I finally sank my first really big ship!
An Illustrious Fleet Carrier(task force) at 23000 tons.
Edit: this is around april of 1940...
A daytime attack, medium heavy seas, four torps set at 3.5 ft, impact, at about 2200 meters. All four torps hit, as well as a fifth shot out the back sinking a Tribal Destroyer. I didn't get to see any of it, as I had turned off event cameras and external cameras(91% difficulty), and I dove for 120m as soon as I launched.
Never even got pinged - they never knew what hit 'em.
Considering the distance to the nearest coast, and the amout of aireal patrols I had to dodge trying to intercept the task force, I think I should also get credit for all the aircraft who are now finding themselves low on feul with no where to land.
