I swapped the prop a few times and I think 1 was a bit better but the boat never got enough speed
Going bigger/more pitch generally meant the engine couldn't rev as much
Going smaller made it rev more without really moving

I'm just thinking of maybe converting the boat to an outboard engine, where they are generally 21 sized
However it's more costly and don't know how the boat would react to so much more weight at the back

Also, at the same time, the boat I have is an inboard outdrive system, and it chews through the dogbone engagement at the back. Probably only a few hours use before I gotta change it over