diameter and rpm change

[cddetelich]

I am in the process of deciding what size prop to buy. I know if I decrease pitch I will pick up RPM, but most of the replacement props are 15.5 in diameter. How much does an additional half-inch in diameter affect RPM?

 

[Silvertip]

Re: diameter and rpm change

If the 15.5 inch prop is the same exact design, from the same manufacturer, same material, same number of blades then that 1/2 inch difference will make very little difference. The issue with diameter has more to do with prop design than just making the the blades bigger. Design of the prop, such as rake, can make the prop 1/2 inch larger yet the pitch can stay the same. The point here is that when dealing with differences in prop diameter, you are probably also dealing with a different prop design so 1/2 inch can make a very big difference. Difference in what is the question. In other words, we can't really tell you exactly what the outcome is. That's why prop selection is such a crap shoot.

 

[Dhadley]

Re: diameter and rpm change

Yep, it all depends on what they do with the extra 1/2" (1/4" on each blade). Generally speaking, very generally, with more diameter you can run a higher X dimension. Some props have more diameter to hold water better - like on a pontoon or an offshore boat in heavy following seas. They don't vent as easy but aren't intended to run a higher X dimension. It's more a useage issue.