GWX and AA gun vs ships

[deamyont]

So, I just sank two fishing boats using only the 2x20mm mounted on my type II. I shot about 80 rounds on the first one, all at the waterline, and as I didnt think it had any effect i turned and shot about 30 on hull (where I thought the engine/fuel should be) the second boat, and to my amazment it exploded. As I turned back, the first boat were almost sunk by flooding... so with other words small ships are easily sunk by guns as small as 20 mm in GWX...

Now here's my question: Im sure this works at patrol- or torpedoboats and such, but will it work on bigger ships aswell?

I assume sinking a tramp steamer or maybee a corvette with the 20 mm is possible as well, as it shouldnt have much thicker hull than a fishing boat, but what about an destroyer? In real life they had a bit of belt armour, and an inch of it should really be able to stop smaller-caliber guns, right?

Anyone know for sure whats possible and not with the AA guns?