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. However, in general, all boats of a class are noticeably similar.
Experimental use: an example is , which used an unprecedented hull design. In this list such single boat "classes" are marked with "(unique)".