Hi I have a 9.8 hp Mercury on my '74 Venture-25 which moves it nicely, up to a point. However after about half-throttle additional power makes little difference. The boat is capable of greater speeds sailing. I suspect that the propeller is cavitating and the bubbles compromise effective coupling of power to thrust. Does anybody know what pitch is optimum for this setup so the prop doesn't stall? .......Bob at griffinaero@yahoo.com