let me start from the beginning. I bought this boat a couple weeks ago. It is a 15.9 Blazer bass boat 1984 model. the motor is a 70hp tohatsu ..model M70A2. I'm not sure what year model it is, thats one of my questions. it is a 2cyl. engine. it seems to run great. there is a manual jack plate on the boat that sits the engine back about 5-6 inches from the transom. I loaded up the kids today and went to the lake. 17 gallon fuel cell is 90% full (in the rear) two batteries in the rear. theres me, my wife, and 3 small kids the boat. the prop is a 11.75x17 SS. my RPMs were around 4200 at WOT. I trailered the boat so i could adjust the height at the Jack plate. The cav. plate "was" about 4-5 inches above the bottom of the boat. I moved the motor down as far as i could and that put the cav. plate at about 1 inch above the bottom of the boat. This slowed the boat down, without any real change in RPMs. I could gain RPMs by trimming the motor up, but started porpuising. I put the extra prop on which is a 12x15 aluminum. It kept the bow real low, and when i tried to trim up, to raise RPMs, it would ventalate. by the way, it has a whale tail on it, and i took them off to see it made a difference, but not anything notcable. another thing is that the hole shot is very slow, and when i am trying to get on plane, the motor does this weird thing where it feels like the prop is hitting something, but that goes away when the boat planes. any suggestions? I think my pitch is wrong. According to Tohatsu, that model of engine should be at 5500-5800 RPMs at WOT.