This from my good friend Clint on another discussion group talking about U-boats in the Channel.
"Before Hitler turned his attention east toward the USSR, which is 1939 through mid-1941 as the time-frame, the Kriegsmarine of Nazi Germany had 57 U-boats which were
CAPABLE of going out to sea. I am not absolutely certain, but I do
NOT believe this counts the 50 existing Type II boats (A through D models), since they were small coastal and training boats only. That said, 8 boats of those larger types were lost in 1939, 24 in 1940, and 9 more lost before June of 1941. Some were replaced of course in that time line, but by mid-June the U-boat strength was pretty small.
6 of the 10 type VIIA (Atlantic boats) had been sunk well before June,1941. There were 24 type VIIB boats build, and they were an improvement in range, but by mid-1941 twelve of them had also been sunk or "lost at sea". There were a grand total of 538 type VIIC boats eventually commissioned during the war years, but only a few had been built by late 1940 and mid 1941 since the very first was only commissioned by Nov. 1940. 5 of the 8 type IX (A) boats built had been sunk by early 1941, and of the 14 IX(B) improved version boats built, 6 had been sunk or "lost at sea" by early 1941. The IX(C) class only showed up in Jan. of 1941, so they really can’t be counted in my estimation of U-boat strength. There were eventually 54 of them produced but not before the time period we are speaking of.
Most of the other classes of U-boats built, by that time, were NOT terribly threatening types. So no matter their numbers, they could NOT have influenced the Channel usage in any significant fashion in the early stages of the Second World War (pre-Barbarrosa). This is why I stand by the "less than 30 combat ready, sea-worthy" U-boats still existed before Operation Barbarrosa, and at the time of Operation Sealion being considered.
I can recommend Richard Worth's ("Tiornu")
Fleets of World War II; and on the net a site called:
http://uboat.net/types/
If you go there (and are patient) you can discover the fates of ALL the "Undersea Boats" built after WW1 until the end of WW2. Pay particular attention to the time-frame of 1939-1941. Does any of this help understand my position on the U-boat weakness of that period? With that limited a supply of ready boats it is no wonder Doenitz was reluctant to allow them to venture into the Channel. With those losses in mind it would appear that the Kriegsmarine DID NOT have even thirty seaworthy "combat" ready U-boats left in early 1940 to the middle of 1941 which is the only period being discussed. Later they did, but when the Operation Seelowe was being discussed, before Barbarrosa, they just flat did NOT have enough of them to risk in the shallow waters of the Channel."