Simon
Head tank.
The head tank is not a storage tank but act like a head buffer where the cooling water from the outboard exhaustsystem enters the fuelcompensating system. I would guess a capacity of something between 25 max. 75 liters, it is situated in the old tower casing. It is in direct contact with the sea, while submerged, via a swan neck pipe on the top. It should give a head at the day/setlingtank up under the pressure hull in the engine room of about 2-3 meter.
The filling of the compensating system is done by the ME cooling waterpumps.
Tore