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Originally Posted by Sailor Steve
When you perform a crash (emergency) dive the tanks are full, and must be partially blown to achieve neutral bouyancy. In a normal dive, and when operating submerged, the tanks are only half (or so) full. Rising and sinking are controlled with the dive planes; this includes surfacing. Either when surfacing or shortly after the tanks are emptied, giving positive bouyancy. If done just before surfacing, this does require compressed air. If done after breaking surface, engine-powered blowers are used.
http://www.maritime.org/fleetsub/chap18.htm#18C |
Sailor Steve, with all due respect, your article appears to be on US Fleet Submarine technolgy.
One must understand that German U-boat technology, contruction and engineering was some what different than US technology. According to Jac P. Malmann Showell (a noted U-boat and Kriegsmarine historian) under normal conditions U-boats did not inject compressed air into their diving tanks to surface but rather sea water was pumped out of the diving tanks to compensate for the expanding air within the diving tanks as the boat's depth was reduced by props and hydroplanes. Once the conning tower and diesel motor air intakes were above the surface, the diesels were started and their exhaust gases were first vented into the diving tanks to blow out all remaining sea water thus raising the boat to a normal surfaced depth.
One reason this might have differed from US subs is that many of the U-boat diving, fuel and ballast tanks
were open to sea water and they had valves to allow for the release of air pressure within the tank and thus the flooding of the tank.