It's been a while (how long ago was 1973?) since I studied these matters, but it
has to do with the kinematic viscosity of the fluid in which the turbine
(propeller) operates. The ideal number of blades for an air-powered turbine is
actually less than 1, but we are limited by the fact that prop blades are only
available in integral numbers. Water requires a prop with a greater solidity,
hence ships with larger numbers of blades. As air velocities approach
supersonic, solidity increases, so turbofan and high-speed turboprop devices
have a larger number of blades.