In the tug I put a box around the engine lined two-deep with horse stall mats, made a huge difference and they are very cheap. That "special soundproofing" stuff is mostly a shuck I think.I've been looking for something to cut that down a bit so you don't immediately go deaf although it's never going to be quiet.
I can't say it was quiet but not unpleasant. A tug should not be silent, after all. And the detroit noise, at lower decibels, is very industrial.
A generator that makes three-phase might be useful in the future, monsewer power ?