NOT TRYING TO ANSWER ANY QUESTIONS...just thinking out loud.
In aircraft props, the fewer the blade count, the better. Also, the larger the diameter, the better. Fewer blades means that each blade has cleaner air to work with, and longer blade span means higher (more efficient) blade speed - more thrust with less drag (better span loading and reduced induced drag). There is some sweet spot between blade speed versus blade area versus pitch.
In planes you increase blade count to reduce diameter, which reduces tip speed (supersonic tips are very noisy and inneficient) and the landing gear can only get so tall before the prop is at risk for clipping the ground. Unfortunately, no matter how cool them 4- or 5-bladed props look, they're giving away efficiency.
In our boats, there's probably some tradeoff between making diameter large for efficiency and the shaft angle you'd have to shoot out the bottom for clearance issues. Also, I've never studied or understood the inside story on pitch versus diameter in boats.
Submarine designers got all the budget in the world and they choose like seven narrow blades. Why so high a blade count? To keep tip speed down for noise issues, while keeping blade chord narrow and angle of attack light to prevent stalls (which make bubbles and noise.) Definitely not optimized for speed.
However, if we had a stainless milled sickle-bladed prop under our boat, we'd be REEEEEAL stylie:
(Message edited by toyotafreak on October 24, 2004)