I have had some difficulty getting this sorted out in my head. Hopefully someone will be able to help!

a) On the type VII, ballast tank 3 appears to be inside the pressure hull. Would this tank be used as the 'flood negative' tank which would be blown after the boat had reached a suitable depth? If so, what was that depth? Am I right in assuming that failure to blow in good time would result in the loss of the vessel?

b) During surface operations, where diving to avoid aircraft was likely, would ballast tank 3 be fully filled to enable a quick dive?

c) At the stern of the type VII, beyond ballast tank 1 there was a 'watertight stern'. Obviously this was not part of the pressure hull and it was not a sealed unit. Was it open to the sea and if so where? What useful purpose did it serve? Similarly there was a 'bow watertight compartment' just beyond ballast tank 5.

d) How robust were the trim tanks? They were situated inside the pressure hull but how would letting water in and out be controlled and at what depth? To attempt to let water in or out at depth would, to my understanding, be potentuially dangerous. Would compressed air be used for the expelling of water?

e) Could the bilge pumps simply pump water directly out of the boat? What was the maximum depth they could be used?

Many thanks in anticipation.