Yeah, I didn't mean that broken ciphers gave away all info, all the time. That is exactly why I added the caveat "when the Allies were able to read German mail."
I had thought such wording would make clear that the Allies weren't always able to decipher and/or reroute.
But , you are still focusing on the number of supplies that was lost by submarines, while this number is subordinate on the number of supplies that arrived in Liverpool .
ONE example :
the period September 1939-May 1940 :
the month with the highest losses was May 1940 : 220,127 GRT were lost and the month with the lowest losses was November 1939 : 53,511 GRT .
Thus : May was the worst month and November the best ? NO
Because in May 5,142,000 GRT arrived in Britain : 5,362,000 were sent and 220,000 were lost
While in November 4,408,000 were sent and 53,111 were lost, which means that only 4,355,000 GRT arrived .
Thus May was better than November .
Other point : it was not the amount of supplies that was lost,that was important, but WHAT was lost . If in November all losses were oil,that would mean that Britain could be in trouble with only 53000 GRT that was lost .
Last point : it was even (theoretically ) possible that Britain could be in big problems without losing even ONE ton of supplies : as everyone knows (or should know ) : all ships that arrived in Britain had to return empty to the Americas or the harbours from where they were sailing.
If in a month 1000 ships left for Britain with 3 million ton of supplies, and if all arrived without losing any GRT ,but if all were lost on their return, Britain would be in big problems .
I know that this IF did not happen , but he indicates that it is wrong to look at the losses of supplies only ,and also that it is wrong to look at the number of supplies only : Britain would be hurt more by the loss of 100000 GRT of oil than by the loss of 200000 GRT of Spam.
It is also wrong to look at the losses of U Boats only,as more U Boats were not a bigger danger than less U Boats .
Question :
was it possible for U Boats to recognize oil tankers ?
If yes, why was not more oil sunk ?
If not : why did Doenitz nothing to make it possible to recognize oil tankers ?