A Prop Question

[Rutcutter]

I know that most feel that if you increase or decrease the pitch of your prop you will see a change of 150-200 rpm per inch of change. What about prop diameter? is there a rule of thumb as to how much rpm will change if you increase or decrease the diameter of your prop? I am thinking primarily about outboard motors and especially 2 strokes.

[Scott1]

I know that most feel that if you increase or decrease the pitch of your prop you will see a change of 150-200 rpm per inch of change. What about prop diameter? is there a rule of thumb as to how much rpm will change if you increase or decrease the diameter of your prop? I am thinking primarily about outboard motors and especially 2 strokes.

I went from I think a 14.5 to a 16, same exact pitch. I lost 600 rpm! That really surprised me. I went from a Mercury Enertia to a Merc Eco

[Bryden24shp]

I know that most feel that if you increase or decrease the pitch of your prop you will see a change of 150-200 rpm per inch of change. What about prop diameter? is there a rule of thumb as to how much rpm will change if you increase or decrease the diameter of your prop? I am thinking primarily about outboard motors and especially 2 strokes.

I went from I think a 14.5 to a 16, same exact pitch. I lost 600 rpm! That really surprised me. I went from a Mercury Enertia to a Merc Eco

Yep, you are increasing the props surface area by a lot. Makes a huge difference. How's that beast running, Scott?

[rancherlee]

Diameter has very little to do with RPM change. Usually smaller diameter props have a higher performance / low blade area design with higher pitches which is really what causes RPM's to climb on a Heavy boat or pontoon. You might get away with a smaller diameter "speed prop" on a performance tritoon with a light load but as soon as a few people get on board the "Speed prop" would loose is ability to grip the water and slip excessively. This is why my DF140 is hard to prop, most props in the 17-18 pitch range I run are mostly 13-13.25", only a few 14" diameter props made for Midrange gear cases.

As for the 150-200rpm per pitch rule, that's out the window with the 11-17pitch props commonly use on pontoons. It's around 300-350rpm

[Scott1]

I know that most feel that if you increase or decrease the pitch of your prop you will see a change of 150-200 rpm per inch of change. What about prop diameter? is there a rule of thumb as to how much rpm will change if you increase or decrease the diameter of your prop? I am thinking primarily about outboard motors and especially 2 strokes.

I went from I think a 14.5 to a 16, same exact pitch. I lost 600 rpm! That really surprised me. I went from a Mercury Enertia to a Merc Eco

Yep, you are increasing the props surface area by a lot. Makes a huge difference. How's that beast running, Scott?

Ha, just the same. I am getting 53 out of her, was hoping for some more with the larger Eco prop I went back to my Enertia 19P and turn about 6300 rpm. The one good thing I noticed, I load my boat up a lot and I don't lose a lot of rpm like many of my friends. I can have 6 adults 2 dogs, coolers and the rest of the stuff on the boat and still run 47mph. I think that is respectable, I may be done chasing the speed now because the performance all around is really good.
How is that new boat of yours running?