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