Jet Boat (Super Light Build)

Discussion in 'Boat Design' started by Red_LaNOS, Nov 6, 2013.

  1. Red_LaNOS
    Joined: Nov 2013
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    Red_LaNOS Junior Member

    Hi guys I am working on a small project at the moment and i realise there are numerous threads for Jet Boats on this site however after reading a fair few of them, none of them seem to answer my questions completely.

    The Hull that i am using will be a Jon Boat, 25kg, 2.4m long.

    This boat is being built as part of a DIY challenge with a few mates. It must be able to travel through shallow water against relatively fast current. Given the conditions i figure a jet drive or water jet are the only means of successfully completing the challenge.
    I have Google'd and Youtube'd extensively the use of fire fighting water pumps as a means of generating a jet of water out the back of the boat for thrust. After much research i realise that the relevant things to look for in a pump for this would be the:

    "Max Suction" (How far vertically above the water the pump can suck water into it) and "Max Head Lift" (How far vertically above the pump can push water)

    The max head lift would give you an idea of pressure the pump can generate and as a result thrust. I think that pressure will give you "acceleration" of the water and volume gives you "mass" so F= MA will give you the thrust in this particular case?
    I haven't seen this being done with much success on Youtube however the hulls people use for these are either really inefficient (barrels etc) or the pump being used isn't a good one and the water isn't being directed through a nozzle or even onto the surface of the water.

    Does anyone have any thoughts on this?

    My other option is jet drive from a Yamaha jetski. I do have access to this however i do not want to put a full sized jetski engine in. Am wondering what kind of motors i can use to power the jet drive successfully given the 25kg mass and 2.4m size of the hull itself. I'm looking into motors such as lawn mower engines, petrol engines from water pumps, 2 stroke 49cc engines for minibikes, whipper snipper engines and maybe even a cheap outboard motor. I realise that unless powered properly a jet drive is actually quite useless since it is highly efficient at slow speeds. Any ideas on this?
     
  2. Red_LaNOS
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    Red_LaNOS Junior Member

    For the water pump idea: I think that pressure gives you Acceleration because pressure affects velocity of the water. Higher pressure = higher velocity of the water. The acceleration comes into it as the water will only slow down as it leaves the nozzle at the back of the boat. This will be negative acceleration of the water but will positive acceleration for the boat in the direction that we are interested in.
     
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is the "challenge" ? You can't match a toy engine to a jet-ski pump, it will just kill it ( the engine, that is).
     
  4. Red_LaNOS
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    Red_LaNOS Junior Member

    The challenge is to create a watercraft capable of moving upstream in shallow water that is moving kinda rapidly. In one section the water moves quite quickly as there is a steep gradient however there are no large rocks to worry about. Apart from that, speed on the flat water as well and ability to cover the most distance before you run out of power.

    The water is moving fast enough that it is quite a struggle to paddle up it in a kayak, even going flat out you will stay in the same place as you near the "rapid" part.
     
  5. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    Afaik the jet doesn't need to hit water. Its simply about accelerating a mass of water backwards and thus causing same force to the boat forward. As you accelerate the thrust gets smaller? Might not be that relevant for you.

    Sizing a pump is hard, the specs are often best case scenarios, they will tell you max flow and max lift but both will not happen at the same time.
     
  6. nimblemotors
    Joined: Jun 2009
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    nimblemotors Senior Member

    [​IMG]build an airboat. works great in shallow water, noise is the only downside mostly.
     
  7. Red_LaNOS
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    Red_LaNOS Junior Member

    I've thought about fan boats but i have no idea how big the fan or motor will have to be to power such a small craft? Was kinda hoping to stick with simple cheap motors like lawn mowers, outboards, whipper snippers etc.


    Was kinda hoping that the water pump or jet drive powered by a smaller motor would work. So even a jet drive powered by a small petrol outboard?
     
  8. Red_LaNOS
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    Red_LaNOS Junior Member

  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I recommend firework rockets strapped to back of the punt, as many as required for good forward motion, repeat as necessary. :p

    Unfortunately, you will attract the attention of the 'fun police'. :D
     

  10. boatbuilder41
    Joined: Feb 2013
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    boatbuilder41 Senior Member

    I have seen a jet drive in a carolina skiff. Although I did not see it up close. I know it did not perform well. Im not sure if it was because of the flat bottom shallow draft of the skiff or the actual instalation of the drive. I would think the problem was that the jet couldn't be mounted low enoughbecause of the motor hitting the bottom. Or maybe the angle of installation. It appeared the water was just blowing into the air creating very little thrust.
     
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