My 2004 3.5 HP 2 stroke Tohatsu pushes my 12' Portaboat to 7-8 mph. My 6 HP 2 stroke Johnson (1975 model) pushes it to 11-12 mph. Both numbers are with 2 people one 200 lbs and the other 100 lbs, from a GPS. Are these numbers out of range? Is my 6 hp under performing? I get the impression that...