...Yes a very informative response. It's a wonderful thing how mathematics can prove / disprove a theory with such finality. I found the last line summary interesting.....
"As an example, a pump with a capacity of 200 gpm flowing through a 2? duct would develop 17.7 lbf of thrust".
If I understood this correctly, this is a pretty dismal way of translating energy into usable work. Kind of like a steam engine...a whole lot of fire for a bit of horsepower.
Joe
From: Cliff Redus <cliffordredus@sbcglobal.net>
Reply-To: personal_submersibles@psubs.org
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Thrust from a pump?
Date: Wed, 3 Jan 2007 17:58:18 -0800 (PST)
Alec,
Below is a derivation of thrust that can be developed from a axial flow pump in terms of volumetric flowrate.
The thrust due to accelerating fluid through a pump can be written as
F=M(V1-V0)
Where M is the mass flow rate, V0 is the free stream velocity upstream of the pump and V1 is the velocity exiting the pump.
But the mass flow rate M can be related to the volumetric flow rate Q as
M=Density*Q
Substituting, the thrust in terms of volumetric flow rate is
F=Density*Q(V1-V0)
But the volumetric flow rate Q is related to velocity in the pump duct ID as
Q=V1*A=V1*Pi*D^2/4
Where D is the duct ID.
Solving for V1, and substituting, the thrust can be written as
F=Density*Q(Q/(Pi*D^2)-V0)
For a thruster oriented approximately normal to the direction of flow, the inlet velocity can be assumed to be zero. The thrust then reduces to
F = 4*Density*Q^2/(Pi*D^2)
Or
F= 0.001766*(q/d)^2
for freshwater where,
F = Thrust, lbf
q = pump volumetric flow rate in gpm
d = pump outlet duct inside diameter in inches
As an example, a pump with a capacity of 200 gpm flowing through a 2? duct would develop 17.7 lbf of thrust.
Cliff
----- Original Message ----
From: "Smyth, Alec" <Alec.Smyth@compuware.com>
To: personal_submersibles@psubs.org
Sent: Wednesday, January 3, 2007 4:20:48 PM
Subject: [PSUBS-MAILIST] Thrust from a pump?
I'm contemplating using water pumps as maneuvering thrusters (NOT for main propulsion). Cousteau's socoupe used a hydraulic pump for the jets, but I'd like to avoid that route if possible due to the excessive noise. DC submersible pumps are convenient but I haven't found anything beefy. I could try mating a trolling motor to a pump head. Submersible well pumps look promising because they're very powerful yet compact, but they require AC, and I don't think I can reverse them so I'd need four pumps instead of two. I could of course hang trolling motors off the boat font and back, but it really messes up the hydrodynamics of what can otherwise be a quite clean design, and the props on my trolling motors make them too large to duct into the MBTs.
Assuming I do settle on some kind of pump, the next question is how strong it needs to be. Does anyone know how to translate GPM (or LPM) into static thrust?
Any ideas or experiences would be much appreciated.
Thanks,
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Alec