At the outbreak of the Croatian War of Independence Soča was being overhauled in the Brodogradilište specijalnih objekata division of Brodosplit shipyard in Split, Croatia.

Velebit was completed as Soča in 1987 at the Brodogradilište specijalnih objekata division of Brodosplit shipyard in Split, Croatia, as the fourth boat in its class.

During underwater operations two battery groups with a total of 256 cells power two 20 kW Končar electric motors mounted on a single shaft that spins a five blade propeller.

[2] Because the class was designed with reconnaissance, small scale minelaying and special operations in mind, it does not possess any offensive weapons such as torpedoes.

[7] In 1993, Brodarski Institut (BI) of Zagreb started a modification program to improve the operational capabilities of the submarine captured two years earlier.

The hull was lengthened by 1.40 m (4 ft 7 in) to create space needed for the installation of a single MTU 105 kW diesel generator, a feature the original Una-class design lacked.

According to the 2007 edition of The Naval Institute Guide to Combat Fleets of the World, Velebit was fitted with a STN-Atlas Elektronik PP-10 active and PSU-1-2 passive sonar and a portable navigational radar may have been used on board along with the GPS.

[4] The claim is contradicted by other sources with news reports stating the submarine was completed without an active sonar, effectively being "blind" underwater.

[13] Acting commander of the Croatian Navy at the time, Ante Urlić, among other propositions, mentioned the possibility of installing a new set of batteries, sonar and overhauling the submarine.