The future of manned submarines is currently in question as a result of rapid developments in autonomy in other fields (e.g. autonomous cars). Therefore, the feasibility of an unmanned submarine, basically a large unmanned underwater vehicle (UUV), as a replacement for a conventional diesel-electric submarine (SSK), was investigated. Characteristics (speed, range, endurance, payload capacity, autonomy, reliability) of a hypothetical generic unmanned submarine design were estimated using expert judgement to predict the state of the art of critical technologies (energy storage, artificial intelligence, etc.) that might be available beginning in 2027. Next, the operational performance of this future unmanned submarine was evaluated using a set of operations that are typical for an SSK. This evaluation showed that an unmanned submarine will not be able to replace the SSK beginning in 2027. Smaller UUVs, however, could be a valuable addition to an SSK.