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To me, most of photos of subs going with significant speed show no noticable smoke at all. Therefore, noticable smoke probably means some kind of trouble with the engine (not necessarily serious but simply weared).
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You are right in that a weary diesel engine produces more smoke, but the WW2 time diesel engines, -like modern ones but less noticeably in the latter- cause a huge smoke cloud when suddenly increasing revs. You might have noticed in a diesel car that travels ahead of you in the highway a sudden smoke cloud when the driver goes from 5th to 4th gear to overtake. I have read a personal account from Reinhard Hardegen where he explains how, during Operation Drumbeat, he suddenly spotted a gunship and inverted course while applying full throttle to engines and reducting gear (yes, subs also have different final ratios and also clutch) and the smoke coming out of the exhaust was enough to cover him so that the gunboat could no longer see the U-Boat. Also, O'Kane narrates similar experiences aboard Tang
IMO the conclusion is: When cruising along at constant speed, smoke should normally not be very noticeable -except in weary engines, as you correctly stated- but in full throttle situations a heavy smoke cloud is completely accurate.