Hi Dougie and Wink, let me build on my thesis a bit. At the beginning of the war Germany had 57 operational uboats of the following types: 2 Type Ia, 30 Type II's, 18 Type VII's and 7 Type IX's. Of the most effective types of uboats, only 25 were operational, and though the Type I's and II's gave a good account of themselves they were secondary players in the Tonnage War. During 1939 Germany commissioned a further 18 uboats, 7 after the war started. Of the number of commissioned uboats in 1939 5 were Type II's and one was the UA. Interestingly, Germany only commissioned 4 Type VII's between 1 Sep - 31 Dec 1939! During the same time period they lost 9 uboats. Taking new construction vs losses Germany had an average of about 50 operational uboats daily during 1939, 60% of those being the marginal Type II's.
Moving to 1940. Germany began the year with 55 operational uboats, two less then they started the war with. During the year they commissioned 55 uboats, 17 being Type II's; therefore they commissioned only 38 uboats of their most effective uboats during all of 1940. 24 uboats were lost during 1940. Using the same logic of adding commissionings minus sinkings we arrive at the average of around 80 operational uboats during the year. Figuring the precentage of Type II's is tough as the A&B's were withdraw from frontline duty starting in May, the C's did remain as operational uboats throughout most of the years and a few of the D's also served as frontline uboats. I good estimate would be 25-30% of the operational uboats in 1940 were Type II's.
I will use Blair's own figures for the total sinkings by the above uboats as statistics vary widely. Blair states uboats sank 147 ships in 1939 and 520 ships in 1940. A total of 667 ships for about 3,690,000 BRT. Given the amount of operational uboats that averages out to about 5 ships sunk for 27,500 BRT per uboat. Now add 100 uboats of the Type VII/IX variety in say an 80/20 mix -available on 01 September 1939. As those uboats were larger and more effective then the Type II's which, as I showed above, made up a significant portion on the Ubootwaffe during that time period, it is not unreasonable to add an additional 550 sinkings for 3,025,000 BRT. Sinkings on that scale would have been decisive.
Really enjoying this,
Ernest