In this assessment, a hundred cruiser submarines armed with six-inch guns could not win the Second World War. Convoy escorts with guns as small as four inches could damage their pressure hulls and prevent diving. A damaged submarine would then remain exposed on the surface, where a destroyer could engage it. The destroyer's more stable gun platform improves its chance of hitting, while the submarine has no armour. An enemy able to escort convoys and exploit that vulnerability undermines the proposed raiding fleet's advantage.
Notable Point · 57:34-58:50
Gunfire can turn a cruiser submarine into a trapped surface vessel
Damage to an unarmoured pressure hull can remove a submarine’s ability to dive and expose it to a steadier destroyer gun platform.