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Ive been a member for a couple years but dont think Ive ever posted. Plenty of good info here. I have a 1985 17 custom with a 1998 Johnson 90- 60 degree looper mounted at factory ht. I know mounting ht has been discussed many times. I have been going thru all 100 plus pages on the engine forum here and haven't found a thread that addresses my question. I want to raise it but hesitate when I consider the change in leverage on the transom. Has anyone had any issues or damage or stress cracks as a result? Thanks.
Logically, raising the engine will decrease the stress on the transom from power transferred to the water due to shorter lever arm. One would need to know the center of mass of the engine to determine stress change due to rough water and roads.
I see your point, but as I look at the change in surface area the mounting bracket makes contact with the transom it appears it would put the same stress on a smaller area. To me less surface area to spread the load would equal more wear and tear on the transom. Maybe my thinking is wrong.
I have a 1966 and a 1964 both 16'. I have been running the 1966 with the motor 3 holes up and the performance is amazing. Just raised the motor to the same height on the 1964 and will be testing soon I hope. It has a 70hp 2-stroke and is the boat I am most familiar with the performance of. The other boat has a 90 hp 2-stroke.
I just looked at the transom thinking about your question and see your point. While running in the water I can't imagine it would be worse, probably better because of reduced drag. On a trailer and a bumpy road I could see potential issue. A support bracket from the lower end to the transom would probably help, or it would not be hard to add 11/2" to the top of the transom with epoxy and wood.
I added woven roving to the inside of my splash well on the 1964. I was repowering 3 years ago and the Yamaha dealer refused to install the motor, as he said the transom was possibly weak. That was way before I raised the motor and in my opinion there was no issue with the transom, he was just looking at the fact that the washers inside the splash well were slightly indented into the gelcoat.
For the record I was very interested to see if the wood was damp at all when I drilled the new holes to raise the motor.It was all completely dry.
