Haijiu-class submarine chaser

The Haijiu class submarine chasers are two (possibly four) modified versions of the Hainan class submarine chasers (Type 037) of the People's Liberation Army Navy.
The main difference is that four ASW rocket launchers have replaced the original two ASW mortars. The biggest improvement, however, it's in the sonar system. Original SJD-3 sonar on earlier Type 037 class submarine chaser is replaced by a 13 kHz (2 frequency ranges) French Thomson Sintra SS-12/DIP 12 hull mounted (HM)/variable depth sonar (VDS). DIP 12, the VDS version weigh 4 tons and has a 200-meter cable with towing speed of 16 kn, and the range is selectable at 2, 4, 8 and 16 km respectively. However, during deployment, it was discovered that the endurance of this class is limited and the capabilities of SS/DIP 12 sonar system could not be fully exploited. There is some disagreement over the type designation of this class, some sources claim it is Type 037I, while others claim it is Type 037 Mod (for modified or modernized).