Admiral Karl Dönitz added two requirements: as the boat would have to operate in the Mediterranean and the Black Sea, it had to be able to be transported by rail, and it had to use the standard 53.3 cm (21 in) torpedo tubes.
Dertil havde admiralen Karl Dönitz stillet to mere krav: da bådene skulle bruges både i Sortehavet og Middelhavet, skulle de være i stand til at blive transporteret over landet med tog, samt benytte de almindelige standard 533 mm torpedorør.