Cat hull, one motor ?

Discussion in 'Powerboats' started by Fanie, May 11, 2011.

  1. Fanie
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    Fanie Fanie

    I Guys

    Can a single motor be fitted between two hulls and expect it to plane ?

    I have fitted an outboard between two hulls and the water wells up and cause quite a bit of spray.

    Would it be possible to make some kind of a false hull that fits on the motor so the motor will think it's running behind a hull ? This false hull coul be like a fin that force the water under it perhaps ?

    Thanks.
     
  2. Fanie
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    Fanie Fanie

    Perhaps a guide plate like this that would force the water to pass under the skid plate... or lift the motor, either way.

    Could it work ?
     

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  3. rasorinc
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    rasorinc Senior Member

    Glen-L recommends this http://www.glen-l.com/designs/house/huckfinn-notes.html The size tells me that you have to generate a lot of water to the prop. I am going to make mine a little longer and taper it in at the front to a zero point. Stan Basically, I'm adding a partial sponson to match the 2 outside ones. Building the Glen-L Bearcat.
     
  4. southatlantic
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    southatlantic Junior Member

  5. rasorinc
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    rasorinc Senior Member

    Fanie, I'm sure someone on the forum knows a math calculation based on your engine and prop size to determine the volumn of water needed to pass. Stan
     
  6. keysdisease
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    keysdisease Senior Member

    Pontoon baot manufacturers do this all the time, take a look at a few of them. This is also common practice with several of the smaller fiberglass catamaran manufacturers too, and yes they plane.

    Steve


     

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  7. Fanie
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    Fanie Fanie

    The motor is a 6hp parsun and it sits between two kayaks. When one accellerates, the water between the hulls well up a bit and results in the problems which can only be lots of drag.

    If one fits a plate as in the picture, that will push (force) the inrushing water down to pass over the prop under the skid plate, I'm sure this could have better results.

    Someone somewhere must have tried someting like this before. The prop is ~200mm diameter, if a plate, say 250 - 300mm wide, force the water down under the skid plate like the hull would have done I'm almost sure it could work. The faster you go the flatter the water would sheer under the plate. An auto adjusting hull.

    Steve, I saw some single motors mounted on pontoon boats, but these all only had a plate in front of the motor like a bow, all it does is to keep water from splashing on the motor. All these were slow going displacement boats.

    Btw, I'm very surprised at the grunt this 6hp parsun motor has. I expected it to be a bit more powerfull than the 2hp Yamaha I had before, but this exceeds my expectations.
     
  8. Stumble
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    Stumble Senior Member

    I don't know about in this particular case, though 6hp seems a bit low to me. But TwinVee specced out a number of their cat hulls with one engine.
     
  9. claydog
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    claydog Junior Member


    Lots of pontoons in my neck of the woods running a single 100+ hp outboard with a very small "doghouse", for lack of a better term, in front of the engine and under the deck. To my untrained eye your idea looks like it would accomplish the same thing.
     
  10. Submarine Tom

    Submarine Tom Previous Member

    Well, you've got ~25 times the power of two paddlers. If mounted correctly (who knows how?) I'm very confident your 6Hp will plane two kayaks joined together. Surely this has been done before.

    Is this one person, two or R/C?

    -Tom
     
  11. yipster
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    yipster designer

    From the mobile a rough thought
    Is that 6hp at the stern?
     
  12. Fanie
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    Fanie Fanie

    Hi Tom,

    The contraption weighs 150kg with the motor and 12L of fuel. It wants to plane but barely seems not to make it. I weigh 100kg's and my friend weighs about 75 to 80kg's. We may want to be three at a time.

    Of course with a 100hp I'm sure getting on the plane won't be a problem :D

    I wasn't planning on getting it on the plane, never expected the little 6hp to be so grunty. If it is possible however, why the heck not.

    The idea is to fish from it, you sit on the trampoline, feet on the trijaks (feelfree's trijaks). It is 4m500 long, 3m wide flat, and about 1m600 wide folded. I don't know how many feet it is.

    Sorry Yipster, missed your post, but here you can see it. At 3m wide it's very stable. I do plan to put a small sail on it as well, daggerboards and rudders, for those looong trawls. Ok and because a few okes told me it cannot be done ;)
     

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  13. Stumble
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    Stumble Senior Member

    Hmm, it looks like you have the motor mounted at exacally the recommended height for an outboard... In practice a lot of boats require the engine to actually be mounted lower than that to compensate for the wake hollow created. Before I spent any money I would think of dropping it a few inches and see the effects.

    You might also consider switching to a larger prop for more torque. I am guessing your prop is cavitating as below planing speeds, bleeding off the power you need to get over the hump. Though this is far from my area of expertise.
     
  14. Fanie
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    Fanie Fanie

    Hi Stumble,

    The prop does not cavitate any, in fact when we sit on it it goes around 4" deeper. Also keep in mind the water wells up between the two hulls at speed, so I guess the skid plate is anywhere from 4 to 6" inches (150mm) below the surface.

    GPS speed at half throttle is 12 to 14km/hr (6kn) and at full throttle only 17km/hr (8kn).
     

  15. Fanie
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    Fanie Fanie

    LOL Aint that the truth :D
     
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