"Keith Hughes" wrote in message
...

snip

this seems like a way to achieve some sort of gearing to me, despite
whatever losses are incurred from backpressure.

It's not a matter of backpressure, it's a matter of reaction mass. It is
Newtons second law of motion, paraphrased; for every action, there is an
equal and opposite reaction. The 'little' stream puts a lot of force over
a small area, whereas the 'big' stream puts a small amount of force over a
big area. In each case, the "force/unit area x area" quantity (total
Force) is the same. As long as the volume remains constant, every
increase in velocity will be offset by a proportional decrease in the area
over which it is applied.

It's not a matter of the water stream "pushing" against the water behind
the boat. Its just like how rocket thrusters work in a vacuum; you shoot
out 10kg of gas at 10m/s over a 10 second period, and you'll get exactly
that much "thrust" in the opposite direction. To be sure, there are lots
of hydrodynamic losses and effects for the boat, but the basic properties
of thrust are the same.

If i understand what you're saying here, it sounds prettymuch
counterintuitive. I may be mis-using some of the terms? let me give an
example just to be sure that i understand what you're saying here, and bear
in mind that of course the numbers im going to use are entirely made up in
my head, so they'd be wrong....

lets say that you have two identical boats with the same pump on each one,
running at whatever flow you like, say 5,000GPH. Boat A has a huge
outlet... say 5 inches in diameter. for arguements sake, because i dont
know how to calculate the speed of the water for that given outlet, lets say
the speed of the water coming out the back is slow. i dont know how slow,
but lets say it comes out at 3 knots.

Now boat B has the same pump, but the outlet is so small, that even though
its using the same pump, the water is coming out at a speed of 20 knots.
what you're saying is that both boats because they have the same amount of
energy put into them, and the same total force.... they'd go the same
speed? is there no relationship between the speed the water comes out and
the speed of the boat ie. it seems pretty obvious boat A could never go
faster than 3 knots, so boat B would never go faster than 3 knots either?

runing pumps in series would allow you to have a smaller outlet and still
maintain the same volume of flow right?

The same volume as what, a single pump with larger outlet? If you mean
use a second series pump to overcome all the frictional losses to maintain
flowrate, sure...but you're now powering 2 pumps. The cost of the higher
velocity, at the same volume, is all the additional power you burn up in
the second pump.

sorry, i think i was just continuing on from something i was writing in a
previous post... i was meaning to say iff you had two pumps in parallel as
opposed to two pumps in series... two pumps in parallel would give you
double the GPH flowing, but having two in series would allow you to have a
higher velocity through a smaller outlet right? you can probably see where
im going with this, but it really does hinge on the question i was just
asking about the relationship between the speed of the flow and the speed of
the boat....

if a higher flow speed allows a higher boat speed, then it would seem
logical to me that you might get more boat speed by running two pumps in
series as opposed to parallel because you could then have theoretically a
much smaller outlet diameter than you could with parallel pumps, and
therefore a higher speed of water being pumped....

you're probably feeling like you're banging your head against a wall here...
but im sure ill get what you're saying pretty soon... all the math that
you're giving me looks right, i think there may be just some basic concept
that im misunderstanding?

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'.

The higher the velocity *at a given volumetric flow rate* the higher the
thrust. It's Newtons formula:

F = m x a

Where F = Force
m = mass (proportional to the volumetric flow rate)
a = acceleration (proportional to the velocity of the water leaving the
pump versus velocity entering the pump)

ok, wait i should have read this first and thought about it more.... so
there is a direct relationship between water velocity and boat speed, if you
can maintain the same volume of water flowing.... right? so the sweet spot
would be just before the pump starts to be slowed by the backpressure? now
this may be pure conjecture on all our behalfs, but assuming you could get
double the pressure (which you probably cant) at the same flow rate by
having pumps in series as opposed to parallel, and for the same current
draw, which boat do you think would go faster, the boat with double the flow
and half the speed, or the boat with double the speed and half the flow?
the total numbers add up the same right, but wouldnt the boat with higher
speed water jet go faster?

Shaun

snip Keith Hughes