 To win this, the U-boat arm had to sink 300,000 GRT per month in order to overwhelm Britain's shipbuilding capacity and reduce its merchant marine strength.In only four out of the first 27 months of the war did Germany achieve this target, while after December 1941, when Britain was joined by the US merchant marine and ship yards the target effectively doubled.As a result, the Axis needed to sink 700,000 GRT per month; as the massive expansion of the US shipbuilding industry took effect this target increased still further.