On Feb 16, 2:43 pm, "Shaun Van Poecke"
wrote:
does having the outlet above the water line really increase the efficiency?
All the jet boats I''ve seen have the outlet below the waterline, but i
could be wrong... is this what all the RC boat builders do?

I was afraid someone would ask that. So I'll admit that I am going
on what I have been told by jet boat and jet ski people. If anyone
knows better about jet pumps, please correct me.

The efficiency of the jet pump is based on the mass of water it
discharges. Any back pressure and turbulence at the outlet only
reduces the velocity of the flow and therefor the rate of the flow.
The discharge ports are below the water line but only when the craft
is not yet up to speed. I think there is a benefit to having a higher
outlet pressure during start up or the "hole shot". Jet pumps also
depend on the design of their intake ports, because at top speed the
forward motion of the craft and the shape of the intake actually
assist in directing the water flow into the pump, much like an air
intake scoop on a dragster. You might milk another 2 or 3 oz of
thrust from that bilge pump if you put a scoop on it. Then again
that would really jack with the drag on a sail boat hull.

I'd go with an old used cheep trolling motor with a busted speed
controller. Clean it, replace the brushes and mount it on one of the
transoms with a hinge that lets it flip down into the water and then
steer with the rudders. Add a simple on/off switch and avoid the
variable speed controller or any other electronics that will just
present other points of potential failure.

Best Regards
Doug
www.submarineboat.com