Diameter size is Different

[Tony T.]

I have a 1990 Johnson model # J15EESR 15 hp that came with a 9 1/2 x 10 RH Alum prop originally new. I was wanting to replace original prop. It seems to be now days this prop new is not used or not really available, and the 9 1/4 x 10 RH Alum has took its place I guess. So I know we are talking 1/4 inch dia difference so how will this effect performance of my outboard if any at all ?

Thanks for any info

Tony

 

[jimmbo]

I doubt any difference will be noticed. Leave the worrying to the propeller engineers

 

[Tony T.]

I doubt any difference will be noticed. Leave the worrying to the propeller engineers

Yeah I understand the engineers probably do have things figured out, but I'm the type that would like to know why ? I mean 1/4 inch difference is all we are talking about, wonder what they found out to reduce the diameter 1/4, thought someone might know ?

Tony

 

[jimmbo]

There are lots of factors in blade design, not just dia and pitch, Blade area, shape, thickness, rake and cup, all factor in. the blade is only 1/8" shorter. Run the 9 1/2 across a couple of sandbars, it too will be a 9 1/4 very quickly

 

[Texasmark]

If you will take a certain prop series and go through changes in pitch you will notice the variance in diameter. Seems that the blade area remains relatively constant and as "rake' increases, diameter drops. As the other guys have stated, part of the prop design. Pitch is your main concern. On props from different mfgrs, yes, each has their own designs which may be different.

 

[jimmbo]

If you will take a certain prop series and go through changes in pitch you will notice the variance in diameter. Seems that the blade area remains relatively constant

In a lot of prop lines, as pitch decreases, diameter goes up and usually the prop blades get larger in area.

 

[Texasmark]

In a lot of prop lines, as pitch decreases, diameter goes up and usually the prop blades get larger in area.

I haven't measured them but yes, when you get them all "squished" out they do look fatter especially with zero rake. In my last 2 years in high school, I worked tug boats in the summer. One day they put our boat in dry dock. Talk about a huge, fat bladed, shallow pitched prop. Boat probably did 10 knows wide open, no load, but boy could she push.