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?

Ill have to have a beer or two before i try to get my head around the
numbers, but thanks for the information!

Shaun

"Doug J" wrote in message
ups.com...
The following was posted by Cliff on the psubs.org group. Personal
submarine and ROV builders consider bilge pumps frequently. They are
often used in ROV's because they are easy, but bilge pumps or jet
propulsion is rarely used in Subs because they are inefficient. If
you do go with a pump, be sure to place the discharge just above the
water line to increase the efficiently. --Doug www.submarineboat.com

Below is a derivation of thrust that can be developed from a axial
flow pump
in terms of volumetric flowrate.
The thrust due to accelerating fluid through a pump can be written as
F=M(V1-V0)
Where M is the mass flow rate, V0 is the free stream velocity upstream
of
the pump and V1 is the velocity exiting the pump.
But the mass flow rate M can be related to the volumetric flow rate Q
as
M=Density*Q
Substituting, the thrust in terms of volumetric flow rate is
F=Density*Q(V1-V0)
But the volumetric flow rate Q is related to velocity in the pump duct
ID as
Q=V1*A=V1*Pi*D^2/4
Where D is the duct ID.
Solving for V1, and substituting, the thrust can be written as
F=Density*Q(Q/(Pi*D^2)-V0)
For a thruster oriented approximately normal to the direction of flow,
the
inlet velocity can be assumed to be zero. The thrust then reduces to
F = 4*Density*Q^2/(Pi*D^2)
Or
F= 0.001766*(q/d)^2
for freshwater where,
F = Thrust, lbf
q = pump volumetric flow rate in gpm
d = pump outlet duct inside diameter in inches
As an example, a pump with a capacity of 200 gpm flowing through a 2"
duct
would develop 17.7 lbf of thrust.

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

"Doug J" wrote in message
ups.com...
On Feb 16, 2:43 pm, "Shaun Van Poecke"
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

im with you there doug, that would suit my budget and my temperament
perfectly. even used trolling motors on ebay in australia attract quite a
premium... Ive seen second hand 40lb motors going for up to AU$250! since
ill probably chop it anyway, buying new is not a big concern of mine, and
ill only want full speed, so drect wiring seems the way to go.

what is the general thought on the life of a trolling motor? are
replacement bushes readily available?

Shaun

Doug J wrote:

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.

Yep, the discharge stream has to displace water already behind the boat,
and that requires work (seen as higher backpressure at the discharge
nozzle, reducing the mass flow rate).

Where you could gain efficiency would be in having a directionally
adjustable discharge nozzle (primarily with a planing hull) so you could
optimize the discharge vector, for maximum thrust in the direction of
boat travel, for different bow angles. For e.g., as the bow rises, the
discharge angle, for a fixed nozzle, rotates downward. The thrust is
now a vector addition of the upward + forward thrusts, with the upward
thrust component being wasted energy. Seems that angling the stream
back to horizontal (and letting the hull do the lifting) would increase
the forward thrust.

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.

Details, details... :-)

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.

Sounds like the best plan to me.

Keith Hughes