As I am at work atm, I am talking out my buttocks now - ok, would be the same at home since most of my books are still in boxes
The lower vents of the saddle tanks were shut as long as they were used as a fuel tank. The consumed fuel was replaced by sea water, which was pumped in from the diving tanks. The reason for keeping shut is simple: the boat movement. If she rolls during rough weather, the vents might come out of the water and fuel could get lost. When the tanks were empty, they were rinsed out, then used as a dive tank. I read a veterans report, that this could only happen when the sea was a little rough, sadly he didn't ste why this is the case. When used as a dive tank, the lower vents were always open, just as the lower vents of the "normal" dive tanks.
So let's check out the facts about fuel consumption: most easy to remember is the range at 12 knots: 12000 km or 6500 NM. At 10 knots we have a range of 8500 NM.
This interesting document gives us the RPM at whioch the engines run at different speeds:
http://www.uboatarchive.net/Manual.htm. (page 14)
So we have 275rpm at 10 knots (langsame Fahrt), 340 at 12 (halbe Fahrt). The fuel capacity for the boat are 117 tons (metric), with 40 cbm or 43 tons in the saddle tanks.
So at 340 prm during surface ride, we have 1.8 tons per 100 NM, at 275 1,37 tons per 100 NM.
During a peaceful cruise the boat would never have burned 1/3 of her fuel during this short distance. As anybody who played SH in 1944 remembers, a Feindfahrt during this time is nearly constant snorkeling or constant dolphin moves.
The interesting question would be, at which rpm the Diesel engines run while snorkeling, though it is highlx unlikely that they operate higher than 340 (halbe Fahrt) - as one could not use the Schnorchel at higher speeds than 6,7 knots. At this rpm she could sail about 2000 NM alone with the fuel from the saddle tanks.
Overall I would say that it is highly unlikely that U297 burned all the fuel stored in the saddle tanks
and had the time to rinse them out - so I'd leave the lower vents shut
Last but not least, I found something about U297:
http://www.militaria-depot.de/u-297-...e=militaria_de
This was stuff from the 2.WO, though there seems to be no pic of U297, we find out that she was declared MIA on january the 3rd. There's also a document signed by Aldegarmann.
Oh and as the nitpicking guys we Germans are, I must also make a correction: U297 was stationed in Horten (near Oslo), not Bergen
