You are under propped.
According to my prop program the ideal fixed three blade prop would be a 19.5" X 10.8", and depending on the amount of area available to swing a wheel
, you could be using something like a 19x 11.5 x3 or an 18 X 12 X 3 with just a slight bit of efficiency loss because the blade area is not as great.
Anything below 18 inch diameter just doesn't have enough blade area to get the power into the water
, and the prop will be cavitating when pushed to higher RPM.
Theoretically, the prop should be large enough prevent the engine from revving to more than its maximum rated RPM, and this means that if the propeller
were clean during the sea trial, the engine should have been able to turn more than 2800. Perhaps as much as 3000 or even higher.
Check the amount of tip clearance (distance between the blade tip and hull) that you have with the 15 inch prop and calculate how much diameter you can swing while still maintaining a tip clearance of about 15% of the prop diameter. Go for the largest diameter you can swing and adjust pitch
accordingly. ( but not over 20" diameter"