First, I\'m assuming you mean the type XXI here.

I remember reading somewhere that the maximum speed for ASDIC operation was 12 knots. That would give the U-boat a five mile lead, assuming that it and its hunter start out very close together and be heading the same direction. Still not much, but it\'s a bit better at least.

If the initial distance is non-zero (say the XXI sneaked into a convoy undetected, and the first notice of her presence that the escorts get is a couple of tankers blowing up), things will look even better. Of course, in this kind of situation the U-boat would be well advised to not use flank speed to escape, but rather to move away silently at something like 7 or 8 knots. After all, the less often she has to surface for charging bateries, the better. Also, traveling at 17 knots would make a lot of noise. All it takes for the escort to at least know more or less where the speeding U-boat is, is a clever escort captain who slows down to (e.g. 12 knots or less) every now and then to listen around.

I would think that the 17 knots submerged just came about without really being aimed for as such, and in practice would only be used for very short periods of time. For instance, to get into position for a really juicy target that can\'t be attacked otherwise, or (more likely) to escape from a well aimed bunch of depth charges raining down on her.

Warning: I\'m not at all an expert regarding sub tactics, so I my ramblings may very well be worthless.