I have seen some evidence of water at lowest exhaust port.
No pinholes in elbow, mains water pressure tested between fresh, raw and exhaust manifold sides for leaks
Apparently no zincs as raw water
side of exchanger is all Bronz - its a beauty when pulled apart!
told me Beneteau
blame yanmar and yanmar blame beneteau installation
meets yanmar specs, just.exhaust pipe run is very long and swan neck is just close enough to engine..wet exhaust rise to engine is marginal but just in spec..
also told me it's caused by seawater forced into exhaust manifold from a big following sea/waves.
I raised my swan neck hoping to fix the issue.. But checked yesterday and I have more corrosion than last time I checked..
I now have a pipe + manual bildge pump from the vetus wet exhaust on the drain plug
to the bildge to drain wet exhaust when sea is rough .
An aussy mate just got his successfully welded up.
Emailed yanmar but not expecting a reply - i will try macs.
It looks(to me) like acid from the sulphur? In the exhaust gasses and sea water mixing corroded the aluminum