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Well that' some interesting info Glenn, I just looked at the 13.25x15 and the 15x16 and the latter is obviously the more efficient of the two. On paper the smaller diameter prop should be giving you 3-mph more speed but it is apparently reaching a point where it is revving but not getting enough bite, and that is where the bigger diameter comes into play. I suspect sic people aboard will cause the 15x16 to drop some rpm, but CC designed these boats to take on a load and still perform. Will be fun finding out.
The 15x16 at 4100 rpm and the 15x17 at 3800 really help tune your best alternative. The stock prop was a (3-blade) 15x16 cupped, which is going to be just a bit over the 15x16, and 4100 seems to me to be a pretty good max rpm for that boat. People who go to the 4-blade props, by the way, are looking for good mid-range performance and not necessarily top speed, because it is rare if ever that a 4-blade will deliver better top end than a 3 blade. At some point it would be fun to try a 4-blade in the 14x15 range, but endeavors like that can be expensive.