The torpedo was the traditional main battery of the destroyer. But the straight running steam powered torpedo was an anti-surface ship weapon, with no ability to engage submerged targets.
To tell the story of the surface launched anti-submarine torpedo, we have to take a brief detour to an air launched ASW weapon.
Submarines in World War II were really more “surface ships that could dive for a little while to avoid radar or visual detection.” As carrier based and long range land based patrol aircraft became available, they became the primary threat to U-boats. Ranging far ahead of a convoy, their mere presence could force U-boats to dive. With an effective speed of only 3-4 knots submerged, even a slow convoy with a speed of 7 or 8 knots could easily evade. But the allies wanted to kill as many U-boats as possible, obviously. So these airborne scouts would also attack, generally with the 325lb depth charge. As noted, the depth charge tended to have a low probability of success.
The Navy, in cooperation with Harvard, Western Electric, and General Electric devised a passive acoustic sensor developed a battery, electric motor (from a washing machine!) and steering system. Melded into one unit, it was the Mk24 Mine, code named “Fido.” The term “mine” was a deliberate ruse to conceal the fact that it was an air dropped passive homing anti-submarine torpedo.
The Mk24 had a 24% success rate (a phenomenal rate in ASW).
With a speed of about 12 knots, and a running time of about 10-15 minutes, when a patrol plane sp0tted a surfaced U-Boat, it would attack. If the U-Boat didn’t dive upon approach, an attack with guns or conventional bombs/depth charges would be made. When the U-Boat did submerge, the attacking plane would fly up the wake, and drop were the wake ended. That put Fido in the sweet spot to pick up the scent, as it were. To conceal that the subs were being attacked by a torpedo, pilots were forbidden to drop Fido until after the U-Boat submerged.
Mk24 was modified in a slightly larger version as M27 Cutie, for use by our submarines in an anti-shipping role. But oddly, neither variant was deployed by surface ships.
In 1950, an active acoustic sensor was introduced on what was essentially an improved Mk 24 body, introduced into service as the Mk 32. The Mk 32 had a greater range than the Mk 24. Rather than being launched from a tube, the Mk2 launcher simply flipped the Mk 32 over a ship’s side.
Eventually, the increased submerged speeds of submarines meant a new, much faster, deep diving torpedo would be needed.
As the Mk 32 entered service, development started on a “universal” lightweight homing torpedo, one that could be launched from surface ships, helicopters, and fixed wing ASW aircraft. The first was the Mk 43 Light Weight Torpedo. Rather confusingly, it was launched from the Mk 32 Surface Vessel Torpedo Tube (SVTT).
The Mk 32 SVTT is usually seen as a trainable triple tube mount (and is typically installed port and starboard on a ship), but fixed twin tube mounts have been used. Compressed air is used to eject the torpedo.
The size of the Mk 32 SVTT essentially fixed the size of future lightweight torpedoes. The Mk 43 torpedo was quickly replaced by the Mk 44, in turn replaced by the Mk 46, which is still in service, and the later Mk 50 and Mk 54 torpedoes.
We’ve focused here US weapons, but as a practical matter, the US was the supplier to virtually all Western nations through the 1960s. Soviet lightweight torpedo was generally similar.
Next up, low frequency long range sonars demand standoff weapons.