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Calculate Thrust for Pump

  1. Aug 10, 2006 #1
    Hello All,

    I am building an ROV and would like to use bilge pumps for the thrusters. I would like to calculate the thrust that a prospective pump is going to create so as to decide which pump to buy. I know that the pump can output 1250 gallons per hour and has a nozzle with a 2" diameter opening. Is there anyway that I can calculate how much thrust this pump will produce? How much more thrust would I gain by going to a 2000 gph pump?

    As you may be able to tell, I'm not a Physicist but rather a Chemist, so please don't hold that against me :wink:

    Thanks for any help.

  2. jcsd
  3. Aug 10, 2006 #2
    as a general formula I worked out that the thrust in newtons is going to be

    (rho) (F)^2/A

    where rho is the mass density of the liquid being pumped in kg/m^3

    F is the volume flow rate in m^3/sec

    and A is the area of the nozzle measured in m^2
    Last edited: Aug 10, 2006
  4. Aug 10, 2006 #3


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    Welcome, coffejor. You might want to experiment with various nozzle modifications as well as just tailoring the pump and motor to your needs. Might I suggest even using variable ones, such as on a garden hose, with remote-control actuators so you can vary the flow to suit your purposes at any given time? It would be sort of like having a transmission.
  5. Aug 11, 2006 #4
    note that the flow rate will be the same no matter what the size of your nozzle is, it will just become far less energy efficient as a propulsion system as the nozzle gets smaller and the exit velocity increases
  6. Aug 11, 2006 #5


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    True. My thinking was based upon the fact that he might at some point want more speed, as in moving from one interesting location to another one some distance away. Once the thing is underway at high volume flow, it can obtain more top-end with a faster discharge.
  7. Aug 12, 2006 #6
    Thanks for all the help, I got through all the calculations and got the force I was looking for.

    I think that I'll experiment with various nozzles on the ROV to see which gives me the best performance, my thought is that the manufactorer of the pumps probably already determined what the optimum flow rate was and thats why they chose the hose diameter that they did, but a little on site test won't hurt either.

    Thanks for all your help, couldn't have done it without u guys.

  8. Aug 12, 2006 #7


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    You're welcome. Glad it worked out for you. One thing to bear in mind, which is why I suggested the nozzle variation, is that these things were not designed for propulsion. They're optimized to get the water out of your boat fast.
    Last edited: Aug 12, 2006
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