Prop size experimenting.
All, What I'm about to share deals with the Yanmar powered E31. I was way over propped (squatting under full power with soot build up aft of the side exhaust when we bought our E31, hull #55 fitted with a Yanmar 3GMF with Kanzaki 2.61:1 transmission. I couldn't even get close to the 3,700 RPM's, factory recommended top speed needed, as proof of proper pitch and diameter, etc. After three iterations including what I started with, I ended up with a 13 X 12 1/2 X1, 3-blade and was able to get 3,700 RPM's right on the nose. Mind you, that's not to say that these engine should ever be run continuously at that speed, it should be at least 80% or less of that. Another E31, hull #06, owner complained to me that his identical power plant and transmission to mine couldn't keep up with a mutual friends E31, hull #19 with the identical set up you have, a Yanmar 2GM20F, and his (hull #19) eventual prop numbers ended up at 13 X 12X 1 RH, 3 blade at my recommendation based upon my own experience. I should mention that he was then able to get 3,700 RPM from his engine. I don't recall what my hull #06 friend started out with but at my suggestion and against the recommendations of two separate prop shops local to him, I asked him to trust me and use my numbers. Again, he got 3,700 RPM's precisely spot on and could finally cruise with hull #19 and not lose ground. Add to that, the owner of E31, hull #44 essentially has the same engine, a Yanmar 3GM30F as I do and copied my numbers. I think he is happy with the result. So there you have it, a difference between four E31's of 1/2" in pitch on one of them and we're all zeroed in. Will that work for your longer boat, I just can't say because I'm not an expert at anything including prop sizes. Here's hoping that my experience will aid you in selecting a better pitch for your prop. Glyn Judson, E31 hull #55, Marina del Rey CA