I'm running a 90 hp Johnson 2 stroke on a heavy built 16'x48" flat bottom with 0.100 gage aluminum hull. It's got a 20" transom and is about 20" deep. It also has 1/8" thick aluminum floors and sides on the inside. The motor weighs around 300lbs, tilt and trim model. My uncle got this thing from a commercial fisherman in South Louisiana. I got it from him and got it running and fixed it up. It has one of those hyrofoil fins in the back which I don't really like the look of. The motor looks huge on there and it's gotten some comments but it's not illegal. Game and Fish guys seen and liked it. Anyhow, here's what I experience with it powered this way. When I first took it out the spray on the back was rediculos, but it performed well with the boat. However, if you trimmed it up too much it started to porpoise like crazy. Runs well trimmed slightly up. I determined that a lot of the spray was coming from water slamming into the hyrofoil, so decided to try it without it. Went out on the MS river with it and boy did I get an eye opening experience. It would not run over half throttle without porpoising like crazy in any position. I promptly went to the bank and put it back on. The cavitation plate was running about even with the bottom edge of the boat, maybe slightly lower. My experience has been that slightly above is best. So before the next outing, I decided to raise it all the way up and leave the hydrofoil on which put the cavitation plate 2 inches or so above the edge of the boat. That completely eliminated the spray. One thing I immediately noticed was that if I got down on it when coming out of the hole it would cavitate a little and would slide in turns. I then dropped it one bolt hole and tried it again. That was the ticket. Hardly any spray and good performance. The things I learned were that you can't run the boat without a hydrofoil and that the transom sits down deep enough in the water due to weight that you can run it higher that normal. The water angles up after exiting the bottom of the boat enough to keep the lower unit deep enough to run well higher on the boat. Not sure what your setup looks like, but I would be tempted to use a hydrofoil and raise the motor up. The hydrofoil helps stabilize the bow and allows you to run the motor higher while keeping the air out of the prop. I'll see if I can post a picture of it.
No was sharing my experience and offering advice. Also, my boat has no horsepower rating from the factory since it's homemade and when you use the formula it computes to 90 hp. Horsepower isn't directly related to dimensions alone. This was built as a work boat for moving large nets around where horsepower was needed. Just needed some tweaking to perform better which is what the OP can do to use what he has.
