SM U-18[Note 1] was one of 329 submarines serving in the Imperial German Navy in World War I. U-18 engaged in the commerce warfare in the First Battle of the Atlantic.

However, the fleet was absent, being dispersed in anchorages on the west coast of Scotland and Ireland.

As U-18 was making her way back out through Hoxa Sound to the open sea, her periscope was spotted by a guard boat.

U-18 then suffered a failure of her diving plane motor and the boat became unable to maintain her depth, at one point even impacting the seabed.

She was rammed a second time by destroyer HMS Garry and eventually, her captain was forced to surface and scuttle his command just outside the Hoxa Gate; all crew members, except one, were picked up by British boats.