I definitely agree that the boat is overpropped and you could be causing
significant damage due to detonation from overloading the engine. Playing with
the secondary opening adjustment also sounds like trouble just waiting to
happen due to possibly running the motor too lean. You need to speak to an
experienced propellor shop about your options. Also, a convertion to an efi
setup although costily might help you burn less fuel at that higher RPMS that
you apparently need to achieve a desireable cruising speed. You would also
reduce engine noise from the secondaries opening with the efi setup.
Jay