Correct me if I am wrong but something as simple as running 25mph and pulling the throttle back to idle and just slowing down can add back pressure enough to cause damage.
I just blew out an exhaust gasket that was brand new couple weeks ago. If it wasn't from back pressure I don't know what could have caused it.
Jim is right, 2, 3"-4" exhaust outlets run into a 2 1/2" single outlet is going to be more restirctive.
Maybe enough pressure to blow an exhaust gasket. Probably not a motor.
I have been on one boat with FAE installed. And people always say at idle it would do nothing to get the fumes away from the back of the boat (as they would just bubble up) but I have experienced it, the fumes are better, its quieter, you can hear your music without some strange grumble.
I see many benefits to install this. I even want one.
When I was a child, I used to talk as a child, think as a child, reason as a child; when I became a man, I put aside childish things.
1 Corinthians Chapter 13 - 11