Excellent, someone that actually understands exhaust bypass. Human nature leads many an yacht engineer or boat owner to come aboard or purchase a new to them boat, see a valve that is neither fully open or closed, and feel it necessary to "fix" it by either fully opening or closing.
On some boats to eliminate that compulsion after "dialing in" the valve setting using a back pressure gauge the valve handle is removed.
Originally Posted by Ski in NC
Common way to do wet exhaust. Often done when exhaust backpressure is high due to too much water in the muffler. So some is diverted overboard. Only about 25% of engine cooling water is needed to cool the exhaust.
Might also reduce the whoosh-whoosh sound of water being expelled from the tailpipes.
But you have to actually throttle that bypass valve. If left full open, exhaust may get too hot and that is a problem you do not want to have. Some previous owner might have said "screw it" with regards to modulating that flow. Just closed it and was done with it.
If running engines at light load, back pressure is a non-issue.