You might enjoy this site and this page. I always wished there was a mod for food allotment, a sort of a days at sea metering - sometimes your fuel could outlast your food in reality.
http://uboat.net/men/foodstuffs.htm
you can also check against the length of patrols and type of boat for historical data.
Me? I have my immersion kit. When the food rack is lean, I head home.
"U-boat crews would often trade with fishermen for fresh fish as well as board their victims and remove any foodstuffs they could carry."
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DID, no cheats, no shortcuts, no non-historic equipment deviations. Boat and crew safety is integral with my immersion style! Follow the historic events during your patrol:
http://www.uboat.net/today.html