Many thanks for your constructive feedback on this problem. To be frank I was very sceptical of the ‘galvanic corrosion’ theory put forward by the engineer for precisely the reasons that you’ve both put forward (i e electrode ‘line of sight’ and both electrodes needing to be in a common electrolyte). However I thought that there would be nothing to lose by giving his suggestion a try, even though I think that chemical corrosion is a much more likely cause. Having said that, if you think about it there is the potential (another pun….) for some galvanic corrosion in the silencer box, with differing metals in the end cap assemblies in contact with a fairly corrosive cocktail of exhaust gas and seawater.
I’m surprised that the problem of end cap corrosion hasn’t been more common. I’ve had to replace the exhaust elbow on Another Fantasy because of corrosion, as well as dealing with the leaky silencer. There are two issues, the inconvenient one of leaks and water in the bilge but a more serious risk from carbon monoxide leaking into the interior cabins. When I first noticed the leaky silencer I installed a CO monitor which so far hasn’t gone off in anger, so presumably if there is any CO leakage it’s at very low concentration.
For ‘Amaretto’s’ benefit the engineer I used at Chichester Marina is Andy Parham, e-mail email@example.com, mobile no. 07834 764528. He’s very helpful.