How far can you go without fuel?

[cardician]

So last night I was patrolling off the eastern coast of England. It's November of 39 so I'm not too worried about warships and airplanes. So I'm moving along at 1/3 and then decide after the 24 hour patrol is completed with absolutely no contacts to head south down the England shore on my way back to Kiel.

First off, the weather is horrific the entire time. Darn English and your bad weather

Along the way I get a few contacts, but in the terrible weather and my IIA I'm never in the right position to be able to catch up with them. Plus I don't want to run the diesels on flank and use it all up. Annoyed I purposefully sail close to the shore (had been at least 100km off). In horrible weather its almost always sound contacts I get first during one of those moments when the ship is submerged beneath a wave. Anyway, warship contact. I want some tonnage and this time I'm in a position to be waiting along its path. I head to intercept its path so I can be waiting for it. I'm now at periscope depth. Suddenly when I'm almost in position the DD starts heading right for me. How it suddenly knew I was there in the middle of the night in terrible weather is beyond me. Regardless, I know I'm in trouble. I decide to take a chance and head straight for it. When I'm at about 800 meters I launch a magnetic eel between .5 and 1m below its hull straight down its centerline. The eel gets just past mid hull and blam! One less destroyer. That was satisfying.

Anyway, I finally head back to Kiel after dodging a few planes that were trying to avenge the destroyer. Just as I get to the canal my officer informs me we're out of diesel fuel. Still at least 50km to go. Uh oh. I decide to take her as far as she'll get me. What happens? I make it the entire way at 1/3 ahead going 7knots. Ok....so what's the deal? Did the crew manage to get the engines to run on something else? Was I really running on vapors the entire time? I mean despite being out of fuel the navigator told me that we could still make it lik 75km or something at our current speed. All in all, still a decent patrol though