You know, I actually tried that too. I backed up a full km at a 90 degree angle, set it to ahead flank and rammed right into her. I hit her in the front, nothing, just a lot of scraping. Then I tried again and rammed her mid hull. This time about 1/4 of my u-boat went onto the deck of the ship. So much so that it actually damaged torpedo bay 2. So I threw her into full reverse backed up. Repaired the damage and that was it. I mean you'd think with the weight of my boat on top of her she'd at least sink a bit more. Nothing. Something is screwy.
Oddly too though I suffered no hull damage. You'd think ramming full on into a ship would at least dent the hull?