“Torpedo in the water!”
The abrupt announcement from Sonar Control caused the usual instant flood of adrenaline in the combat watchstanders aboard the USS Morton. Each silently and guiltily prayed that someone else in the ASW squadron would be the target. The men collectively held their breath for a moment as Sonar rattled off the distance, bearing, and course of the torpedo.
“It’s headed for Fluckey.”
After a split second’s hesitation for those who weren’t the target to silently thank the gods of ASW, the group’s ships reacted and began the envelopment.
Tactics for this kind of shallow water ASW operation had been learned or, in many cases, relearned, the hard way and the price of that learning had been blood and ships. Now, though, the three other ships of the group - the Morton, O’Kane, and Gilmore (somewhat ironically, the Morton class ASW corvettes had been named for famous WWII submarine commanders) – began their well practiced envelopment maneuver. What had previously been a four-ship, line abreast, hunting formation now peeled off in different directions to encircle the point where the torpedo had originated and which contained the enemy submarine. The fourth ship, the Fluckey, which now had a torpedo pursuing it, had turned away to run at maximum speed. That was how these envelopments worked. The unlucky targeted ship turned and ran while the remaining three ships attempted to surround the submarine and fix it in the center of a triangle.
As Fluckey turned to begin her run from the torpedo, she also launched one of her Mk54 lightweight ASW torpedoes back down the bearing of the incoming torpedo. This was intended to distract the sub while the three remaining ships began their envelopment and to, hopefully, cause the sub to break any control wires to the incoming torpedo. There was also the off chance that the ASW torpedo would actually hit the submarine. It happened occasionally, but not often.
As the three encircling vessels began to reach their assigned points of the triangle they shifted from their hunting mode of randomly alternating between active sonar and passive listening to continuous active searching. It was imperative to find and fix the sub’s location and begin attacking before it could settle on its next target.
By now, the torpedo chasing Fluckey was entering its terminal approach and it was apparent that Fluckey was not going to evade the incoming torpedo. This was not unusual as the fleeing vessel in this scenario typically only managed to evade the torpedo about half the time. This did not, however, mean that Fluckey was doomed. Aboard the fleeing ship, the ASW mortar, an adaptation of the old Russian RBU-6000, trained back down the vessel’s wake as the ship, itself, abruptly slowed and turned to allow the ship’s hull mounted sonar to get a precise targeting fix on the incoming torpedo. The data was quickly transferred to the mortar which twitched slightly to its final firing position and a volley of six compact ASW rocket propelled depth charges arced up and out to fall in a pattern around the torpedo’s predicted location. The charges quickly sank a dozen feet or so and exploded as one on a timed fuze. As often happened, the incoming torpedo was violently deflected off its path and, by the time it settled back down it’s direction had changed sufficiently that the small on-board sonar in its nose, with a very small and narrow field of view, was unable to reacquire the Fluckey and the torpedo headed off in a safe direction to eventually expend its fuel and sink to the bottom.
This tactic required perfect timing since slowing and turning the fleeing ship ensured that there would only be one, or at most two, attempts to deflect or destroy the incoming torpedo. If the attempt failed, as it did about 20% of the time, the fleeing ship was almost certain to be sunk. In the cold, hard math of combat, this was acceptable as the ASW corvettes were cheap and easily replaced. Understandably, the crews did not share that feeling.
Breathing a sigh of relief, Fluckey cautiously turned back toward her sisters to eventually rejoin the hunt.
Meanwhile, as the rest of the group continued their search, the ships acted as gatekeepers, preventing the sub from slipping out of the triangle no matter which way it turned. They slowly began constricting the triangle and herding the submarine towards the center.
Depending on the submarine’s behavior, this search could take many hours. Passive sonar had proven ineffective in shallow water due to the ambient noise overwhelming and masking any target noise. Non-nuclear subs were notoriously quiet and hard to find in shallow water. Ultimately, though, active sonar had proven effective. The “quietness” of a sub didn’t matter when active sonar was applied. It was usually just a matter of time before the sub was localized.
Of course, the drawback to active sonar was that it provided the submarine with a precise fix on the hunting ship’s positions and it was not uncommon for the submarine to fire back at its attackers. O’Kane had encountered this a few weeks previously and had saved itself by turning directly into the torpedo and closing at full speed. The ship had managed to get inside the torpedo’s safety range limit and then taken advantage of the proximity to the sub to launch a full volley of contact fuzed mortar charges which had achieved a single hit that damaged the sub, forcing it to the surface where the group’s 76 mm guns quickly ended the encounter.
The three searching ships continuously shared their data to assemble a composite tactical picture and triangulate the submarine’s position. Aboard the Morton, the group ASW Commander decided that the tactical picture was solid enough for an attack. Gilmore was currently in the best position and he transmitted the order for a short range ASROC launch. Gilmore’s launcher trained out and the rocket fired. The torpedo splashed down on top of the sub’s position and began its search pattern. The submarine immediately went to full speed and turned away from the torpedo. Unfortunately for the sub, it turned towards O’Kane which quickly launched a Mk54 torpedo to meet it. The sub was now sandwiched between two searching torpedoes and running out of options. This was exactly why the corvettes were always deployed in squadrons. Numbers were the key to the corvette’s success.
A final, desperate turn by the sub resulted in it heading towards Morton who also launched a Mk54. At this point, with three torpedoes surrounding it, the sub’s fate was sealed. O’Kane’s torpedo made the first hit and Gilmore’s ASROC torpedo quickly finished the sub.
This story is an exploration of corvette anti-submarine warfare tactics. The tactics are based on a compilation and analysis of several papers and articles dealing with ASW, sonar characteristics, shallow water acoustic characteristics, and ASW weapons. An example paper is listed below (1).
Notably, aviation assets are not involved. I’ve described the force structure and CONOPS into which corvettes fit and the rationale for not including helos on the ships. This scenario describes a set of tactics that could make that option viable.
This is a single scenario. In the real world, there might well be other assets, both surface and aerial, that could be called upon for assistance. It is not intended that this scenario and the ASW corvette be considered the only means or even the best means of conducting anti-submarine warfare. It is simply one means using one type of platform and is intended to show how a low end platform can, with proper tactics, perform effective ASW without the benefit of helos.
To summarize, the following tactical elements were demonstrated:
- Line abreast sweep
- Envelopment – contacting vessel runs while others envelop
- ASW hunting in squadrons
- Short range active sonar
- Continuous active sonar
- Triangulation from multiple ships
- RBU / Hedgehog as anti-sub weapon
- Sail into short range torpedo contact to get inside torpedo safety limits
- RBU as anti-torpedo tactic
I hope you enjoyed the story format!
, “Passive and Active Sonar Prosecution of Diesel Submarines by Nuclear
Submarines”, Erik J. Nelson, March 2008 Postgraduate School