World War II submarines were surface ships that could travel underwater for a short time. Diesel engines gave submarines a high surface speed and long range, but the speed and range of the boats was greatly reduced underwater. This is because they relied on electric motors which ran off of large batteries. These batteries where weaker and required recharging after use. Recharging the storage batteries meant surfacing to run the diesel engines which required oxygen. Because of this combat patrols consisted mostly of surface operations.