Jeff Scott, an associate aerospace engineer of the non-profit group Aerospace Web Organization, state that the more blades, the greater the air-moving capability and efficiency of the propeller, A 5-bladed prop moves more air than a 3-blade prop, if both are spinning at the same rpm.
However, since energy cannot be created nor destroyed, the air-moving capability comes at a cost. The more blades, the greater the total air drag, since each blade produces its own drag. It takes more power to spin a 5-bladed propeller versus spinning a 3-bladed prop, according to self-employed aerospace engineer Dr. Martin Hepperle, who owns MH Aerotools Engineering in Braunschweig, Germany. The Ace Propeller Company points out that this holds true for boat propellers as well.
Common logic would tell you the faster a propeller spins, the greater the air movement. In reality, this is not the case. Propellers "top off" in terms of air-moving efficiency. Increasing the rpm above a certain point will not increase the air movement efficiency. This holds true for all propellers, regardless of the number of blades. Dr. Hepperle notes that if a prop exceeds its designed rpm range, the problem of cavitation sets in. Cavitation is a phenomenon in which the air or water separates from the blade surface, leaving vacuum pockets. When these pockets collapse, prop damage occurs by the force of the implosions. The Watts Up Marine Service states that prop speed and pitch determines the extent of cavitation. Pitch refers to the angle of the blades to the central hub.
In order to extract the maximum efficiency from a system, the prop and the engine have to be matched. First, engineers look at the engine horsepower. They design a prop based on the maximum torque of the engine. The overall goal is to move as much air as possible with the least amount of drag and slower prop speeds, to avoid cavitation. The greater the horsepower, the greater the amount of blades required. Scott points out that even a 5-bladed prop is insufficient for the Tupolev Tu-114 airplane, with its massive turboprop engines. Engineers extracted maximum air efficiency by designing two 4-bladed counter-rotating props, placed back-to-back. In effect, the airplane has 8-bladed props on every engine.