Here's the other side of the coin. In 1973, I bought a 1968 boat that had
Salsbury Hydrovacs installed by the original owner. I sold that boat in 1983, and
the mufflers were still in perfect condition.
I will add that they worked well beyond my expectations. It was hard to hear
the engines, even at idle at the berth, and never had exhaust smell near the
back of the boat when running. We operated on plane most of the time.
Perhaps the gent with the problem was using the wrong size unit.
Not sure, but I think the units go only to 225 hp. when used on a 4-cycle
diesel. I don't believe any are made for higher output engines.
JDL

I operated a 32' wooden powerboat for 20 years with a 225hp gas inboard and
one of these units. I loved the quietness and the barely audible burbling
exhaust at idle, but - to be fair - I never operated the boat without the
unit.

Keith