Extract from my book "Type XXI U-Boat (Electroboot) The story of the world's first true submarine capable of operating primarily submerged. : Part One" available on Amazon.com


Preserving electrical current ‘amperes’ was precious and all unrequired lighting, electrical drives, ventilation fans, trim, drain pumps were switched off. The crew sweated it out in the darkened interior, as the unseen fingers of active sonars searched for the steel casing of their Unterseeboote. There were eight battery powered hand lamps inside the Type VII, mounted at fixed locations, with their power switches marked with rings of luminous paint.
Illuminating the crew stations, they cast deep shadows inside the darkened compartments. The Unterseeboote contained a maximum of thirty-six hours, of breathable air for its crew. After that carbon-dioxide poisoning would begin to take its toll on their clinical and mental physiology. More oxygen was consumed by the crew if it worked in an animated state, under duress. Those off duty were therefore sent off to recline in bunks to conserve oxygen. Each crew member had his own battery pocket lamp and the sleeping areas soon shone with tiny beacons, adding to the surrealism.