Theres nothing wrong with a five blade prop on this set-up, and im not sure youll gain any speed with a three blade, at least not in the overall performance of the boat. I ran a 22p four blade on my 90 model, and loved it. The old Mariner Mag III was a stout engine in its day, but liked to turn around 6,000rpms to keep it from fouling plugs. The 22p four blade(Predator) would run about 6,000rs winter or summer, and run in the low 60s, When it was new ! As boats age they tend to lose performance, both from the engine losing pep, and the boat not being as solid as new, and maybe taking on some moisture. Bottom line, try whatever prop you chose to try, but id try a four blade with the proper pitch and let her eat ! D.