What is performance difference between these two props

[oldrudedude]

I have two props:

- One is -----------> 12 3/4 X 23

- the other is -----> 13 3/4 X 15

- The 23 is a much more agressive pitch but the diameter is smaller.
How do I calculate top speed difference and/or hole shot performance?

The 13 3/4 X 15 is presently on my 16.5 deep V bowrider with early 70's 100hp Johnson.

The 12 3/4 X 23 is from a blown 140hp Johnson.

Both are aluminium three blade props.

Gearbox appears 2:1 ratio.

I saw an online calculator but it doesn't mention prop diameter. Why not?

OK, I know someone is going to ask: What is present RPM at WOT? Well, I don't know. That is why I installed the tach today. Haven't had a chance to try it yet. The engine was originally on an early 70's 17.5 Crestliner Muskie (my friend called it Marg the Barge). Perhaps that is why the motor has such a low pitch prop. The motor has good hole shot on the present boat but I am getting low to mid thirties for top speed. I am wundering if the 12.75 X 23 is too much prop for that old motor.

If I swap my two props what differences would I expect to see?

 

[guy74]

Re: What is performance difference between these two props

that is a big jump from 15pitch to 23pitch, motor might not even get it to plane with the 23. At the least the 23 will be lugging the motor really bad, unless you are already very "under-propped" with the 15.

 

[oldrudedude]

Re: What is performance difference between these two props

I have the factory service manual for the motor. The manual lists the 12.75 X 23 as one of alternative props, but that is near the upper limit. The motor was originally supplied from the factory with 13.25 X 17.

 

[guy74]

Re: What is performance difference between these two props

The 12 3/4 X 23 would be right at home on the motor, on a very light highspeed hull. Prop pitch is always a compromise, and a balance between horse power and drag. I have ran props with pitches from 13 to 19 on my 60hp Johnson, the difference in top speed has only been 28mph-6200rpm on the 14 X 13, 34mph-5100rpm with the 12 3/4 X 19. I find that as an overall good prop for my setup is the 13 3/4 X15, it gives me 32mph at 5800rpm, and a decent holeshot. I would guess that on your setup, you will be happiest with a 15, or 17 pitch prop for a heavy deep V, depends on the useage. Lower pitch for watersports, where you need a great holeshot, higher pitch on a cruiser that you want to keep the rpms and noise down on, just as long as it doesn't lug the motor.

 

[oldrudedude]

Re: What is performance difference between these two props

The specified full throttle rpm for this motor is 4500 - 5500. I guess lugging would be below 4500. I am more of a cruiser than a water sport dude although I did buy a set of water skiis last year.

Guess I will try both props with my new tach and see what I get. Thanks for the input.

 

[guy74]

Re: What is performance difference between these two props

Yep, it doesn't do any harm to try them both out, and doesn't cost anything to try. Please post your results so we all can learn from you experience.