I hit a ship with 3 eels (though 1 may have detonated prematurely) and it didn't sink for over 48 hours. It was listing to port and the stern was submerged under water and she was stationary in the water. 1 eel hit in the engine room and the other astern, it was a approx a 4500 ton merchant. Seeing it was getting beyond riduclous and I had no more eels left (IIa) I decided to fire all my 20cm flak rounds at it. Well, the cargo exploded but nothing much else. So on the third day I left it and returned to base. I have to add that the sea was dead calm...that probably didn't help either as even the smallest waves would've brought her under.
As for 20cm flak ever having sunk a merchant ship, I have managed it only once. Was one of the really small merchants. It was hit with 1 torpedo if I remember correctly and was just dead in the water without listing. I fired the flax and after the cargo exploded and secondary explosions too and then there was an almighty explosion and the thing just broke in two. I'm sure it was the flak as it was so different from the effects the eels have.
