An
old rule of thumb with propeller pitch and engine speed: you get 450-RPM change with 2-inch pitch change.
The rule would suggest that if you reduced pitch to 15-pitch from 17-pitch the engine speed should increase about 450 RPM. If that obtains, you would then hit WOT engine speed of about 4900-RPM.
Your boat speed already seems a bit poky for a modern 50-HP engine on a 15-foot hull. The SSL must add a lot of weight. My old SPORT 15 with an ancient 1976 50-HP engine (rated at the power head, not at the propeller) ran about 34-MPH top speed.
With your initial data, and adding the GEAR RATIO (2.09:1), the propeller slip calculates to 23, which is too high. SLIP ought to be around 10 at best speed performance.
If you changed to a 15-pitch, if the engine accelerates to 4900, if the SLIP remains 23, then boat speed would (still) be 26-MPH. If the propeller SLIP could be worked down to about 12, then the boat could hit 30-MPH with the 15-pitch.
Drop to 13-pitch; if the engine accelerates to 5700 RPM, if SLIP is down to 10, then boat speed will be 30-MPH. That sounds like a reasonable goal.
The present propeller is not letting the engine accelerate into its power band. The typical four-stroke-power-cycle outboard engine needs to be able to accelerate to the very upper end of its recommended full-throttle speed range in order to produce its rated horsepower output. The 17-pitch is too much load for the 50-HP. Also, the 17-pitch is not turning at the propeller shaft speed it was designed for--it is turning slower--and that makes the SLIP increase. Your "wild...guess" did not produce good results.
All this means that going down to 13-pitch should:
--let the engine accelerate to the upper RPM range where it can make 50-HP
--let the propeller shaft speed turn where the propeller is going to get more efficient (less SLIP)
--and the two factors, more horsepower and more propeller efficiency, combine to produce more boat speed.
Also, in the smaller pitch ranges the pitch changes are usually in 1-inch pitch increments. I'd line up three test propellers in pitches of 15, 14, and 13 for sea trials. If not available, try a 14-pitch.
That was the pitch I recommended to you in my first reply.