Right, be we're talking about heavyweights for ASW, which actually lasted longer than you might think. Some FRAM DDs (the ones without DASH) and some DLGNs has Mk 25 fixed 21-inch tubes, mostly for Mk 37. When Mk 37 became obsolete thanks to faster Soviert subs (fast diesels and specially nukes) these tubes went away but not until the 1960s. The Garcias, Brooke's and even Knoxes were built with Mk 25 tubes and some sort of plans to fit the MK 48 to these ships persisted to about 1970 (definitely dropped by 1972). The Mk 48, BTW, was designed from the outset to be both as submarine and surface-launched torpedo.
The idea of surface-launched heavyweights for ASW may have revived a bit around the mid-1970s because a) Mk 46 was not looking all that good at running down fast Soviet subs and b) there was promise of improved ASW sensors like ERAPS that could exploit the range of a heavyweight from a surface ship. So we see designs like the ones I linked that have tubes for Mk 48. I'm not suggesting this was a great idea, but it was a fairly common proposal for quite a while into the mid-1970s or even early 1980s, even after Mk 48 development was officially limited to subs in 1972.
These tubes would not be the very heavy installations found in subs, because they didn't have to hold against ocean pressures, didn't have to expel the torpedo against water resistance, and didn't have to get the torpedo going all that fast. It was enough to push the torp over the side and away from the hull so gravity and the torpedo's own motor could do the rest. (Indeed, you don't want it to be too fast, becuade a fast/shallow entry can cause the torpedo to broach.) I can't find a weight for the Mk 25 tube , but its predecessors were basically aluminum cylinders (probably fiberglass wrapped aluminum by these 1976 proposals) with fairly lightweight muzzle caps and breeches.
Here's a picture of Mk 25 and Mk 32 tubes together on a FRAM DD (Allen M. Sumner) circa 1960:
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And here's a Mk 48 being fired from a DEG, probably around 1969-70, from a better than average history of the MK 48 and surface ships on The Drive.
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