[Jimbuna]

I tend to find that the submarine in a convoy (usually the British S class) lags a bit at the rear of the convoy and it is usually they're reports that warn the escorts of a hunter approaching from the rear.

My remedy for this is usually to take the sub out with my deck gun or eel(dependant upon your position) then go deep on a following course to that of the convoy.

If an escort comes sniffing you are usually safe at depth and in silent running mode.

Once they leave the scene to take up their convoy station again you can surface and hopefully pick off the rearmost ships in the convoy, one by one....repeating the previous operation if and when an escort comes sniffing again.