hi! how do you compare the performance of a 2 blade to 3 blade and 4 blade prop? and what prop (number of blades) perform better for a slow and draggy craft? i will be using this in an airboat, but airboaters usually use only 2 blade props. it will be for a 25cc homelite with a 46" hull, so i want to know what kind of prop will be appropriate for a big and heavy craft. more pitch, less pitch? more blades or less blades? i know it's a bit off topic here in this forum but maybe you guys already run using these prop which boaters don't use much. just want opinion.
thank you very much.

Hmmmmmmmmm (scratching head) Are we talking about props for airplane or boats?

Steve

props for planes of course. that's what you will use in an airboat.
although these boats will not go as fast as the planes.

The GENERAL rule of thumb is: When going from a 2 blade prop to a 3 blade prop, either go down 1" in diameter and keep the pitch the same (for instance, go from an 11x6 2 blade prop to a 10x6 3 blade prop ) OR keep the diameter the same and go down 1" on the pitch (for instance gro from an 11x6 two blade prop to a 11x5 3 blade prop ).

Your choices, to a great extent, will be determined by what size 2 blade you are now using and what sizes/pitch are available in a 3 blade prop. A lower pitch will provide more thrust (get you "out of the hole" faster and provide a slightly faster engine response time ), but knock down the top end, while a higher pitch will give you more top end at the expense of your "hole shot".

In any case, be advised that a 3 blade prop is substantially more expensive than a 2 blade prop.

Hope this is of some help.

ernest2 (& Campy), in going to a 3B from a 2B, Campy is correct about reducing the diameter 1" for the same pitch, but if you keep the same diameter, the pitch should be reduced by 2" (this is for props in the 10" - 13 " dia range).

And one other thing that hasn't been mentioned; three bladed props are less efficient than two blades, and four bladed are less efficient still.

I'd guess, that for an airboat you'd want something with a lower pitch/higher diameter. An airboat is more likely to take advantage of the higher torque available than in the higher top speed, essentially because of the much higher parasitic drag of water.

That's my guess. Anyone else?
Dennis-

thank you very much for all your answers. helps a lot. i guess 2 blades are really efficient compared to 3 or 4. now, can a stock homelite handle a 17x6 APC? or is it better to leave it to 16x8?

I am Building an airboat at the moment and looking to put a 4 blade prop on it
due to the fact i am running only a 14 inch prop
with a 39cc mac engine
i will get the steapest prop i can find jam it on there and see what it does
a airboat needs the max air flow it can get

If you were to put a tach. on it i think you would find they would turn pretty close to the same RPM. The 17-6 would pull better.