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.