Simon
I should assume the mufflervalve would be in the next section of the exhaust outlet, which means the linkgage would lead to the lower part of the muffler valve casing matching the shaft turning the muffles flapvalve against the exhaust outlet, thus having the waterpressure behind the valvedisc. See my rough sketch of the group exhaustvalve. As to the pressurehull passing , I am not sure if the group exhaust housing goes Right through to the outboard exhaust pipebend or if it stops at the pressure hull flange. In the latter case a coolingwater pipe has to bridge the flange. A typical hullflange has a steel ring thicker than the pressure hull welded to to same. On this flange the are the flange boltholes not drilled Right through. Below is a sketch showing the system. In the case of the group exhaust doublewall casing the flange had to be intergrated in the casing ( a complication) but the coolingwater bridgecould be avoided.
Tore
