Under the normal conditions diesel engine has no visible exhaust even if it operates on HFO. Visible "black" exhaust may appear in case the diesel is significantly overloaded or its proper adjusting lost. "White" exhaust appears when the fuel oil (MDO or HFO) contains water or compression piston rings/valves guides/turbocharger's seals are weared and lubricating oil enters the cylinders and exhaust line.
Also there is feature in SH5 when both diesels are vibrating during operation. It is "cool", i like it by the way, but IMHO it is not realistic. As both diesel of U-boat drive the propeller shafts directly, they are to be mounted on their foundations without any flexible intermediate parts. So during the diesels operation you should not see their vibration only, but feel the viration of the whole U-boat. Would be cool if it can be modded also
