They didn't close the funnel........... the four boilers were placed outside the pressure hull in free-flooding compartments. The Kaiserliche Marine discovered by accident that, contrary to popular belief, a boiler would NOT explode when suddenly submerged in cold seawater- a picket boat sank while under steam and when salvaged the boiler was still in working order! The mind boggles at the effect of exposing the boilers to salt water every time the sub submerged though- corrosion would have been a HUGE problem..... not to mention the very real likelihood of boiler water contamination with sea water and all the problems that would cause for the Chief Engineer and his subordinates. Wouldn't surprise me one bit if the boilers would have needed replacing at every overhaul between missions.
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