[the dark knight]

Might be my imagination, but it seems like on a merchant with a very thin hull plating (compared to say, the armor on a destroyer), the AP ammo just does not do the job..... With the HE shells, you can light a tanker, or a small merchant up with a few rounds, and BOOOOM! they are exploding. But using the AP on a tanker, it seems like the damage is not as great. Maybe it is just my imagination, maybe not.

In the real world there is something called over match. If the shell diameter (caliber) over matches the armor it is trying to pen, it will pass through, and there is a possibility that shell will not explode. This happened from time to time with the 88mm L/56 found on the Tiger I when fired at the Stuart tank. The Stuart has a max armor of 2 inches in the front, the Tiger fires a 3.5" shell. Fails to detonate, but still will kill crew and cripple the tank. This also happened in Bismarck's torpedo bulges during her fight with the HMS. Hood. Shells from the Prince of Whales penned the bulges, but failed to detonate. Could have been faulty fuses, could have been due to the thin steel of the blister. So, with thin lightly armored ships (or tanks) HE is your best bet. It will explode on contact, blast the skin of the ship (or tank) wide open and kill/damage anything behind the skin of the ship. Historically U-boats did not carry AP ammo, or Star Shells. Only HE since the deck gun was to only be used against merchants.