Confirmed Submarine kills against ships during WWI.

Confirmed Submarines kills During WWI. 
1914, September 5
HMS Pathfinder is sunk at the start of World War I by U-21, becoming the first ship to ever be sunk by a self-propelled torpedo fired by submarine.

1914, September 22
 German submarine U-9 sinks three unescorted British armoured cruisers HMS Aboukir, HMS Hogue and HMS Cressy in approximately one hour.
1914, October 18
German submarine U-27 sinks HMS E3 in the first ever successful attack on one submarine by another.

1914, October 20
German submarine U-17 sinks SS Glitra in the first submarine sinking of a merchant ship during the world wars.

1915, May 7
German submarine U-20 sinks RMS Lusitania killing 1,198 and leaving 761 survivors. The sinking turned public opinion in many countries against Germany, contributed to the U.S. entry into World War I.
1915, May 25/27
 In the morning of May 27 German submarine U-21 sinks the British pre-dreadnought battleship HMS Majestic off the Gallipoli peninsula. Two days after the same U-boat sunk the pre-dreadnought HMS Triumph near Gaba Nepe.

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