experimented with the idea of the Schnorchel (snorkel) from captured Dutch submarines, but saw no need for them until rather late in the war. The Schnorchel was a retractable pipe that 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. Problems occurred 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. Speed was limited to , 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 that the Schnorchel mast could be detected beyond visual range. Several other pioneering innovations included acoustic- and electro-absorbent coatings to make them less of an ASDIC or radar target. The Germans also developed active countermeasures such as facilities to release artificial chemical bubble-making decoys, known as Bold, after the mythical kobold. Classes Type I: first prototypes Type II: small submarines used for training purposes Type V: uncompleted experimental midget submarines Type VII: the "workhorse" of the U-boats with 709 completed in World War II Type IX: these long-range U-boats operated as far as the Indian Ocean with the Japanese (Monsun Gruppe), and the South Atlantic Type X: long-range minelayers and cargo transports Type XI: uncompleted experimental artillery boats Type XIV: used to resupply other U-boats; nicknamed the Milchkuh ("Milk Cow") Type XVII: small coastal submarines powered by experimental hydrogen peroxide propulsion systems Type XXI: known as the Elektroboot; first subs to operate primarily submerged Type XXIII: smaller version of the XXI used for coastal operations Midget submarines, including Biber, Hai, Molch, and Seehund Uncompleted U-boat projects Countermeasures Advances in convoy tactics, high-frequency direction finding (referred to as ("Huff-Duff"), radar, active sonar (called ASDIC in Britain), depth charges, ASW spigot mortars (also known as "hedgehog"), the intermittent cracking of the German Naval Enigma code, the introduction of the Leigh light, the range of escort aircraft (especially with the use of escort carriers), the use of mystery ships, and the full entry of the U.S. into the war with its enormous shipbuilding capacity, all