Mystery ship

[GreyBeard]

January 1940, surfaced, returning from assigned patrol area BE59, now in quadrant BE62. About 0100 hours the WO hollers, "Ship Spotted." Almost exactly 180. I come about and begin my pursuit. The ship turns out to be an intermediate tanker (about 3400 tons) making 13 knots. At about 1200 meters I proceed to shell the bejeezus out of her with the deck gun. I've got my VIIB in reverse to slow the closing speed between us. After a short time the deck gun crew stops firing and refuses to continue. I can still fire shells manually. The tanker is stiil clearly visible through the uzo. I look on the nav map and notice the tankers icon is fading and then disappears completely. I can still see her through the uzo and watch her sink. I receive no credit for this sinking. Anyone have an idea what causes this?

GWX3, a handful of other mods (but none that give a compatibility warning when enabled with JSGME) and SH3 Commander. My 3rd patrol in this campaign, no mods were made to sub at start of 1st patrol.