Silent Hunter 5 Tonnage war

In Silent Hunter5 I have reached "Tonnage Wars" March-April 1943.

In this section of the game you have to sink 100,000 tons of merchant shipping. My best so far is around 50,000 tons.

I find that by setting my torpedo depth to 5 meters & firing directly from astern of the target ship making my torpedo hit the ships propellers stops the ships so that I can surface(at a distance) & sink the ship with my deck gun.

Even doing this & replenishing my weapons from a milk cow I still cannot reach 100,000 tons & the short time span of this mission will not leave enough time to return to port to re-arm then return to the patrol area.

Has anybody completed this section, or is it set so that you can never win?