Submerged In Dover as I type this (GW1.1)

I've only been playing this game for a week or so, with the GW1.1 mod, and just had my best patrol so far. I've downed a couple of small warships, two large merchants and two large C3s. I was on my way back to Wilhemshafen and as I passed Dover I thought I might as well use the 4 torpedos I still have sinking whatever I might find in the local harbor.

Sneeking inside was a bit tricky as there are countless armed trawlers and V&W destroyers roaming around. I got inside and spotted my very first Fiji, then my first Dido and then, I couldn't believe my eyes, my very first passanger liner.

A quick glance at the renown scores that I can get for each ship settled it for me. I want to down the passanger ship, and get out of there ASAP as I've been spotted and am being pinged at this very moment. I should have enough time to let lose my two last front torps and at least one from aft before reinforcements can arrive.

My question now is, just where do I aim at this behemoth? The communitymanuals link that shows weak spots doesn't have the passanger liner on its list. It's the early 40s and from what I've read here so far the magnetic trigger aren't to be trusted, at least not just yet.