Like the British X class boats, the Type XXVIIA was designed to carry explosive charges to be laid beneath enemy ships, but it was markedly smaller and had substantial differences from the X class. It dispensed with a dual diesel/electric propulsion system, relying instead solely on electrical power in the form of a AEG torpedo motor, on the basis that since it would operate submerged there was no need for a diesel engine. However, this resulted in a very low endurance of at .