kfbr392 wrote:If an opponent can be reached in an underwater start-up, even if the battery capacity is used ruthlessly, then run, carry out an attack in a start-up and force it if necessary ... 20 percent battery capacity is still enough to do a longer Wabo tracking (4-5 hours) ...
These two quotes that you provided (thanks!) appear at first contradictory. The second envisions attacking on the surface initially while the first encourages using the batteries "ruthlessly" to reach a convoy.kfbr392 wrote: The XXI should attack surfaced, use eyes, FuMB, FuMO (Hohentwiel), and Balkon-GHG to get a picture of the situation. Stay surfaced as long as possible.
In fact they're probably not contradictory: the ideal is a surface approach but underwater approach is possible if the sub is forced under.
Given the proliferation of Allied airpower later in the war, and given the anticipated response to T21 (e.g. more B-24's over the North Atlantic), T21 probably would have been forced to approach submerged in most circumstances.
That's not a firm obstacle to devastating the convoys, however: U-boats had a detection range of around 10 miles depending on weather etc. So if you have 100 boats at sea it's pretty easy to form a picket line that will guarantee contacts. It'd be harder to concentrate in large wolf packs but even 2-5 T21's would be a massive headache for escorts. Five T-21's could launch 90 torpedoes in half an hour, which means most of the convoy is sunk.