Increasing transom height on jonboat?

Question

I got a jonboat, and it's got a transom for a short shaft, but I got a long shaft 25hp yamaha outboard. I'm pretty hevy, i put lots of fuel in the back (about 12 gallons) and the four stroke engine is also heavy. The problem I am having is that the boat is really heavy in the back and increased weight is affecting the depth my boat in the back. Previously I evened the height of the cavitation plate with the bottom of the boat, however, I was having problems because when my boat went to plane my cavitation plate was still in the water ( could not see it on the surface). Then I increased the height of the cav. plate (motor) to be four inches above the bottom of the boat, about half an inch of the prop was above the bottom of the boat. Everything worked fine, no circulation problems, and the cav. plate was on the surface of the waterwhen I went to plane.. When I took my motor into service the tech told me that the motor is too high.

Should I decrease height eventhough its working fine

Answer 1

Yes the cavitation plate should be even with the bottom of the hull. no more then an inch in either direction up or down.

Answer 2

If your prop is above the bottom of the boat it will also cause more cavitation and cause premature corrosive problems. To remidy your situation try moving your gas tanks, battery, etc. forward. You shouldn't really need 12 gal of gas for a 25hp motor unless you plan on running wide open all day.