just bought a 07' mercury 8hp 4 stroke 2.08:1 ratio 5000-6000 rpm range. put it on my 1969 feather craft 10'8" alluminum johnboat . i get 15.8 mph wot with a 7.5 pitch. 550lbs total weight. my question is what is the estimated rpm with 10% slip. or what speed would be achieved at 6000 rpm with...