The answer may lie in the precise language used, thus:

- the u-boat had a design life was four years is not the same as

- the average life of a front line u-boat was four years.


In any case, I am unaware of a precise design life value for u-boats built before the war broke out,vs. the early war years, and say late war years. Did it change, and did they recognize the change officially (and/or unofficially).

Best wishes,

Dan