Need to understand the diameter thing with props

[Bull54]

I have a 1985 35hp mercury, it has a 10 3/8 X 13 prop on it in ALUM. Blades have some bad nicks in them and I am going to replace them. I have a 13 spline prop.

They list a 10 X 13, 10 3/8 X 13, and also a 10 1/2 X 13".


Between the 10-10.5 and the 10.38 what does that change form the way I am running now say. Is it more hole shot?, more top end?, neither?

From my understanding all 3 props will fit my motor, just depends on which one I want to run, but this diameter thing is hard for me to understand. I mean I know it is the size etc, but WHAT does it actually DO?

Would appreciate it if one of you guru's could put this in English for me

 

[Dhadley]

Re: Need to understand the diameter thing with props

Diameter is generally a function of design. Generally speaking the more diameter a prop has should mean more blade area. IF the blade geometry is the same between the props you're comparing. Generally speaking the bigger diameter should allow for a higher X dimension (engine mounting) which should mean less gearcase drag. None of this is written in stone.

It's not so much diameter that counts but what the builder does with the diameter or blade area. It's very possible one prop at 10" diameter can run higher than another manufacturers 10.25" prop for the same motor.

 

[Bull54]

Re: Need to understand the diameter thing with props

Thanks for the reply. Any idea how we are supposed to figure all that out? Is there a chart etc for this or is it just trial and error and a lot of expense going through them?

 

[Silvertip]

Re: Need to understand the diameter thing with props

That little bit of diameter change will not affect you in the least. If you look at three props from the same manufacturer, same model, but with only a pitch change, a 15P prop will have a diameter of 12-1/4 inches. The same prop model in a 17 pitch will have a diameter of 11-3/4 inches. A 13P prop in that same series will have a diameter of 12-1/2. As was mentioned, diameter is a function of design. Just buy a prop that is intended for your engine and with the pitch you want. Diameter will take care of itself. There are prop charts. You looked at them. You very likely saw props from three different manufacturers, or three different models from the same manufacturers, or three different pitches from the same manufacturer. See where this goes? Every try on about six pair of shoes of the same size and realize four of the six don't fit worth squat. It's a function of design.

 

[Bull54]

Re: Need to understand the diameter thing with props

Thanks I think I got it now Appreciate the help!

 

[Texasmark]

Re: Need to understand the diameter thing with props

I agree with the function of design Silver mentioned. I have 3 props of the same type from the same mfgr in 17,21, and 24"P. In observing them side by side, it seems that the diameter reduces as the pitch increases, as does the rake and it's like taking the blade and laying it down along the hub as the pitch increases.

Now you are mentioning 3 props with the same pitch and different diameters. You say "they list" and I assume you are looking in one mfgr's catalog. I find that confusing, but I know nothing about it other than what you have told us......confusing when you ask yourself "what's the point"? When you take the area of the smallest and subtract it from the area of the largest and take a percentage of the difference vs the total area there isn't much change. Now if these were from 3 different mfgrs, or one was SS, the second bronze, and the third alum. then that would make sense.

My 2c,

Mark