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Keith Hughes

wrote:

Keith Said:

snip

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...

OK, I'm confused. Are you talking about *just* a bow thruster operation?
If not, why would you add another mechanically lossy system instead of
just using the marine engine? No matter what system you bolt onto the
engine, it will be less than 100% efficient at energy conversion, so
you'll just lose power in the process.

For bowthruster operation, should be easily doable.

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.

My understanding is that they are considerably more efficient than
typical outboards, since the "ring" around the prop eliminates a
significant amount of prop slip relative to having an open prop.

Keith Hughes