Another great book; "The U-Boat War In The Atlantic"

[Catfish]

^ Ah i see, thanks. But not quite true, they used to indeed go surfaced with decks awash and Diesels running at daytime and calm weather, in the mediterranean (to evade air attacks by faster diving), but not during attacks or at night – afaik.
Diesels running at half speed would easily be able to press out the exhaust fumes from the outboard exhaust orifices, even if the deck was awash. The intakes were mounted at upper tower level (those openings with the grids).
I take it decks awash is a perfect recipe for disaster during an attack, when the bow and tower undercut waves in any but very calm weather. This would be especially bad for the larger type IX boats, their deck acting as a gigantic dive plane