Sounds like it's the prop design if you fall off plane at that high speed. I use a 14 1/4 x 15, 4 blade bay pro 3 on my 18 with a 375* 150 Johnson. Fully loaded, I see 5500 rpm and 40 - 42 mph. I can plane around 12 mph. This prop is a stern lifting design.
I had a 14 1/2 x 17 viper that would achieve the same top end but would not plane below 20 mph. This prop is a bow lifting design.
Most stock aluminum props don't provide much stern lift. I would try a 13 x 15 stern lifting prop. Stock omc sst, mercury black max or yamaha cleaver props have that design.
__________________
" I'm the one thats got to die when its time for me to die; so let me live my life, the way I want to".
J. M. Hendrix
|