Royal Navy planners in the 1930s used capability efficiency to ask whether enough units would exist to guarantee a capability when needed. One objection to very large submarines with big guns was their massive crew requirement. The same manpower could crew three ordinary submarines, which could also be obtained for the cost of one large boat. In the Navy's view, those ordinary boats were nearly as effective as the oversized submarines. The illustrative choice is between three 80-percent solutions and one 95-percent solution, with a clear preference for the three boats.
Chapter · 1:44-2:26
Why the 1930s Royal Navy preferred more usable submarines
For the crew and cost of one large submarine, three ordinary boats offered the Royal Navy more units that were each nearly as effective in its judgment.