In December 2018 a Vietnamese fisherman operating about eight kilometers off the coast of central Vietnam, spotted a large circular object floating nearby and, curious about that it might be, attached a rope to it and brought it onto a local beach. Once on the beach, someone realized it appeared to be a torpedo (it was seven meters long, 533mm in diameter and had a propeller). Part of it was painted red and it had Chinese language writing on it. The police were called and they brought in a naval officer to confirm that it was a Chinese training torpedo that had not been retrieved after use. The torpedo was put under guard until it could be removed for further examination. Before removal, many cell phone photos were taken and found their way to the Internet. The torpedo looked like an American MK-48 meaning it was probably a training version of the Chinese Yu 6 torpedo. This weapon entered service in 2005 and was known to be based on an older Mk 48 Mod 3 that Chinese fishermen retrieved sometime around 1980. Chinese engineers spent over two decades reverse engineering key aspects of the Mk 48 while also retaining some characteristics of the older Russian torpedoes they were using.
The Mk 48 Mod 4 entered service in 1982 and there was a remote chance the Chinese fishermen found one of these that had been lost. China is also known to have stolen a lot of data on U.S. weapons (mainly via Internet hacks) since 2005 but tech appears to have gone into the Yu 9 torpedo, which may still be in development and may supplement late model Yu 6s rather than replace them.
Most modern torpedoes have training versions in which the warhead is replaced with an inert (non-explosive) payload that includes electronics to record the performance of the torpedo as well as an electronic beacon so that the torpedo (which is rigged to float after use) can easily be found. Sometimes the beacon fails and you have a lost torpedo. Since Vietnam and the U.S. are now on friendly terms American technicians will probably get a look at the torpedo the Vietnamese found, provide additional confirmation of what it is and collect data on how the Chinese adapted older Mk 48 tech to build their standard submarine torpedo.
The MK-48 entered service in 1971 as the Mod 1 and has been continually upgraded since then. The MK-48 is a 533mm (21 inch), 1.7 ton weapon with a range of up to 74 kilometers (at 50 kilometers an hour) and a top speed of 102 kilometers an hour (at a range of 38 kilometers). The MK-48 can be controlled from the sub via wire guidance and has onboard sonar to assist in finding targets and avoiding underwater obstacles. There are numerous electronic devices on board to get around countermeasures. The MK-48 has a 295 kg (650 pound) warhead and uses a proximity fuze. Maximum depth is about 800 meters.
The MK-48 is still used by the U.S. Navy as well as Brazil, Australia, Canada and the Netherlands. The Mod 4 was withdrawn in 2007 having been replaced by the Mod 5 starting in 1987. The Mod 5s were all withdrawn by 2011 as the Mod 6 entered use. The Mod 6 is still around but the Mod 7 entered service in 2016.
Since World War II only three submarine-launched torpedoes have been used in combat to sink something. Only one was launched by a nuclear sub (a British boat). The other two were launched by Pakistani and North Korean diesel-electric subs. No MK-48 has ever been used in combat.
Practice torpedoes are not the only military items showing up in fishing nets, washed up on beaches or spotted at sea and brought in. The waters of the western Pacific are increasingly populated by AUVs (Autonomous Undersea Vehicle) set loose to collect technical data on the water all the way from the surface to the sea bottom. Increasingly these AUVs are being caught by fishing nets by accident or seized by warships on purpose to make a political point or, eventually, interfere with legal data collection that is nevertheless very useful in submarine operations. Twice in 2016, Filipino fishermen in the South China Sea have caught American AUVs in their nets. These torpedo like devices are clearly marked as to what they are and the American embassy will send someone to pick them up if found. These AUVs are silent, very small, and able to operate on their own for up to a year. The first models were two meters (six feet) long and weighed 59 kg (130 pounds) and built to operate completely on its own collecting valuable information about underwater “weather”. What this AUV does is automatically move slowly (30-70 kilometers a day) underwater, collecting data on salinity and temperature and transmitting back via a satellite link every hour or so as the AUV briefly reaches the surface. This data improves the effectiveness of sonars used by friendly forces, making it easier to detect and track enemy submarines. That’s because the speed of sound traveling through water varies according to the temperature and salinity of the water. Having more precise data on salinity and temperature in a large body of water makes your underwater sensors (sonar, which detects sound to determine what is out there) more accurate. This data can also assist submarines in better avoiding detection.
The first of these navy AUVs could dive as far down as 200 meters (620 feet) and later models were able to go down to 1,000 meters or more. These AUVs use a unique form of propulsion. They have wings and a small pump, that fills and empties a chamber. This changes its buoyancy, causing it to glide down, then back up. This maneuver moves the AUV forward. Equipped with GPS and a navigation and communications computer, the AUV is programmed (or instructed via the sat link) to monitor a particular area. The small pump uses less electricity than a propeller (to move it at the same speed). Thus these UAVs can remain at sea for up to a year on one battery charge. Before the battery runs out the owner has to direct the UAV and a ship to a rendezvous where the AUV will remain on the surface and the ship will haul it aboard, replace the battery and perform any other needed maintenance. The U.S. Navy currently has over 2,000 of these AUVs in service or on order and plans to keep increasing this robotic ASW fleet as long as they keep demonstrating they can do the job. The U.S. Navy apparently has a lot of these AUVs operating in the South China Sea.
It’s not just AUVs Filipino fishermen come across. In 2015 and 2013 fishermen found Chukar target drones. The first one had a curious history because it landed intact in late 2012 and then drifted over 2,000 kilometers from Guam until it reached the Philippines. The Chukar is designed to float so that it can be recovered and reused.