I just had another 59,000 ton patrol, and yes, it's the middle of 1941 and the merchies are getting armed. So far, it doesn't seem like all of them...on this run, I had BdU report a neutral convoy (wha?) in the Western approaches. I figured it couldn't be all neutral ships, so I intercepted, and there it was, a mix of Greek, American, and British ships without an escort in sight.
It was daylight, but I went in on the surface anyway, since I figured if anyone shot at me, it'd be the Brits, and there weren't many of them (I'd already found out some merchies were armed after a gun duel with a 6,000 ton cargo ship earlier). It turned out that there were only two ships armed, but they both started shooting at me...I didn't shoot back immediately as one of them was Greek and the other American. I deck-gunned a British cargo ship and torpedoed a medium sized tanker, but then the damn neutrals started getting damaging near misses. I thought about trying to use the deck-gun to disarm the nearest (the American), but he was too far away for that kind of precision, so I increased to flank and torpedoed a pair of C2s on the way out, both Brit. Around this time a corvette showed up and started shelling me with much more accuracy than the merchies, so I dived, used two torpedos to finish off the two cargo ships, then went deep and silent.
The corvette depth charged me for about 20 RL minutes, but apparently he couldn't hear me very well...none of the charges was close enough to do damage.
I blundered into another convoy later, but due to a poor job of positioning myself, I only sank another 6,000 ton cargo ship. I still had six torpedos left though, so I cruised my patrol zone south of Iceland for awhile, and then had to use all but one of my remaining torps to sink a giant cargo ship that I found.
I think your previous assesment is right Dracho: A 70,000 patrol is possible given the right mix of weather and targets, but you'd have to be damned lucky...