“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!
___________________________________
(1)Naval Postgraduate School, “Passive and Active Sonar Prosecution of Diesel Submarines by Nuclear
Submarines”, Erik J. Nelson, March 2008
