Usually you're suppposed to avoid destroyers simply because (from a U-boats perspective) they aren't worth the effort given your primary objective; to sink enemy shipping and starve out Britain. Destroyers are numerous and losing a single destroyer to the Allies isn't a big hit, however the torpedoes which you expend in the act of destroying said destroyer (which can be many if you aren't practiced or lucky) would be much better spent nailing big fat merchants. That and the fact that a merchant isn't likely to sink you whereas losing a single U-boat, to Bdu, is a terrible loss.
Also more destroyers is even a better thing if you think about it, and provided you can evade them whilst destroying merchant cargoes. See Destroyers are huge fuel hogs and if you can take out alot of oil tankers, in conjunction with other U-boats, the shortened supply of oil, ever drained by the high demands of destoyers on the high seas, would eventually cripple Britain's ability to field its defensive weapons and would shut down industry and so on.
However if you're a hot shot that likes to brag to your Kaleun buddies go right ahead.
As for tactics that can vary. Some people have very realistic appraoches whereas others exploit certain quirks in the game engine. For evasion that depends on the year. But obviously th easiest way to evade is to avoid. And that isn't hard if you pay attention.
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