The Project 633 class (known in the West by its NATO reporting name as the Romeo class) is a class of Soviet diesel-electric submarine, built between 1957 and 1961. A Chinese variant - Type 033 - was built in China from 1962 to 1984.
The Una-class submarine (also known as Type 911) was a class of six midget submarines built for the Yugoslav Navy at the Brodogradilište specijalnih objekata (English: Special objects shipyard) during the 1980s. They were designed with the purpose of laying small minefields and transporting naval special forces, with or without their submersibles, in shallow waters that were inaccessible for larger submarines. Due to their mission profile that called for a small design as well as the need to stay undetected, they lacked torpedo armament and a generator for battery recharging.
During the Croatian War of Independence and the dissolution of SFR Yugoslavia, five of the six boats were relocated to Boka Kotorska where they served with the SR Yugoslav Navy. One, Soča (P-914), was captured in drydock by Croatian forces. It was later modified and entered service with the Croatian Navy as Velebit (P-01). All of the boats have since been decommissioned.