I asked the same question, as we expect to have a fairly high capacity fire
pump aboard. Turns out that aside from Glenn's and Brian's explanation
(which are correct, you want low speed, very high volume for a thruster and
a fire pump does the reverse) that a "standard" hydraulic bow thruster is
off the shelf and relatively cheap (to the extent that anything in our world
is cheap), particularly since we have the prime mover and want hydraulics
aboard anyway for windlass, docking capstans, crane, and get home chain
drive (that's a bonus which was cheap).

Also, one of the advantages of the bow thruster is that it eliminates all
the single point of failure issues in the steering system -- the bow
thruster is proportional electric controlled, and can be run, if necessary
from the autopilot, continuously, at sea. That eliminates the tiller head,
tiller arm, two heavy blocks and tackle, and the two hearties to heave on
them if the steering failed. And, of course, even that doesn't save you if
the rudder itself breaks or jams.

It is possible to buy venturi jet thrusters that might work from the fire
pump, but they won't solve the steering issue, and they are generally for
larger vessels.


--
Jim Woodward
www.mvFintry.com


..
"Ron Thornton" wrote in message
...
Jim Woodward's comment on hydraulic bow thrusters resurrected a thought
I had a long time ago and never followed up on. If you are going to pump
a fluid, why not pump seawater to nozzles in the hull for thrust.

Regards, Ron