Two blade props are nearly as efficient as a 3 blade
. What you gain in performance with a three blade will be eaten by loss of speed under sail in light to moderate conditions. The three blade props are smoother.
A properly pitched prop, either two or three blade won't make a big difference in available power.
Prop pitch and diameter are determined by available space for the prop, transmission
reduction gear, and horsepower/torque curve of the engine. Diameter is limited by space and how fast the prop is spun. A large diameter prop cannot be spun as fast as a smaller prop because the water passing the tips starts to cavitate above a certain rotational speed. Smaller diameter props tip speed is lower for the same rpm so can be spun faster with a higher pitch to make up for smaller diameter. Most diesels have a fairly hefty reduction gear so they can swing a larger diameter prop because they develop more torque at lower rpm than gasoline engines. The old Atomic 4 had less torque, direct drive or low ratio gear reduction and made max hp at a higher rpm so tended to have smaller diameter props. Assume that's why the PO went with a lower ratio transmission
(less reduction/higher shaft speed) and stayed with a fairly small diameter propellor when they converted to diesel