Keith Said:
.... Looking at the RULE site, their largest bilge pump is 8000gph,
or 133gpm. That calculates to 7.8 Lbf thrust, with a 31 amp draw at
12VDC. Comparing that to a Minn Kota Endura 30, with 30 Lbf thrust,
at 30A/12VDC gives a good comparison of the relative efficiencies.

THAT makes a lot of sense, (a 3 or 4 to 1 ratio) and gives us some
real-world numbers to think about... And the following implies that a
decent experiment could be done by using a maximum-outlet-diameter
smooth hose to the outlet, and then fitting different experimental
nozzles:

I was reading a page by an RC boat builder who use a bilge
pump for drive on his boat. he used a fishing scale to measure the trust
produced by the boat, and found that making the nozzle on the outlet
increased thust, but only to a certain point.

Yes, and that certain point is where the flowrate begins to decrease as
a result of the additional head pressure caused by restricting the
outlet. There are other issues that arise when the outlet is
sufficiently large that it represents a significant percentage of the
width of the boat, which you can do with an RC boat, that just don't
arise in 'real' boat applications.

Let me try an approximation based on the above, looking at at my idea
of running a large? inboard pump connected to my existing marine
engine thru an air-conditioning compressor clutch, and piping it thru
a control valve to vary bow thrust port-to-starboard.

30 Amps at 12 V gave maybe 7 pounds thrust. That's about 1/2
horsepower. Say I can use 2 HP (What I understand a car air-
conditioner uses) to a pump with the same losses as the example Keith
showed. So maybe that's 28 pounds thrust. That sounds like plenty
for a 25 foot boat...

Question: How much thrust do typical electric bow-thrusters give in
the smaller sizes?? (We'd expect them to be more efficient).. BTW,
they are expected to be used at close-to-zero hull speed, so the
thrust measurement is reasonable here.

Maybe I can try some of this out this Summer on Lake Champlain
(Vermont) . (Now I'm boatless :-( on the Med this year, but moving to
the shore of the South China Sea for the next 2 or 3 years where I
WILL Mess With Boats!).

Interesting discussion!

Regards, Terry King ...On The Mediterranean in Carthage