For the most part never any higher than 128x, although last night while crossing the Atlantic to the US coast I used 256 and might have tried 512 had I not set the max compression to 256 in Commander. Of course as the war progresses I slow things down on the surface, usually 8 or 16 while charging batteries or snorkeling then crank it up when back under. Anything higher in late war and I've found that the planes will have you for dinner.
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