Any prop, regardless of how many blades it has, should be pitched to allow the engine to reach full rated RPM. It is especially important on gas engines without governors that they do not over rev via props with too low of pitch. Too high of pitch will cause an engine not to reach full rpm and is called lugging and will shorten the engines life, faster the more it lugs. Diesels can handle under pitched props okay, they will run up against tje governor and turn no faster. But your wasting the engines available power doing so.
Two blades are less efficient that one, three are less efficient that two, and so on. Efficiency of a prop is based on distance traveled per revolution. 100% would cause a 12 inch pitch prop to move 12 inches per rev. A three blade will slightly decrease efficiency which will be directly attributable to fuel economy/range, but only if both props are of equal quality. A good three blade will outperform a cheap two blade.
Only reason for more blades are reduced vibration and higher thrust at lower rpm. On a small clunky 1 or 2 banger diesel anything that gives smoother running is usually quite noticeable. I believe Main Sail wrote an article on the subject that is quite technical and well worth the read. Some like the thump of a two blade, others will spend $$$$ to get rid of that thump.
