As my reading and experience has indicated, the primary reason for a four-bladed prop versus a three bladed prop in a single engine installation is to smooth out or reduce the propeller vibration imparted to the hull. As an example, check out the four propeller installation on the Iowa class battleships. The outboard props are "in the breeze" clear of any keel or other hull structure ahead of them, and they are three-bladed. Contrarily, the inboard two propellers are immediately behind twin skegs of considerable size and are four-bladed. While wandering around in the dry dock under the Iowa while I was a crewmember, the chief engineer pointed out the differences in the props and informed us all that the reason was four blades behind the skegs had proven to reduce the vibration imparted to the hull. I think the diameters were the same for all four.
As a bit of an aside, for maneuvering purposes, we used engine orders as simply port or starboard engine ahead or astern to such and such a speed as that was how the engine order telegraph on the bridge was install. So ordering port engine ahead 1/3 and stbd engine astern 1/3 had both port props spinning at ahead 1/3 and both stbd props at astern 1/3. The only time this was ever not the case in my three years aboard was when I lost the outboard stbd engine while entering Guantanamo, Cuba and I had to conn at slow speed with a breeze coming from stbd. It was actually quite difficult to maintain course. Loosing an inboard engine would have not been as serious.
So now I own a boat with a single engine with a five-bladed prop. I guess Mainship figured that if a little bit was good, a whole lot was better, especially with a tunnel over the thing.
