The Losos class was designed for special operations and engaging surface ships located offshore, and is thus very durable and almost completely silent.
The hull is made of a titanium alloy, which helps with signature management because it is not magnetic. The non-magnetic alloy would greatly reduce the effectiveness of enemy magnetic anomaly detectors or magnetic limpet mines against this type of vessel.
Only two Losos-class submarines were built: MS-520 and MS-521. Original planning called for a total of 12 Project 865 Piranha submarines to be constructed; this was eventually reduced to six, then just the two. Launched in 1986 and 1990, respectively, they are in reserve but are expected to soon be discarded.