Coast Guard photograph, PHC Ken Mather, USCGC Sherman (WHEC-720) in her original configuration with a 5″/38 and dual hedgehogs, April 1969.
This is hardly a current topic, but it is one I have seen discussed several time, most recently in comments on a post about the new Canadian Arctic/Offshore Patrol Ship (AOPS), e.g. why the Coast Guard used the 76mm Mk75 gun on the 378 FRAMs rather than the 5″/54 Mk45.
It is true that, although about four tons heavier, the early model 5″/54 Mk45 mounts were a drop in replacement for the 5″/38 mounts we had on the 378s. It would have appeared an easy choice, but in order to accommodate the additional weight of the Harpoons and CIWS, perhaps the Coast Guard had no choice but to go with a lighter gun. That gun (and its associated firecontrol system) was certainly seen as a significant improvement over what we had had in essentially all respects including reduced manning and maintenance.
If we look at the weights as built that were removed:
one 5″/38 (20.5 tons) and two hedgehogs (14.4 tons), totalling approximately 35 tons
and compare that to some of the weights added
one 76 mm Mk75 gun (8.2 tons), two Mk141 quad harpoon launchers (11.8 tons total), eight Harpoon (6 tons), and a Mk 15 Phalanx CIWS (6.8 tons). Total approximately 32.8 tons. Plus more weight for the addition of the hangar. Essentially the difference was a wash.
If instead, we had used the 5″/54 Mk45 mod 0 (24.5 tons) in addition to the harpoons and CIWS, the total weight would have been 16.3 tons higher or 49.1 tons.
There are other weights that might be added in calculating the total weight devoted to armaments, but obviously I don’t think this was an excessive amount, there are too many examples of smaller ships with far more weapons. I previously noted, (“OPC-Design for Wartime, Build for Peacetime”) that as built the little 255s had 140 to 150 tons of weapons.
Even so, an additional 16.3 tons topside (even if that is only about 0.5% of the full load displacement) might have been too much for the 378s. I don’t know. Perhaps a former 378 engineer or DCA could enlighten the discussion.