Empirically speaking, Hard convoys appear to zigzag on average by 15 degrees every 15 minutes, but the deviation from this average can be quite high, especially for the exact timing of zigzags.
Since a typical 4-bearing takes around 15 minutes, its utility is rather limited when facing a Hard convoy. You are much better off using Eyeball Mk1 as well as any and all techniques that do not require a prolonged period of observation.
Another practical implication is that you are well advised to time your attack to occur right after a course change (but not before the convoy settles on it properly) to minimize the chances of a whiff due to a random zigzag after torpedoes are in the water.
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