With the German U-boats and Japanese submarines prowling between America and any potential future battlefield, the United States needed a cheap and easily constructed cargo ship to quickly replace the inevitable losses. On 27 September 1941, the SS Patrick Henry is launched: the first of 2700 Liberty class ships constructed during WWII. These large welded cargo ships formed the backbone of the American merchant fleet. They were the prime targets for German U Boats and surface raiders and the US Merchant Marine took the highest proportion of casualties of any service during the war. By 1943, a new Liberty ship was being launched every 8 hours.