Dunkirk destroyers

[Parachute]

By luck the start date of my latest patrol coincides with the Dunkirk evacuation. I headed straight over from Wilhelmshaven, and after a few attempts managed to get through the CTD ring which seems to surround the harbour by sitting it out on the bridge at x16 & x32 time compression on my approach.

As soon as I spot the harbour lights in the distance, I drop to periscope depth and move at ahead one-third. However, even if I keep the periscope down for the entire approach, at least one of the stationary destroyers detects me well before I can get anywhere near torpedo range of the juicy targets, and since there's no room to dive it seems to be pretty much game over as soon as this happens.

I feel like I must be doing something stupid, because I've been able to slip between convoy columns in the past without being noticed by the escorts, and doing that required getting much closer to them and moving at higher speeds than I'm doing here. I'm a relatively new player so I imagine there are some differences between these scenarios I'm unaware of. Could anyone explain what's going wrong? And is sinking the troop ships at Dunkirk pointless anyway because it's impossible to get away afterwards without being sunk by the destroyers?

I'm using GWX amongst other mods.