"The 15 horse was at the top o
"The 15 horse was at the top of the horsepower range for that powerhead. That model has a 16 cu in motor, which they also used post-94 for the 9.9's (so you can bump a 9.9 up to 15, but there is nothing to bump the 15 up to).
You can make some minor gains, depending on what you are looking for as an outcome. A high flow prop hub, trim tabs, hydrofoil etc may change the handling (marginally to noticably) and a high energy coil swap will help it idle better and save a tiny bit of gas - but your "top speed" is limited.
If your rig is on the heavier side you may want to look at less pitch as compared with more.
I run a 4 blade 11 pitch (Solas Alcup-4) on my 91/15 horse, but that is on a 14' aluminum that has a dry weight of 220lbs - my whole rig, including me and the wife, gear/gas/battery/junk weighs in at under 1000 pounds. Even on that set-up I swap down to a 9 pitch, 3 blade (Quicksilver) during the hottest part of the summer when the motor is labouring to "breathe".
The prop pitch must be matched to the rig. A larger pitch on a heavy boat will do nothing but bog down the motor. If your rpms are not getting up where they should be (about 5750 on your motor), then you have too much prop.
So you are kinda limited in what you can do. As mentioned you could try a couple of things, but the only real change you would notice is "less money in your wallet". For a noticable performance increase you would need to move up to a 25 horsepower (or greater if the boat allows) and tack on the extra weight.
Your 15 should weigh in at about 75 pounds, the 20/25 models about 115lbs (short shaft)....."
