Why do large ships, submarines and other propellers have odd or even blades of 7 or 9?

Why do large ships, submarines and other propellers have odd or even blades of 7 or 9?

The rudder of modern submarine, whether cross rudder or X rudder, is in front of the propeller and is centrally symmetrical. This means that:

1) the rudder will disturb the flow field in front of the propeller, resulting in periodic changes in blade load, which will bring vibration and noise.

2) If the propeller is also centrally symmetrical, there will always be at least two blades of the above-mentioned vibration and noise synchronization, which enlarges the vibration and noise, is not conducive to the safety of the propeller structure and submarine stealth; if the propeller is not centrally symmetrical, there will be no synchronous blade, which is safe to the propeller structure. And the impact of submarine stealth is much smaller. As for surface ships, although there is no "rudder in front of the propeller" problem, when the draft is too shallow or bowing, part of the propeller blades will appear on the surface of the water. When the blade enters the air from the water, the load changes dramatically (and vice versa). If the propeller is symmetrical in the center, it is possible that two blades are just one outlet and one inlet, which will lead to superimposed vibration amplification and endanger the safety of the propeller structure. However, some surface ship propellers are very deep and do not need to worry about coming out of the water, so there are also centrosymmetric propellers (such as four-blade propeller, six-blade propeller).