What affect does prop diameter have? I know a bigger diameter will lower rpm but does it/can it make the prop more efficient?

Yes it can make it more efficiant. More diameter can be great if the boat can handle it.

And what determines if a boat can handle it? Are there handling characteristics as well?

The reason I ask is my Viper 23 is still the best prop I have tried but I running more rpms than I would like to(52-5300)

I want to try a Mirage + but I don't think my boat will pull a 25. But I noticed a M+ 23 is a 14.625" prop compared to my viper which is a 14.25". So, if the M+ will lower my rpm AND be more efficient it may work. Does that make sense?

Yes, the Mirage plus is a larger diameter so this will effect your RPM. Additioanlly stepping up to a 25P, from your 23P will decrease your RPM in itself, about 400 RPM without the diameter in the equation.

Julie

Julie, I don't think my boat will spool a 25. I am thinking about trying a 23 pitch M+. So, my question is both being 23s, how much rpm difference will I see between the M+(14.625") and the Viper(14.25)? Any difference in handling.

And the more important question, will the M+ be more efficient. Right now I am running about 11% slip, but I am not trimmed out as high as I would like to be because I am already at 52-5300 rpm. I was at 8% slip (before the motor swap) trimmed out properly.