For what it's worth, it would seem to me that as the dia. increases so will the area within it. At some point there has to be a breakover point where it is better to have another blade. As the dia. increases so is the area of air being acted upon. There will be a given amount of thrust at a given power setting so rather than increase dia. to an unreasonable size you go with more blades which will act on a given amount of air the same as a larger dia. prop. Now what the break over point is depends on the total load of the prop. Everything is a contributing factor from the max HP of the engine and its available torque to turn the prop. To the load being moved to the lift of the wing. Everything, so its not a simple answer but it can be figured out. Just not by me, im no math whizz however you can find the area of the prop and you can find the amount of thrust it will produce with your engine and you can figure your max weight. So with a little math that im not good at you can find the max dia. prop you can swing without killing your engine. When the thrust of a given prop dia. is no longer enough and you have HP and Torque to spare, you can add a blade and get more thrust when you cant add to your dia. and do the same thing. At that point the bennifit is greater than the added drag. That is the way I see it anyway......
