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Originally Posted by ReM
The combination of being able to read Enigma and the DF-ing (HuffDuff) of the heavy chatter needed by U-Boats to coordinate their Wolfpack attacks gave the Allies distinct advantages. A lot of this was so top-secret that some archives have only been opened during the 1990's, some 50 years after events took place.
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Well, I used to be in the Signals Intelligence business (Ex-US Army 05H, Electronic Warfare Signals Intelligence Morse Interceptor), and I can tell you that while HF/DF would have been valuable in an immediate tactical sense, it wouldn't have been of lesser utility for future planning. Allow me to explain..
Direction finding can only tell you, within a certain margin of error, where a target is located at the moment you DF them. At close range, such as with HF/DF sets mounted on ships, this isn't as much of an issue. In a convoy situation, if you have a couple of ships that have DF gear, you can pretty much nail the location of the u-boat transmitting (provided, of course, that you know the relative positions of the two receiving ships with reasonable accuracy!). Even if you only have one ship equipped with HF/DF, you will still get a 'line of bearing' to the target, that you can run down.
At larger distances, say from the US and the UK to the middle of the Atlantic, the irreduceable errors inherent in HF/DF, caused by the action of the ionosphere, make finding a particular boat a matter of chance. At those ranges, a circle of 50 miles would be a very good error, with 75 or more probably the average result. You also run into another problem: HF/DF in a 'strategic' (ie., fixed land stations) sense, even when combined with traffic analysis and radio fingerprinting techniques, has little to no predictive value. In other words, unless you have assets within range to prosecute the contact when you DF them, you don't know if they are going to head North, South, East, West, or stay in that same spot, which makes targeting them with a Hunter/Killer group problematic. It can still be done, but it isn't anywhere near as efficient as cryptanalytical results.
Cryptanalysis has predictive value. If you can successfully read the messages you are intercepting, many if not most of the time you know what that u-boat is going to do in 24 to 48 hours in the future, or more. You know where he is going to be, so you can place assets like planes and Hunter/Killer groups in the area to meet them.
This allows you to free up some escorts from convoy duty to actually kill the u-boats before they come in contact with the convoy. That keeps the merchant losses down (because a boat in contact with a convoy could still get lucky, but a boat sunk before it even gets anywhere near a convoy has no chance). You can't use them effectively though if you don't know where the u-boats are going to be ahead of time, which really is only possible if you can actually read the traffic between BdU and the boats.