Hi Steve,
Our boat has both wing tanks. The port one is for fuel and the starboard one for water. The isolating valve for the water tank is towards the aft end of the engine bay. The breather is a hose leading off from the top of the wing tank at the forward end which remains curled up inside the cockpit locker. When filling I uncurl the hose and lead it out of the locker so that the overflow water goes in the cockpit and ultimately the cockpit drain.
The main tank breather goes into the sink.
There is only one filler pipe which is on the starboard side. This feeds into the main tank. When it’s full the pressure in the pipe pushes the water up into into the wing tank.
On a couple of occasions during our last sailing season when the boat was rolling at anchor (not heavy rolling) there were loud percussive bangs emanating from the main tank. It was obviously an air bubble so the solution was to drain off a little water. This had never happened before and I have yet to discover why they happened this year. Maybe next year…
Peter