I'm not picturing retrofit, which appears to be what this idea meant. I mean, replace the OTL Type IX with a *Type XII which has OTL Type XXI's battery & hull features (& maybe bow & stern sonar arrays?), without the other glitch-prone junk, & start it when a/c begin to make Biscay transits hazardous, around when the flak boats started appearing. Agreed. I also notice nobody's addressed the issue I raised before: what about corvettes & slow convoys? It's all well & good to talk about DDs & a/c with FIDO, but the majority of convoys were escorted with corvettes that (TTL) wouldn't be able to keep up with a dived boat any more, either, & there simply aren't enough DDs to escort all the convoys. Where are they going to come from? And what happens to all the enormous amount of freight that now isn't safe to be sent? That alone is going to bugger things in Europe (& elsewhere!) pretty nicely. That could give real benefits to Japan. It looks like it means the PQ convoys have to stop entirely. Picture how pissed Stalin will be. How much harder will the Eastern Front turn out to be? (OTOH, how much farther from the Inter-German border will they be at war's end...? Postwar may be better.) OTOH, it might mean more shipping has to depart Pacific ports, which has knock-ons for simple delay. Iran convoys would seem to be affected, if they go by way of the Pacific. How much does delay of supplies alone affect ops in North Africa? And if the slow convoys have to stop, what about Bomber Command (just for a start)? How much will ?ops have to be curtailed? How much will food supplies in Britain be affected?