i believe about 10% prop slip is most efficient, less slip would mean a larger prop with more surface drag as it rotates. while more slip results in lots of turbulence.
my guess is that with a variable pitch prop, and a multi speed transmission you could get some benefit; only in situations where one boat needs to perform various different tasks,say a tow boat that should get to a stranded boat in a hurry and then have lots of low speed power for pulling and maneuvering a larger boat back to the docks
if all you do is waterski or commute, just tune the prop and engine for hull shape and ideal speed and skip the losses from the trans altogether.
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