wrote:
Keith Said:
... Looking at the RULE site, their largest bilge pump is 8000gph,
or 133gpm. That calculates to 7.8 Lbf thrust, with a 31 amp draw at
12VDC. Comparing that to a Minn Kota Endura 30, with 30 Lbf thrust,
at 30A/12VDC gives a good comparison of the relative efficiencies.

THAT makes a lot of sense, (a 3 or 4 to 1 ratio) and gives us some
real-world numbers to think about... And the following implies that a
decent experiment could be done by using a maximum-outlet-diameter
smooth hose to the outlet, and then fitting different experimental
nozzles:

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.
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...

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.

Maybe I can try some of this out this Summer on Lake Champlain
(Vermont) . (Now I'm boatless :-( on the Med this year, but moving to
the shore of the South China Sea for the next 2 or 3 years where I
WILL Mess With Boats!).

Interesting discussion!

Regards, Terry King ...On The Mediterranean in Carthage


To repeat my earlier post, my 30 lb Endura at 30 amp/12 volt pushes my
16' skiff or 14' Hobie at about 3.5 mph. My 50 lb Endura at 42 amp/12
volt pushes the same skiff at 4 mph. The force required as hull speed is
approached (for a displacement hull) increases astronomically. 28 pounds
might snuggle a 25 ft hull up to the dock, but it wouldn't do much in
open water.

BS

Shaun Van Poecke wrote:
ok, Ive heard this idea brought up before and seen it poo poo'd, but can it
work to some extent?

Ive got a 14' cat that i store next to the beach in sydney harbour and i
like to get out for a sail when i can. Unlike most cats this size, it has
heaps of flotation - 3-4 adults and still not sinking anywhere near as much
as a hobie 14 with 2 POB. It has an outboard bracket on the back which was
on it from the factory, but i've destroyed 2 admittedly cheap and nasty
second hand outboards on it so far. I have a new honda 2hp 4 stroke on my
dinghy that i dont want to put on the cat because it would probably suffer a
similar fate.

What happens is that because your moving so fast, the outboard gets
completely drowned, as do the people sailing for that matter ;-) an
outboard might work on a lake or something, but sydney harbour gets choppy.
The thing is, this boat was designed for island hopping the whitsundays and
the manufacturer claims enough flotation for 4 people, plus camping gear, a
stove, and food and water for a week! I cant see how they get the outboard
to survive...

i do ok if i get the outboard started and there are no waves and i just
motor round on it. a 2 or 3 is plenty to move along. its rated for a 5 at
maximum. the trouble is, as soon as you sail if there's a bit of chop, that
motor is getting a drink - then you'll never get it started. I want a bit
of propulsion for getting into some of the finnicky bay areas rather than
spending a couple of hours tacking to death, 5 minutes of propulsion would
get me there. propulsion would also be nice for getting back in if the wind
dies, and finally as a backup just in case something breaks. i like to go
out when the wind is big, and ive already had a couple of incidents with
snapping a gooseneck fitting, and destoying a couple of jib cleats.

so here's what im thinking; a 1250GPH bilge pump in each hull, intake hoses
going into the water, outlets hooked up to hoses facing rear. these draw 5A
each and are cheap to buy. A 40-50AH sealed gel cell would give me a lot
more power than i need. I cant see myself using ot for longer than about 30
minutes to an hour at a time. Is this going to move me at all? at least
more than trying to paddle the boat by myself (which gets me almost
nowhere!) would i get any benefit from using smaller diameter hoses to
create a stronget 'jet', or am i just unnecessarily loading the pump for no
real gain? am i miles off in the GPH stakes.... do i need a lot more
pumping to get any effect?

im not looking for a speed boat here, just *movement* even 1 knot would be
something. benefits to this system are that its fully submersible,
completely unobtrusive, almost silent, would cost me about $100, starts
instantly when i need it.

thanks,
Shaun


Much bigger than what you all are talking about, but this is electric
propulsion done right:

http://www.asmomarine.com/2005/asmo_uk/00.shtml

There are a number of others selling electically propelled boats too.
Just google up electric boat

BS

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

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

Jets (be they axial like real waterjets or centrifugal like a bilge
pump) are more efficient than larger diameter propellers ONLY when the
boat is moving fast enough that the drag from propeller
strut/shaft/and rudder becomes a significant part of the drag values.

In the real world that works out to be around 25-30 knots. A bilge
pump will never beat a small trolling motor propeller at displacement
speeds.

