I long for a bow thruster on my boat, but it would end up in a flooded
potable water tank, or alternatively, inside a large black water holding
tank.  It's annoying, but bow thrusters always end up down in just about the
most inaccessible spot on your boat (for the obvious reasons).

It occurs to me that I've never seen any bow thruster implementation other
than a transverse pipe cut through the bow with a propeller in the middle,
but the recent discussions about modifying trolling motors for bow or stern
thrusters got me to thinking about alternatives.

Why not run pipes from the engine room forward to the bow, and mount a
high-volume pump in the engine room where you can get to it for maintenance
and repair?  It seems like a reasonably good idea, but I've never seen it
done, so there must be a major problem with it, eh?  Would friction loss in
the pipes reduce the delivered horsepower too much?

It my case it would be just as easy to run permanent piping to the port and
stbd bow, and much easier to locate the motor in the engine room.

For most on the list I know the HP requirement would be less - in my case
I'm told I'd need just shy of 50 HP in a traditional bow thruster, with an
18" - 20" tube.  I'm wondering if I could use smaller piping and up the
pump's HP to deliver the same thrust through what is essentially a water jet
application?

Comments?

Jeff Patton
Perseverance
Seattle

Jeff Patton wrote, "Why not run pipes from the engine room forward to the
bow, and mount a high-volume pump in the engine room where you can get to it
for maintenance and repair?  It seems like a reasonably good idea, but I've
never seen it done, so there must be a major problem with it, eh?  Would
friction loss in
the pipes reduce the delivered horsepower too much?"

I researched this last summer when I posted about mounting a water cannon on
my boat for grins, giggles and security. In my case I needed a 25 HP pump to
run the water cannon and if I routed that flow to water jets I could get the
thrust of a 7 to 8 HP bow thruster. Water jets are inefficient thrusters
because you are achieving thrust by conservation of momentum. Thrust is
proportional to exhaust velocity, but mass times exhaust velocity squared is
the kinetic energy of the exhaust stream that can not be converted to useful
work. I showed that as you reduce pressure and increase flow rate, you
eventually achieve the same efficiency as a conventional bow thruster. At
this point the pump wants to looks like a propeller and the water pipe is
the same diameter as the bow thruster and all you have done is moved the
pump by adding a lot of pipe:-(

It occurred to me that a venturi nozzle, which mixes the jet exhaust with
the surrounding water could approach the efficiency of a bow thruster and
retain the benefits of a high pressure low flow rate system (small pipes).
The ideal arrangement for these jets would be back to back like a bow tie
inside a transverse tube (sounding like a bow thruster with a remote pump)
but you would still need the tube:-( At this point I got busy and left the
concept die the death it truly deserved.

Cheers;
Mike Schooley
Designing "Portager" a 36' Trailerable Trawler