Re: prop for an 115 2stroke johnson?
To get more speed, you'll have to go stainless. I'd stick with that pitch, depending on the style and number of blades. If you go down to a 15 aluminum, you'll get more RPMs and about the same speed. If you go up to a 19 aluminum, you'll be lugging the motor to death. You kinda almost already are. Where is the antiventilation plate in relation to the bottom of the boat? Even is normal. Up a hole for speed, down a hole for dig.
Most aluminum props are generic for general-purpose duty. You need a specialty prop. That pretty much means stainless. IMHO, Merc has the best and biggest variety and you can get a hub kit to fit your motor. I'd consider a hole up on the transom with a Laser prop. Only question is 16 or 17. Not sure if both are available. Unfortunately there's only so much you can do for a riveted aluminum hull.