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View Full Version : Prop Diameter Size vs Pitch



syncronized77
10-18-2010, 04:50 PM
Hey guys,

I'm trying to figure out this whole prop thing still….which has a higher effect... a larger diameter prop or higher pitch. For example….

45mm (1.7") diameter w/ 1.45 pitch = 2.48

VS

42mm (1.65") diameter prop w/ 1.6 pitch = 2.640

Which of these are faster, has more torque, has a higher amp draw?

teach
10-18-2010, 06:37 PM
Well I have to admit I have asked the same questions over and over. There is no real answer. In a perfect world the 642 would have higher top speeds with more amp draw. There are just to many things that kill theories though. Prop slip by itself jerks everything out of whack. I think the only way to tell is go play.

I wish I had a real answer for you.

jac4412
10-18-2010, 07:04 PM
I'll give it a shot with my understanding. The basic answer is it is all dependent on your system. To make it very basic you will notice a difference in a boat at high speeds. In a boat at lower speeds (30-40) the difference in the propellers there are typically minimal. A prop as you know is dependent on pitch (with lower pitch= more pull= need more power to go fast but taxing the engine at higher speeds, with higher pitch= less pull= more speed= less low end, need the boat to get on plane... and stay on plane). Also the diameter effects it with a lower diameter= greater speed= more rotations but less pull and vice versa with a larger prop. The first prop will have more torque and amp draw but again, I need to know your system, it also depends on how you drive it. I'm not sure if youve ever driven a speedboat but playing around with props is the half the battle when you want a good ski run. Trick is to get the optimal RPMS for your speed. If youre running a bit high it could be you need a bigger prop, or a smaller one.

But, back to your question again as teach said there are MANY factors that will go into this. Best way to work on a rc boat (or as I know and have learned) is to try several props... Look around at what people say is a good size for your boat and go from there. Check temperatures and notice speed/pull. I mean at our level a prop can be effected by so much a scratch. I've seen people with broken props pulling two wakeboarders and going 40 mph in a mastercraft... big difference even though they should be relative. Then again.. it would be nice to have a v8 in a MG...

jac4412
10-18-2010, 07:05 PM
Oh and the 2nd one SHOULD be faster... but again so many factors... just gotta try it out really!

syncronized77
10-18-2010, 07:17 PM
Yeah...I figured prop selection would involve more variables than just prop diameter and pitch. And I understand that trying out different props is really the best way to figure out what works with which application....I'm guess I'm just trying to get my head around what prop size and prop pitch actually effect in regards to speed, torque, and amp pull....

And from your informative post above, I've learned alot!! :bowdown: I had no idea that lower pitch props provided more torque! I honestly thought torque was mostly calculated by what motor you ran.

I'll probably have to reread your post a few more times though to figure it all out....all in all I was just wondering if its possible to simply pick between 2 different prop sizes by just looking at the specs of them...but I'm guessing that comes with more experience lol

jac4412
10-18-2010, 08:17 PM
Well again its all dependent on your system. Thats the key with small boats. And a correct prop. Here is a good explanation for full scale boat props:

Pitch is the forward movement of a propeller through one complete revolution measured in inches. Lowering prop pitch will increase acceleration and pulling power. A higher pitch prop will make a boat go faster as long as the engine has enough power to keep the rpms in the optimum range. If the engine doesn't produce enough power to run a higher pitch prop all performance suffers and engine damage can result.

So in our case, again the best way to find a prop is to check other peoples setups, as well as your own.... Decent temperatures and good speed usually mean that prop is ideal for your setup. Hot temp and great speed is bad.... Low speed but cool motor isn't what your looking for either. You'll figure it out!

Rumdog
10-18-2010, 09:57 PM
Here is my take, in general. Every hull needs a prop with a large enough diameter to push it efficiently. Too small equals cavitation, and slippage at speed. Too large dia. causes handling issues like prop walk/roll/torque whatever you want to call it. Too small a dia. for your motor, will not load it enough for it to make power. Too large a dia. will overload a motor, and cause large amp draw. Once you find the sweet spot with diameter, for your hull, and electronics, then adding pitch will increase your speed. Too much pitch on certain hulls can also cause cavitation, and too much lift in the transom. I always start out with 4 series props. either the x, or m. The m series, being de-tongued will let the motor spin it a little more easily. It will also decrease lift a bit. The difference in speed between a x445 to a x645 can be very significant. So can the difference in amp draw.

SlickZERO
07-21-2015, 08:28 PM
I have a stock Lucas Oil Cat except for a HW/Turnigy 120 A ESC. With a X442 it runs 45.2 ave mph. With a M545 it runs 51.2 but bogs in the corners and takes about 1/4 straight to get up to speed. So which is the "right" prop? For racing the X442. If for fun I want to race some guys straight down the lake then the M545.