Why U-boats could not replace an invasion escort fleet

Even a larger U-boat force is judged inadequate to protect an amphibious fleet. A submarine surfacing to fight a torpedo boat with its deck gun risked damage that prevented it from diving, leaving it vulnerable to another attacker. Using torpedoes against small boats would consume weapons rapidly and burden the wider supply system. Torpedoes occupy considerable space, and submarines carry relatively few. An illustrative load of 20 torpedoes supports ten attacks if easier targets require two weapons apiece, or five if maneuvering warships require salvos of four. Further salvos would reduce the number of targets that could be attacked. The same concern about exhausting torpedo stocks appears in Cold War logistics. Against a motor torpedo boat, the cost includes replacement effort and production time as well as the weapon itself.