New York: Random House, 1998: 171. His ideas were rejected promptly by the Kriegsmarine in 1934 as too unconventional and fanciful but Walter did not give up and in 1937 he showed his revised plans to Kapitän zur See Karl Dönitz who was at that time commander of a U-boat training flotilla. "With a speed of about twenty knots and a range of about 7,200 yards, the T-3 Falke was believed to be effective against ships traveling at speeds of up to twelve knots." Underwater a U-boat is slow, un-maneuverable and almost blind. U-Boat Worx The craft can travel up to 3 kt (3.45 mph) underwater, is rated to dive down to 100 m (330 ft), and pilot and passenger can look forward to up to eight hours of operational autonomy. 1993. Due to its slow underwater speed, the U-boat had to be ahead of the convoy and already in a good attack position. Using the small flood openings and assuming the voltage drop problem was fixed, the XXI could make 18 knots on battery and, of course, have a longer range at medium speed. Enemy sonar and hydrophones could also detect a submerged U-boat.

The current U-boat of that time (IIA) had a maximum surface speed of 11 knots and could make about 7 knots underwater.

Depending on the size of the flood openings, the top speed varied by almost 1 knot. The Columbia Encyclopedia, Fifth Edition. Submerged attacks are more difficult but are necessary during daylight or attacks against warships. New York: Columbia University.

6.2 m/s: torpedo, in naval warfare. Hitler's U-Boat War.