"Keith Hughes" wrote in message
...
Hi Shaun,

Ive been reading up a bit on pumps, but some of the math is beyond me. i
do know that its possilb eo hook must pumps up either in series, or in
parallel. in parallel you quite logically get a doubling of flow in
gallons per hour or whatever, while in series you combine the 'heads'
whatever that means. i think it means head pressure?

Yep, head is pressure. Basically, you have one pound/sq.inch for each
27.68" of water column (height).

i know a lot of the losses in small pumps are from pumping 'up'.

That's kind of a misconception regarding 'head'. Pumping up, down, or
horizontal, the flowrate is dependent on the total backpressure on the
discharge line (but of course, 10' of vertical pipe does have more total
backpressure than 10' of horizontal pipe - of the same size).

most small pumps are rated by how high they can pump water, and the
rating for flow goes down as the height increases. installed in a boat,
i would try to keep the whole thing on the level with the shortest hose
runs possible. on a beach cat, i would have a thru hull on the side of
the hull with maybe 6 inches of hose going to the pump, then another foot
of hose going to the outlet.

Keeping the tubing runs as short as possible is certainly the right
approach to reduce frictional losses. One problem with the inlet on the
side of the hull (or any hull surface tangential to the water flow) is
that you get Bernoulli effects as the boat speed increases, that tends to
create a vacuum in the suction line (the same concept that makes paint
sprayers - the kind that use air hoses - or end-of-hose garden sprayers
work. The high speed stream across the diptube end creates suction to
raise the paint/roundup into the discharge stream).

i think youd have to start with two pumps in each hull, both running off
a common larger diameter inlest, and through a Y joiner to a common
outlet. this would give you some options. you could run the pumps in
parallel, or in series. then you would have to experiement with various
reductions in the outlet to see what the smallest diameter nozzle you
could use without losing flow would be. this is probably how you would
use 'gearing'.

if you used too large of a diameter nozzle, you really wouldnt get any
force at all.

Don't confuse "velocity" with "Force". Just like with a garden hose where
you have, say 80psig, you can pinch the end to get a higher velocity
stream, but you get less flow (i.e. less mass). Since the force = mass x
acceleration, the force however is the same (you only have 80psig to start
with). The same is true for pumps, as you note above, when you create
more backpressure (pinching the hose), the flowrate goes down. If you move
100gpm of water through the system, the force is the same whether the
discharge is 1" or 3", only the velocity of the dischage changes.
Remember, PSI is pounds per square inch (i.e. force per unit area), so the
1" discharge stream may be at 10 times the pressure of the 3" stream, but
the 3" stream has 10 times the cross-sectional area of the 1" stream.


Keith Hughes


in the very simples sense though, if i had the same volume of water flowing
through both a very large and a very small outlet, the speed would be much
greater for the smaller outlet right? this seems like a way to achieve some
sort of gearing to me, despite whatever losses are incurred from
backpressure. runing pumps in series would allow you to have a smaller
outlet and still maintain the same volume of flow right?

While there would obviously be a sweet spot for any given pump, having more
velocity at the outlet seems like it would probably result in more real
world 'thrust'. 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.

Shaun