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The Germans took the idea of the ''Schnorchel'' (snorkel) from captured Dutch submarines, though they did not begin to implement it on their own boats until rather late in the war. The ''Schnorchel'' was a retractable pipe which supplied air to the diesel engines while submerged at [[periscope]] depth, allowing the boats to cruise and recharge their batteries while maintaining a degree of stealth. It was far from a perfect solution, however. There were problems with the device's valve sticking shut or closing as it dunked in rough weather; since the system used the entire pressure hull as a buffer, the diesels would instantaneously suck huge volumes of air from the boat's compartments, and the crew often suffered painful ear injuries. Waste disposal was a problem when the U-boats spent extended periods without surfacing. Speed was limited to {{convert|8|kn|km/h}}, lest the device snap from stress. The ''schnorchel'' also had the effect of making the boat essentially noisy and deaf in sonar terms. Finally, Allied radar eventually became sufficiently advanced such that the schnorchel head itself could be detected. The U-boats had a radar detector but the Allies changed to centimetric radar which the Germans did not discover.
The later U-boats were covered in a sound-absorbent rubber coating to make them less of
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