The M class followed the K-class programme with large submarines usually carrying a single 12-inch Mark IX gun. The development continued the ambition to combine long-range independent submarine operations with a cruiser's role. That combination raised a problem beyond the machinery and armament: much of a cruiser's peacetime deterrent value came from being visible. A submarine's value depended on its ability to remain hidden, so the two roles rewarded opposing characteristics.
Notable Point · 2:06:28-2:08:15
M-class guns and the conflict over cruiser visibility
The gun-armed M class continued the cruiser-submarine idea, whose peacetime visibility requirement conflicted with a submarine's reliance on concealment.