Difference between 2 and 3 blade propellers

[jlanders001]

So the way I understand it, 3 blade propellers are used for more acceleration/racing and 2 blade are used for best top speed? Do I have that right?

[Fluid]

It is far more complex than that. The fastest R/C boat in the world ran a three-bladed prop. More oval race-winning boats run 2-blades. A three-blade has more blade area for the same diameter, so you can run a smaller diameter 3-blade for the same amount of thrust. Smaller diameter means less prop torque reaction, sometimes less prop walk.



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[skater368]

Better acceleration, more bow lift

[rscheffer]

Better acceleration when the prop gets a grip you mean.

From standing start a 2 blade is much faster because the 3 blade cavitates much more .
Lift etc is al about the blade form.
Diferent props give a very different amount of lift.
Special hydro props are made to give more lift to get the back of the boat more out of the water usefull for riggers .

There is a lot to gain in using the right prop but finding one can take a while.
Alround the k props and octura props are very good.

[Fluid]

"Bow lift" is a full-scale boating term. We don't use it with R/C boats, we talk about the prop "lift" raising the transom. High-lift props are normally used with hydroplanes to keep the transom up. Low-lift props are more common and work better (usually) for cats and monos.

The number of blades is of much less importance than blade shape and size. The Octura "X" and "M" props are low-lift props which will give minimum cavitation off the beach and will often give the best acceleration. They are available in 2 and 3 bladed versions. When using "X" props with equal load (smaller diameter with 3-blades) on the same I see no difference in lift or cavitation between 2 and 3 blades.


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[jlanders001]

Great info. Interesting and makes sense.

So what would you guys say that the average diameter is for props used for a dual powered FE 36" cat?

Edit: Forgot to say that I'll be running 2 4092 motors as I'm sure that makes a difference, correct?

[Snowride]

I may not have a ton of experience in RC boats but in the real life size stuff I have over 40 years. 3 bladed props will give you a better hole shot and do not cavitate any more than a 2 blade. 3 bladed props will be smoother as they are naturally better balanced and smoothness increase with added blades therefor a 5 blade is the smoothest with great hole shot but hampers the top end. The least amount of edge cutting through the water (read friction) + the faster prop. That being said the fastest prop would be a 1 blade but balancing just won't happen so the fastest is a 2 blade.

[jcald2000]

Most full sized boats are not turning 30,000 rpm at the prop.

[madmikepags]

Hey Snowride, there is not a good correlation between full size and rc boats, most of our rc boats use surface drives and most real boats (except racing boats) use submerged drives. The power to weight ratios are so different that there isn't much similarity and rpms aren't even close, and RC boats when running properly have very very little hull and prop in the water, not to bash your experience but I also have over 35 years of powerboat experience (and very little has transferred) and am friends with a team formula driver and mechanic and they are befuddled with the way my RC boats run.

OK lets take an example between a x447/2 and a x447/3 the 3 blade will be faster than 2blade and the 3 will have better acceleration from a standstill and out of a corner. The 3 blade will draw more amps too, but a better comparison would be a 452/2 and a 447/3, The top speed will prob a little faster w the 452. The 447/3 will have better acceleration off a start and out of the corner, The 452/2 will still give you less amp draw, and be smoother and easier to drive, w/ 3blade prop you will always have 2 blades in the water w a 2 blade only 1 is always in the water. Asking the questions are good but there is nothing like going out there and trying the theories in real situations. With datalogging soeedos and new and cheap GPS units it is so much easier today to get information than it was just 5 years ago. And like Fluid said this is just simplifying it, there is so much more to props than diam and pitch and # of blades. Ask Ken haines about the 642 I loaned him for his N2 hydro, his boat never ran better or faster than w that prop, he actually offered me $500 for a $13 prop!!!! there was so much done to that prop I can't even explain it!!!

[jlanders001]

I really hope you took the $500.

[angrycat]

I really hope you took the $500.

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