What was the First British Ship sunk in World War 1?

The large British passenger liner RMS Lusitania, in May 1915. Over 1500 total were killed. For more than fifty years after the war the British kept secret the fact the the ship was carrying tons of explosive guncotton and millions of rounds of rifle ammunition to England in her cargo hold, which made her a legitimate target. The German submarine commander could not have known this though. Outrage over this incident caused the Germans to cease for a time "unrestricted submarine warfare", and moved the US closer to entering the war against Germany. When the Germans resumed "unrestricted submarine warfare" at the start of 1917 they knew this move would likely cause the US to enter the war, but they were taking a gamble that they could win the war before the US could get an army to France and change the outcome.