boat Design' started by bmikkalson, Oct 12, 2009.

  1. bmikkalson
    Joined: Oct 2009
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: MN

    bmikkalson Junior Member

    Hey, 1st post here. But have learked around alittle bit.

    I am thinking of using a small 14 foot flat bottom duck boat as the hull for my project.

    The question I have what is considerd to small in width for a air boat? 4 feet 5 feet???? I plan on mounting the motor real low and use a belt drive for the prop. To keep the center of gravity real low.
    Estimate HP of motor is around 40-50 myabe more if I can get my hands on a Vtwin Harly motor.

    I am more interested in jetting around the river so handling is pretty important. I am planning to keep the whole package small so it accelerates from a stand still quick.
     
  2. messabout
    Joined: Jan 2006
    Posts: 3,168
    Likes: 333, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    You will need generous bottom width to help offset the torque reaction of the prop. The HD twin will shake the boat into splinters in short order. You can get a rice burner engine for less money and enjoy a far smoother operation and probably less maintenance, less noise, less fuel. A decent 600cc engine will make more Hp than you can use, a 750 or 1000 cc Yam, Kaw, Hon, Suz will blow the boat up to decidedly dangerous speeds.

    Sounds like a fun project if you do not make an unfortunate engine mistake.
     
  3. bmikkalson
    Joined: Oct 2009
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: MN

    bmikkalson Junior Member

    what would you consider a generous width??? 5 feet?


    perhaps I should look into a 2 stroke 440 or such. Has any one ever used a snowmobile clutch set up?
     
  4. kroberts
    Joined: Mar 2009
    Posts: 318
    Likes: 12, Points: 18, Legacy Rep: 210
    Location: Chicago area

    kroberts Senior Member

    bmikkalson,

    I don't have any experience with an airboat, but I have had 2 hovercraft.

    The first one was a Universal Hovercraft UH-12r, for which they no longer advertise plans. That was 12'6" long, 6'2" wide. The second is a UH-18sp, which is 18' long and 7'6" wide. The second is pointed like a traditional boat shape, the first is relatively rectangular. That makes a difference, since the center of lift moves with the shape. That matters for boats too, but not as much I think.

    I think the harley motor is a really expensive proposition, and you probably won't get that much extra out of it besides the sound, if that is what you want then great.

    You want a light weight prop, large diameter. The light weight gives you less torque roll, and the large diameter gives you lots of static thrust, which is thrust at zero boat speed. That amounts to acceleration in practical terms. You want to turn it slowly to reduce noise and prop tip erosion. My UH-12r used a hopped-up Rotax 583, which per book claimed 98 hp and per mods was probably somewhere between that and 120.

    Prop tip speed for a hovercraft we generally try for 350 fps or less to prevent tip erosion and noise.

    None of this helps you with the advisability of using a duck boat for an airboat hull. All I can think is that maybe you would need to reinforce the engine mounts somehow, but I don't know anything about how the hull bottom shape affects things, but your biggest thing is going to be the nose-down force from fan thrust. I imagine the airboat hulls have more depth in the nose.
     
  5. bmikkalson
    Joined: Oct 2009
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: MN

    bmikkalson Junior Member

    Thanks, lots of good info. the nose down force, is that what keeps the fan from flipping over the boat?
     
  6. kroberts
    Joined: Mar 2009
    Posts: 318
    Likes: 12, Points: 18, Legacy Rep: 210
    Location: Chicago area

    kroberts Senior Member

    Ok,

    With any boat, there is a center of mass. If you have a cable holding the boat from the exact center of mass, then you could turn the boat at any angle, including upside down or straight up and down, and the boat will stay there. There is no heavy side.

    There is also a center of ... well, on a hovercraft we would call it the center of lift. That is where the shape of the boat comes into place. The pointed nose will displace less water than the wide rear, for example.

    Then you have the other forces acting on it, like thrust. With a standard outboard motorboat, the prop pushes forward but the center of thrust is under the boat. So the nose tends to be pushed upward, since the center of mass is going to try to stay put, and the motor is trying to spin it around and flip over backward.

    With a hovercraft or airboat, it's opposite. The center of thrust (the shaft of the prop, in both cases) is now pushing from above the mass, so the force of thrust tends to drive the nose downward.

    I suspect that one of the designers will get on and say that airboats have a higher side near the front than a normal boat, to counteract that nose-down force.
     
  7. lewisboats
    Joined: Oct 2002
    Posts: 2,329
    Likes: 129, Points: 0, Legacy Rep: 1603
    Location: Iowa

    lewisboats Obsessed Member

    Catch the intro for CSI Miami...you should be able to estimate sizes fairly close from there.

    Edit: Never mind...here is a Screen Snapshot. From the looks I would say about 14-15 ft and 6.5 ft wide with not much flare so maybe 9" or less narrower at the waterline. The front is similar to a Jon boat with that thing sitting on top...presumably to protect from the swamp grasses and such. I notice lots of sealed tankage for buoyancy and general walking about on.

    [​IMG]
     
  8. bmikkalson
    Joined: Oct 2009
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: MN

    bmikkalson Junior Member

    HMMM, it appears to me that making my own hull may not be the way to go. I do not really feel like doing the hull 5 times. There is lots of science to this that I really don't want to solve. hahahh Any one recomend a good home buil hull??
     
  9. hoytedow
    Joined: Sep 2009
    Posts: 5,812
    Likes: 374, Points: 93, Legacy Rep: 2489
    Location: North of Cuba

    hoytedow Carbon Based Life Form

  10. messabout
    Joined: Jan 2006
    Posts: 3,168
    Likes: 333, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    There are a lot of airboaters here in Florida. They are considered by the general public as the lowest form of life. (Actually I think some of them are pretty good people.) They are, out of necessity, a bit clannish. The biggest problem of airboaters vs public is noise. Most of the airboats use aircraft engines such as Continentals. The extreme guys use big inch V8 engines, some of which make 500 HP. There is even some state legislation that requires the airboat engine to have mufflers. Engines are not the problem, props are. They make one hell of a noise when turned fast. Go out to the local airport and listen to some Cessnas or Mooneys and the like. Make that kind of noise on the local lake or river and you get some unhappy, unsympathetic, residents. So much for social commentary.

    You can compensate for the force vector that induces bow down attitude. Simply angle the thrust line slightly up..which may need some experimentation. About the boats; Most of them are FRP, a few aluminum. and very few wood hulls. The current crop of them have spoon shaped bows, they are usually ...maybe 7 feet wide or more. Many of them are surprisingly short, as in 12 feet LOA. The ones with the Continentals simply mount the engine and prop at some elevation that allows the prop to clear the transom by a foot or so. Big *** Chevy, Ford, or Mopar engines are often, but not always, mounted at a lower position and use collossal Gilmer belts to drive the prop. Red neck engineering can be observed on some of these boats but a lot of them are very well and thoughtfully done.

    You might be able to Google up some specs about these boats. One of the brand names is Gilileo, another brand is Combee I think. I believe that there is an airboaters association here in Florida. If you can contact them they will be able to give you information and encouragement.
     

  11. bmikkalson
    Joined: Oct 2009
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: MN

    bmikkalson Junior Member

    hahah, ya some of those boats are pretty red neck.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.