Hi Simon et al,
I hadn't considered a water jacket around the exhaust, makes sense on several levels, on the VIIs does the jacket finish at th end of the silencer or go all the way to the end of the exhaust?
There is definitely an issue with the water pressure on the exhaust as I said previously on the sub surface exhausts on the IXs (experimental) and the XXIs (production) they did away with the silencers all together. I assume that the exhaust would have to have a pressure differential valve in it otherwise the engines would flood, (no valve) or the manifold gaskets would blow, (manual valve). I am not saying that there wasn't a manual valve, but there would have been an automatic pressure differential valve as well.
regards
Jon