Why could German Type VIIC U-boats stay submerged for only limited periods?

German Type VIIC U-boats were limited in their ability to remain submerged primarily because they relied on batteries to power their electric motors while underwater. These batteries had a finite capacity, meaning the submarines could only stay submerged for a little less than a day when traveling at a speed of four knots.1

Because the diesel engines required oxygen for combustion, they could not be used while the vessel was fully submerged. Consequently, the U-boats were forced to surface periodically to recharge their batteries using their diesel engines. It was not until the later introduction of the snorkel device that U-boats could draw in fresh air while remaining at periscope depth, which allowed them to recharge their batteries without fully surfacing and significantly extended their underwater endurance.12