Wow, what an A+ response! Thanks Brian for yours as well. With this information I can now go out and do some virtual shopping to see how much I'd have to spend to accomplish the same thrust with the two approaches. Off hand I'd say the well pump solution would likely be cheaper, but mating trolling motors to pump heads makes use of two brand new trolling motors I happen to have sitting in the garage. thanks, Alec ________________________________ From: owner-personal_submersibles@psubs.org on behalf of Cliff Redus Sent: Wed 1/3/2007 8:58 PM To: personal_submersibles@psubs.org Subject: Re: [PSUBS-MAILIST] Thrust from a pump? 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, Alec The contents of this e-mail are intended for the named addressee only. It contains information that may be confidential. Unless you are the named addressee or an authorized designee, you may not copy or use it, or disclose it to anyone else. If you received it in error please notify us immediately and then destroy it.
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