 For compactness the ballast tanks were wrapped around the batteries, low down and sharing the flat surfaces of the battery tank.The Kingston valves linking the ballast tanks to the sea could be left open, a practice known as "riding the valves", and the water level in the tanks controlled solely by the vent and blowing air valves.The drawback was that the ballast tanks, open to sea pressure, had a flat surface to the crew compartment and were thus restricted in strength to the bursting strength of this flat plate.