Got a 100 HP 2-Stroke Yamaha on my 17' Hourston, does about 40mph; 35ish with 4 guys and gear. While many warn of overpowering your boat, I don't think under-powering is good either - if it turns to **** out there you need the extra torque to get your bow up and outta those waves. Personally I wouldn't go with smaller than a 90HP on a boat that size...then again if my boat had come with a 60HP I probably would have run it like that till the motor kicked...anyways, 150 might be pretty taxing on the transom - has it ever been rebuilt?