wrote in message
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I know I have discussed this before but i finally got a chance to look
over hydraulic motors. The idea is to use a gasoline engine inboard
on a boat and transmit the power via hydraulics to a hydraulic motor
that is submerged. This way you can have great flexibility of tilt
and trim etc. When we discussed this before, somebody said that
hydraulic motors have poor efficiency but efficiency ratings (rotary
power into hydraulic power) is commonly 75% and some are as high as
95%. RPM at the output might sound like an issue but I see rpms
listed as high as 2800rpm. Such a drive would have a very simple
transmission.

A guy tried something like that on a fairly large (36-38') sailboat at the
marina I have my boat at.
It was on the hard and everytime I walked by it, I was trying to figure out
what the strange protrusions were on the hull.
It had one on the rear, but also two more port and starboard of the keel,
about 5 feet from the stern. They appeared to be capable of rotating, as
well as providing power to a center shaft of some type.

Curiosity finally got to me, so I asked the lead mechanic at the marina what
it was all about. Turns out the boat owner was a hydraulics engineer of
some kind and had designed a hydraulic propulsion system, driven off his
small diesel power plant. It included the two, rotatable, hydraulically
powered "rudder" motors, plus a larger, main propulsion motor.

Apparently, it worked, but was prone to all kinds of problems. The boat was
on the hard in order to remove the entire system and return it to it's
original configuration.

However .... the Grand Banks we had was equipped with a hydraulic motor that
served as a stern thruster. It was mounted on the transom, obviously below
the waterline, and was powered by a pump driven by the main engine, with
stainless hydraulic lines running from the pump to the motor. Worked fine.

I'd hate to spring a leak in one of the hydraulic lines though, especially
if the Environmental Police or USCG happened to be nearby.

Eisboch