As Colin impliies, generator noise is often much more critical than main engine, as generators are often run when stationary.
There is little doubt that wet exhaust is quietest - our Fisher panda came with wet exhaust and was almost imperceptible when running.
However, unlike our trouble-free keel cooled DAF615 the system was regularly shut down by FP's (perhaps hypersensitive) controls when its raw water system ingested weed in the often less than pristine waterways we travel. Problem was, this, as intended, prevented damage to the plastic parts usually included in wet exhausts downstream of the water injection point.
Ùnfortunately, keel cooling is normally considered incompatible with wet exhaust.so last year whilst out of the water at SFR we had the FP converted to keel cooling (Blokland box) and dry exhaust.
The dry exhaust has been a problem since. The muffler selected by the shipyard is only just adequate on board, where sound is noticeable but bearable. SFR retained the original exhaust outlet, discharging horizontally about 5ocm above water on the curve at the front of our tjalk.
When moored against a hard surface unacceptable noise is reflected back, and the sound discharged forward and to port is unacceptable, the exhaust acting like a megaphone and broadcasting across the water sufficiently to cause passers by to wince, and if we are moored, neighbours to complain (especially in Holland where "quiet enjoyment is treasured and the period between offence and direct action complaint measurable only in nanoseconds).
I will try Colin's muffler this winter, probably with a vertical exhaust outlet.
If this does not do the trick I think my only solution is to revert to wet exhaust, but using a dedicated pump and generous filter(s) for the water source (I want to retain the keel cooling).
Charles Fitzhardinge
De Vrouwe Anna Maria