Can a pump-jet be used for underwater craft?

Discussion in 'Propulsion' started by samcool111, Mar 18, 2012.

  1. samcool111
    Joined: Jul 2009
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    Location: China

    samcool111 Dr.

    I recently got a small pump-jet, or water pump, for ship models.
    It's just like those in big ships know as the water jets or so. But smaller, design to be used on ship model.
    That thing is not big, only 0.25 meter length. That thing seems to be only used for surface crafts model. But I wonder can that be used for a underwater craft model?
    I got an old underwater model, how about fit the water jet on it?

    I'm not good at those jet pumps or water jets. I'am quite confused about them. Wish to get some hint...
  2. tunnels

    tunnels Previous Member

    A water jet looses its push once the nozzle is under the water . with air round it , it has grunt . without No air maybe half amount of grunt !! Some one on this sight will be able to enlighten you why this is so !!. get the garden hose and a bucket a!!,as the water fills the bucket the hose has push but once the water level rises and the end of the hose goes under water!!!!!ooops you have become gruntless !! :confused:
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    It is in fact quite simple.
    In a closed space the water pressure exerts equal force in all directions. If an opening is made in one plane, there is a resultant force away from the hole because the water pressure cannot exert force to the missing wall and drops to near zero. When submerged, the surrounding water partly replaces the missing wall, thus preventing the pressure to drop completely.

    Above water, high pressure and flow speed provide more thrust, under water you need low speed, high displacement. A propeller does exactly that!
    1 person likes this.
  4. tunnels

    tunnels Previous Member

    Thought there had to be a simple answer !! so there were are !! :p
  5. samcool111
    Joined: Jul 2009
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    samcool111 Dr.

    It seems quite simple. But... I finally got what you mean.
    The water jet will not work well when underwater. The performance is bad.
    But, I saw some big naval submarines used the pump jet or pump proplusion on the net. That may be another type of the pump? OR the underwater version?
    Can my old water pump be modified to that type of pump jet ?
    Or, I had to buy a propller....
  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Try it with a positive displacement type pump.

    Then try it at a constant power input.

    If the garden hose nozzle exit is thick and blunt, the exiting jet can create a suction on the backside of the nozzle surface. This would happen on a standard garden hose nozzle. Use a thin nozzle.
  7. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    There are some small submersibles that use water jets for propulsion. I do not know what type of pump they use. But the type you have would not be very efficient underwater.
  8. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    A ducted fan, or Nozzled prop would be best
  9. Frosty

    Frosty Previous Member

    A jet works be every motion has an equall and opposite motion.

    This is equall to the speed it is repelled, not very fast pushing against water!!

    All you will have is a propellor and a bad designed one.

  10. samcool111
    Joined: Jul 2009
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    Location: China

    samcool111 Dr.

    The propller may be a good choice. I will try to buy one.
    The hole thing is about the knowledge of hydrodynamics. Quite interesting.
    But I would try the pump anyway, may be interseting too.
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