The perils of war at sea were many but nothing compared with deadly efficiency of the German U-boats. The Germans started the war with 30 submarines and ended it with 111.
In 1917 Germany stepped up its U-boat campaign and declared 'unrestricted sub warfare' on 31st January. In the following months the U-boats more than doubled their disastrous effect on allied shipping merchant and naval alike. April 1917 proved to be the most murderous month with over a million tons of allied shipping lost to the devil in the deep.
One in four merchant ships that left port failed to return and P&O losses in 1917 were far greater than in any other year of the war. In declaring ‘unrestricted warfare’ the Germans now considered hospital and all merchant ships as legitimate targets.
