You are dropping a LOT of pitch going from 19p props to 15p props.
just plug the numbers in to the prop calc . . . boat setup is pretty straight forward. Once you get the engine height set right and weight distributed in the boat. Finding the right props is just math + a small amount of voodoo.
Pitch is number of inches your boat moves forward per rotation of the prop.
19p = 19 inches
15p = 15 inches
You have some slip . . . usually less with 4 blade props or large diameter props, but generally around 10% for recreational boats.
Gear Ratio on the f115 = 2.15:1
Prop Calc
http://www.go-fast.com/Prop_Slip_Calculator.htm
What was the max RPM with the 19's?
