Garth Mobey has kindly replied to this issue and I am adding to the forum on his behalf.

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Re: Diving procedures and trim:

I can only refer to my own experience, having served for four years on Daphne Submarines, which are based on the type VII. With the Daphne, ballast tank three is outside of the pressure hull. Inside number three and on either side of the pressure hull are the trim tanks, three to a side, looking like grossly oversized scuba tanks lying on their sides. Each set of three are inter connected, open to the sea from beneath, and there is a high-pressure air supply to the top from inside the boat. Trim is controlled by either admitting seawater to these tanks or blowing gently from the reserves of compressed air within the boat. These trim tanks, as some people call them, should more correctly be called buoyancy tanks, because they actually control the positive or negative buoyancy. The water in the fore-and-aft trim tanks on the Daphne are also controlled by blowing air into them, thus moving mass forward or aft, thus keeping the boat level. I should also point out that port and starboard trim, or "heel," can be controlled by changing the balance of water in the buoyancy tanks on either side of ballast tank #3.

The ballast tanks always hold close to the correct amount of water for proper control, and I can only imagine that this was the practice on the type VII. I have only ever read of the expression "flood negative" in reference to the US submarines, and I believe that this may be because their ballast tanks - all of them - are within the pressure hull. Correct me, please, if I am wrong. It is possible that the external bow and stern watertight compartments on the type VII were in fact the fore-and-aft trim tanks, and perhaps something has been lost or added in the translation of diagram text into English.

All of these tanks on the Daphne sub are extremely robust, the maximum operating depth of this particular design is 300 meters - which we did on a very regular basis with no problems at all, no strange noises, either.

As for diving quickly, this is a function of the volume of air which can be vented from the ballast tanks and the rate at which the air would be replaced by seawater from the open bottom of the tank, hence the very large vents on the Daphne. It was usual for us to be doing a little more than six knots before a dive, so the diving planes could also be used to gently point the boat downwards. In excercises, though, we could assume diving angles of up to thirty degrees. Usually these steep angles were assumed after the boat had reached about thirty metres so that the aft tanks could not break the surface and empty themselves, leaving the boat with it's screws helplessly thrashing air. I do not know whether these steep angles could be achieved with the type VII. I must also mention at this point that once the boat was dived, very little fiddling around with ballast is necessary, because the diving planes control attitude and depth, even at very slow speeds. Differences in the apparent specific gravity of the boat due to changes in water temperature and/or salinity can easily be compensated for by the control center Chief, who admits or blows water from the buoyancy tanks in increments until there is no need to use large diving plane angles to maintain the required depth.


As for the need to dive quickly to avoid aircraft, the situation simply does not arise, running the diesels underwater is no problem with the snorkel. I seem to recall that this was introduced in late 1943, correct me if I am wrong, but it certainly would have been a problem before the introduction of this equipment, as well as the various types of radar detectors also fitted later on in the conflict.

To answer your question about bilge pumps, on the Daphne they pump water directly out of the pressure hull.
I should imagine that this was the case with the type VII, too. Another way this may have been done, and this is pure speculation, would have been to fill internal tanks with bilge water and then blow the contents out with compressed air, these tanks obviously being strong enough to withstand the pressure required to achieve this.
As a matter of fact this is how sewage is dealt with on the Daphne, with all sorts of special safety and vent valves to prevent that most awkward of situations, a wet bum!

I hope this helps,

Garth