The type XXI was the stopgap design based off of the failing type XVIII boat. Both were Walter designed. The type XXI's stearn and rudder were quite different from the proposed type XVIII.
Both designs had the figure eight hull cross section. On the type XVIII it was proposed for much of the lower section of the hull was to be tanks to hold the h2o2 to power the Walter turbine. The XXI utilized these tank areas for batteries, and a whole hell of a lot of batteries. The diesles of the type XXI were not directly connected to the propeller shafts and instead turned generators which charged the batteries, which in turn powered the drive motors for surface propulsion. Very similar to the American designs except faster underwater.
The type XXIII was a replacement for the type II, and was also a Walter design with the figure 8 hull. But was never equipped with the turbine. It was just about the same length, but smaller on the inside, faster underwater, but not as fast as the XXI, and definitely not as deep diving. It was also armed less than the type II. (two torpedoes, compared to five of the type II)
The type XVIII was in fact produced, but the hulls that were to be the type XVIII were redesignated for the NEWER, type XXI design. So no true examples of the type XVIII ever met the water. (thank god, for the crews of these explosive boats, and the crews of the convoys that would have been decimated by them)
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