681－687, 689－692, 694－696, 698－709 and 721－742 Type 037 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). Type 037I Haiqing-class submarine chaser The Type 037I submarine chaser (NATO reporting name: Haiqing class), is a follow-on class of submarine chasers to the preceding Type 037-class of the People's Liberation Army Navy. There are two designation sof this class, the base Type 037I and the improved Type 037IS. The earlier boats were built by the Qiuxin Shipyard of Shanghai, and later by Huangpu Shipyard. The ships are armed with two 6-tubed anti-submarine mortar launchers. The earlier boats are fitted with two manually operated twin-barrel 37 mm guns, but they have been replaced by two automatic 37 mm guns on the later variants. They also have various other mounted heavy machine guns for patrol duty. These weapons focus on a general-purpose patrol boat, with only a limited anti-submarine capability. One Haiqing class submarine chaser was purchased by the Sri Lanka Navy and was commissioned in 1996 as , serving in the Sri Lankan Civil War. Ships in PLAN service by pennant number: Type 037I; 688, 693 , 694 and 697 Type 037IS; Approx. 611－614, 619, 631, 632－635, 710－713, 743, 744, 761－763 and 786－789 Type 037IG Houxin-class missile boat The Type 037IG missile boat (NATO: Houxin class) were first built by Qiuxing and Huangpu Shipyard from 1991 through 1999 for the People's Liberation Army Navy (PLAN). In addition to the PLAN,