Alternative Jet Propulsion

Discussion in 'DIY Marinizing' started by JOBBER, Mar 4, 2012.

    Joined: Sep 2008
    Posts: 8
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    Location: carol stream, il 60188

    JOBBER Junior Member

    Does this have any possibilities as a “poor mans” jet boat engine. It would be used on a 10-12ft displacement craft. The premise would be to hard pipe a suction and discharge arrangement. The suction filter screen would have to be larger than the line size shown as an accessory. This would minimize clogging and allow enough water to be available while the craft is moving. Using cam quick connect fittings would allow the discharge to be directed for steering. My questions are:
    Would it even work?
    Would flow volume or pressure be the deciding selection factor?
    Would pump discharge size or a restrictive nozzle be more beneficial method of propulsion?
    High-Pressure Water Pump, 2in. NPT Ports
    116 GPM @ 40 psi, 111 GPM @ 60 psi, 93 GPM @ 80 psi and 90 GPM at 94 psi.
    (8,120 GPH ÷ 60 = 135 GPM > no where above???)
    26 feet of suction lift
    Impeller = aluminum & Volute = cast iron
    No solids (can be used for pond water with proper strainer)
    Self-priming (after priming port is filled)
    Adjustable throttle
    Fuel capacity 1 gal.
    Weight 60#
    Honda GX160 Engine (commercial grade)
    Semi-Trash Pump, 3in. NPT Ports
    264 GPM (max) – TDH 45.5psi (no pump curve available)
    26 feet of suction lift
    Impeller = cast iron & Volute = cast iron
    3/4” Ø solids (can be used for pond water with proper strainer)
    Utilizes a check valve to hold a prime while the pump is not running
    Multi-directional discharge??
    Adjustable throttle
    Fuel capacity 3/4 gal.
    Weight 65#
    Honda GX160 Engine (commercial grade)
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Jets work by accelerating water. The thrust is the opposite reaction to the displaced water. It will move a boat, very slowly and inefficiently.
  3. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Sorry, but your proposal is not going to work. In order to create thrust with a reasonable efficiency, you need to induce a small velocity increase to a great flow of water.

    You are doing the opposite; giving a high velocity increase to a small flow, and doing it with a huge amount of piping that is causing a lot of friction losses. There are a number of threads under jet propulsion, where the principles are explained, take some time and study. Then make a specification: what are you going to propel, at what speed. That gives you the thrust and operating conditions that you have to meet.
    1 person likes this.
  4. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    It does work because in the manual you are warned to properly anchor the pump, as it will topple over or moves when free standing.
    But it will not be very efficient (see post #3).

    If you want to experiment, buy a Chinese clone from this pump. In Europe the going price for such pumps is about $200. Last week I bought only the petrol engine, brand new, for $108. For $400 you can buy the pump with an electric starting diesel engine.
    Joined: Sep 2008
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    Location: carol stream, il 60188

    JOBBER Junior Member

    Thanks for the input

    Thank you for the input.
    You're rigth there is alot to know about jetpulsion theory.
    I did some searching and found these.

    It seems it can be done only as stated, "very inefficiently".
    This person actually put together a how-to and testing report.

    Inadvertently found this also. Someone always has an idea.

  6. baeckmo
    Joined: Jun 2009
    Posts: 1,157
    Likes: 131, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    These proposals pop up every now and then, so let's make a rough calculation to show what it's all about.

    You have a pump with a few operating points given, we check this: 111 gpm/60 psi. In SI-shape that is ~40 m of water column (= 392 kPa) and 0,007 m3/sec. The pipe inner dia is 2", say 50 mm, which gives a flow velocity of 3,6 m/s. An estimate of the friction losses requires knowledge of the actual piping system, but let's assume there is about 2 m of piping plus a couple of bends an inlet and some other paraphernalia, all in all adding up to an equal pipe friction loss of ~1,5 m aq.

    Available head (H) over the nozzle is then 40-1,5 m aq. Since H=v^2/(2*g), the nozzle velocity is ~26 m/s. With a flow of 0,007 m3/s and a contraction factor of ~0,95, the nozzle dia will be ~19 mm.

    Static thrust is mass flow times velocity change. With water density 1000 kg/m3, the mass flow is 1000*0,007, ie 7 kg/s. With the velocity increase from zero to 26 m/s, you have a total static thrust of 183 N. The "hydraulic power" is pressure times flow; in this case 392e3*0,007 = 2744 W. If the Honda is delivering max output here, the input power to the pump is 3,53 kW. The pump efficiency is then 2744/3530=0,78.

    Now, if you are moving with, say 6 knots (~3 m/s), there is less velocity increase over the nozzle (a GOOD inlet converts about 65% of the incoming dynamic energy into pressure in the pump). In a simple system like yours, there is no recovery whatsoever, so you will have a thrust at 3 m/s that is ~23 m/s times 7 kg/s; ie 161 N.

    The total effective propulsive power is then 161 * 3 W (=483 W) and you spend the full engine power of 3530 W to accomplish this, resulting in a lousy 13 % total efficiency.

    With a decent propeller, say about 9" dia, the same propulsive thrust would consume just over 1 kW! In my world that is "Does not work!"

    In addition, I have serious doubts about the performance of those pumps, since if you check the pump hydraulic power at the various operating points given, the engine will either be overloaded (bogging down to lower rpms) or the efficiency has to be over 100 %, which I doubt......

    So, there are better ways to propel a boat than those irrigation pumps.....
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