Rocket U-boat – a planned ballistic missile submarine; project abandoned Type XVIII U-boat – a U-boat designed to use air-independent propulsion; several were under construction when the war ended Type XXI U-boat "Elektroboot" (Electric boat) – the first U-boat designed to operate completely submerged, 118 were built but only 4 were completed Type XXIV U-boat – a planned U-boat designed to use air-independent propulsion Type XXVI U-boat – a U-boat designed to use air-independent propulsion; several were under construction when the war ended