1990 15' Fisher Marine Jon Boat

My 1990 15' Fisher Marine V hull with a 1990 15 HP Mercury Stick Steer will not stop plowing the water. on the Mercury there are only 1,2,3 Trim selecting holes. It has a 9" dia. X 9" pitch aluminum prop on. I can put the motor in the 3rd slot (which is the highest slot) and I can get a whapping 16 MPH out of it and it does not plow as much water. but, it will cavitate when you turn to the left. due to direction of prop rotation. It does well coming up out of the water also in 3rd slot. when I drop it down to the 2nd slot it starts plowing more water and slows down to 13 MPH. but, it does not cavitate any. need help deciding on what to do on changing prop size go up to a 10 , 11 or go down to a 8. I have no way of knowing RPM. but in 3rd slot motor does not seem to be to high in RPM range. Thanks

You need a tachometer to get the prop pitch right.
https://www.amazon.com/Penny-Tachom...UTF8&qid=1397616805&sr=8-4&keywords=tiny+tach

Forget speed until you get the prop right and then adjust height and trim to get the best speed without ventilation.

Also get a Dole-Fin or similar hydrofoil. If the engine is marginal (for the loaded weight of the boat) it will help get the hull on plane quicker.
https://www.basspro.com/Doelfin-Hydrofoil-Stabilizer/product/27337/

Prop selection without a tach is expensive and frustrating.

Thanks for the RPM info. I order the Tach today. will respond back as soon as I get some INFO.