The fixed pitch
propellor is most efficient at one speed, its design speed and design rpm
. At all other speeds it is less effcient, but can still be more effcient than a reversible, as reversing
a propellor will always be inefficient, unless the
propellor is designed to be as efficient forward
as backwards, which in turn means the blades have no twist, thus are much less effcient than
a good fixed pitch
propellor, when going forward.
The solution is of course to have vertical propellor shafts and 90 degree gearboxes,
to make your inboard act like a small outboard
without a reverse :-)!