By D. A. (Midwested) on Tuesday, December 3, 2013 - 01:26 am:
Grant,
On some ships that is true. Diesel engines operate
most efficiently in a narrow speed range. So it's
best to keep the RPM of the engines constant. But
to vary the speed of the ship, something has to
vary. It can be done by changing the speed of the
propeller by driving it with an electric motor.
The electricity comes from running an electric
generator from the diesel engine. Until more
recent developments in solid state electronics
this was not very cost effective.
A more conventional method used when many of this
class boat was manufactured was to use what's
called a variable pitch propeller. Different power
methods were used, some electric and some
hydraulic. Either way, the thrust of the propeller
was changed, not by changing the rotational speed
but instead by changing the angle of the propeller
blade. You can even cause the thrust to reverse
direction but the speed of the diesel engine
remains constant. The James Barker actually had a
gear box that accomplished the speed reversal but
did have a variable pitch prop.