Simon.
Group exhaustvalves.
Having received further pictures I have to revise my last theory slightly. The revised system drawing is shown below. I guess there is a small detail which may be a bit unclear, I don`t believe there is a connection at the crossing point, engine coolingwater pipe and the pipe to the drain- crossover selector cock. In fact on the photo the pipe is not shown, I am sure it is there but as a separate pipe going somewhere else. Otherwise I hope the sketch is self explaining.
I have again messed up your nice drawing in an attempt to show the complete arrangement. Most of the changes are based on photos and drawings. Otherwise the system is following basic requirements like keeping pipes, flanges, packing and stuffingboxes which are not pressureproof behind the main overboard seavalve. Further the coolingwater should go from low inlet to high outlet. The main coolingwater outlet from the engine is shown being a fairly large pipe, the full amount of coolingwater can not flow through the narrow coolingspaces of the transmission flange and end cover. They are cooled in series and the pipes are much smaller as shown, howewer the only connection not confirmed by documents is the small pipe branch off from the main cooling waterpipe ( or exhaustmanifold) to the transmission flange. I have indicated two possibilities on the drawing. The system is elaborate and many pipes are stacked on top and behind each other, this is not easy to draw. The collection of valve and pipeoutlets, drains and vents is really cramped in the aft bulkheadarea and would require another study I guess.
Tore