Walter:

Do you have any info on how the allies could have built a faster FIDO, say up to 20 knots or so? That would have been enough to kill a XXI. I presume tha main problem is not propulsion but the higher background noise level of faster movement affecting the primitive, first generation acoustic seeker.

Also, about a XXI fleeing a FIDO, I agree that it would be blind and a sitting duck while on schnorkel. But even if it dashed away and survived, it would use much of its battery charge doing so. A Fido travelled around the same top speed as a XXI (12 - 15 knots for a Fido, I believe, and about 15-16 knots (depending on how clean the hull and the state of the batteries) for a XXI. So a XXI would have to go full speed to escape, and that would drain a full charge in 45 minutes, less if the batteries were not charged, or not in tip top shape.

In other words, surviving a Fido attack would leave the XXI badly drained and probably in need of a schnorkel - with those same Avengers watching and waiting.

BTW, surface running from an aircraft would be fatal. The aircrew would strafe the boat without mercy, killing any crew on deck. A sub is delicate, and vulnerable to damage - 20mm cannon fire will puncture a hull, and even sustained strafing from .50 caliber machine guns could damage enough systems to make a sub undiveable, which is the same as unsurvivable. Unfortunately many a sub got to demonstrate this during Doenitz's 'surface and fight it out' period.