According to the author on "How to Make Propellers", take the desired 2 blade length and multiply it by the constant .933 for a 3 blade length conversion, or .86 times the 2 blade length for a 4 blade conversion, and you will get the "same " performance from the reduced length/more blades. A 60 inch 2 blade, will be 55.98 inch 3 blade, or 56 inches; 4 blade will be 51.6 or 52 inches.
Multiple blades are quieter, and climb better with only a small increase in drag incurred. The big engine planes use multiple blades when ground clearance prohibits the use of a longer blade. The reasons for using a 3 or 4 bladed prop are: 1) increased ground clearance; 2) the need for longer blades to harness the hp, but increased blade length causes the ends to break the speed of sound; 3) inherently balanced by the multi blade setup/easier balanced, quieter operation. Colin Rainey brokerpilot9...@earthlink.net EarthLink Revolves Around You.
