In the Overseas patrol Submarines, practically all of the fuel was carried in external tanks. The tanks were riveted and tested to 20 lb/in, but they leaked to such an extent that they could be considered a failure. Various reasons, such as defective plating, manhole covers and bad equalizing arrangements, were blamed for the leaks, and many fruitless attempts were made to overcome the faults. In fact the trouble throughout had been caused by the rivets and, in an era when welded ship construction was in its infancy, the externals were ultimately rebuilt in welded construction.
Of the 19 Overseas Patrol Submarines built, 18 served in the Second World War; 12 of which were lost on active service. The last submarine of this type to be taken out of service was the Vickers-built Otus, in April 1946.
