Flak against ships

[Commander Mysenses]

Originally Posted by Luken

Well, anti aircraft gun ammo has great armor penetration

Well yes, the 88mm Flugzeugabwehrkanone certainly was pretty good against armour. But the 20mm Flak 38 might struggle with 1 - 1.5 inch of steel plate on the hull of a tanker. Especially as they would be using fragmentation (FLAK) rounds against incoming aircraft, not likely they would find much use for AP rounds for their 20mm AA.
You could tear up the light superstructure pretty good with frag rounds and start some fires in the cabins, but the heavy crude oil in the holds would need some serious heat to get going... even then it would take days to sink.

You'd be better off boarding and scuttling an unarmed tanker, using the FLAK as cover.