Got this off some BB somewhere, sometime.
1939:
Allied and Neutral ship tonnage sunk by German and Italian submarines (#ships, GRT)
Sep39 48/178,621
Oct39 33/156,156
Nov39 27/72,721
Dec39 39/101,823
Tot39 147 (36.75/month)/509,321 (127,330.25/month)
British merchant ship construction capacity from 1939-1941 did not exceed 1.2 million GRT per year.
US merchant ship construction in 1939 was 0.242 million GRT.....