I've seen Cal 20's pushed by a big trolling motor for a sailing
school. Works o.k. in calm water but in any sort of breeze it's not
enough thrust. The Cal 20 with a 3.5 HP outboard was a LOT faster and
would end up towing the ones with a trolling motor.

For a Thunderbird, a typical 5-8 HP long shaft outboard is the only
solution that makes sense.

Evan Gatehouse

"Evan Gatehouse2" wrote in message
...
Jets (be they axial like real waterjets or centrifugal like a bilge pump)
are more efficient than larger diameter propellers ONLY when the boat is
moving fast enough that the drag from propeller strut/shaft/and rudder
becomes a significant part of the drag values.

In the real world that works out to be around 25-30 knots. A bilge pump
will never beat a small trolling motor propeller at displacement speeds.

I've seen Cal 20's pushed by a big trolling motor for a sailing school.
Works o.k. in calm water but in any sort of breeze it's not enough thrust.
The Cal 20 with a 3.5 HP outboard was a LOT faster and would end up towing
the ones with a trolling motor.

For a Thunderbird, a typical 5-8 HP long shaft outboard is the only
solution that makes sense.

Evan Gatehouse

hi Evan,
there may have been some crossed wires here.... the bilge pump/trolling
motor solution was for a 14 foot beach cat.

Shaun

Keith Said:
OK, I'm confused. Are you talking about *just* a bow thruster operation?

**Yes, ONLY Bowthruster, maybe also piped to SternThruster ?? Just for
smooth docking/undocking or dead-slow movement in calm water.. I can
and do bring my 22' Inboard CuddyCabin to shore / rock points with the
canoe paddle. But having a fully-controllable couple of horsepower
should allow total maneuverability. The marine engine would be in
neutral, just running the pump...

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

Right! Apparently this is a significant factor in dead-slow
operations. There are huge (Kort Nozzles ?) on big tugboat propeller
installations. Interesting idea...

We've had some excellent information in several above posts. What I'm
getting from all this is:

- Inboard pumps to various outlet nozzles can be effective for
maneuvering and very slow speeds.

- Efficient forward-motion propulsion up to 3 or 4 knots is much more
efficient using external propeller (such as 'trolling motor' types).

My idea is a different one: I have conventional marine propulsion for
running underway, but I wish I could have good fine-control
maneuvering / docking / dead-slow sightseeing based on an inboard pump
driven from my regular marine engine. Especially with a conventional
fixed-propeller / rudder type boat that steers for c*** in reverse,
and is very difficult to turn in a short radius, this would be "Nice".

Regards, Terry King ...On The Mediterranean in Carthage

wrote:
Keith Said:


We've had some excellent information in several above posts. What I'm
getting from all this is:

- Inboard pumps to various outlet nozzles can be effective for
maneuvering and very slow speeds.

- Efficient forward-motion propulsion up to 3 or 4 knots is much more
efficient using external propeller (such as 'trolling motor' types).

I'd say that covers it.

My idea is a different one: I have conventional marine propulsion for
running underway, but I wish I could have good fine-control
maneuvering / docking / dead-slow sightseeing based on an inboard pump
driven from my regular marine engine. Especially with a conventional
fixed-propeller / rudder type boat that steers for c*** in reverse,
and is very difficult to turn in a short radius, this would be "Nice".

I certainly agree with that. Our Catalina 30 is in a very tight slip,
and we get a lot of prop-walk with our 3-blade prop. A couple of
thrusters would certainly be handy at times. Cheers,

Keith

On Sun, 11 Feb 2007 07:33:28 -0800, R Swarts
wrote:


Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

BS

It's always the maker's claimed thrust that raises questions in my
mind. They take the thrust reading stationary, as far as I know.

Anyway: if you maintained 30 lb thrust at 3.4 MPH
that would need 203 watts or more
i.e. 17 amps at 12 volts
to provide about 0.27 HP.

Brian Whatcott Altus OK

On Sun, 11 Feb 2007 09:53:07 GMT, "Shaun Van Poecke"
wrote:

....
thats a good question. here's one in return; how many hp do you think i
generate with a 3 foot paddle considering that i have to move rapidly from
one side of the boat across the tramp to ther other side otherwise i go
round in circles?

shaun


I'm late to the party. A paddler is probably putting less than 60
watts into it. Less than a tenth HP.

Brian Whatcott Altus OK