Submarines of the United States Navy are built in classes, using a single design for a number of boats.
Minor variations occur as improvements are incorporated into the design, so later boats of a class may be more capable than earlier.
Also, boats are modified, sometimes extensively, while in service, creating departures from the class standard.
Experimental use: an example is USS Albacore (AGSS-569), which used an unprecedented hull design.
In this list such single boat "classes" are marked with "(unique)".