Three months earlier, on 4 February, the Germans had established a U-boat blockade around the British Isles and had declared any vessel in it a legitimate target.

Nevertheless, to keep America out of the war, in June the Kaiser was compelled to rescind unrestricted submarine warfare and require all passenger liners be left unmolested.

This rock held one of the key navigational markers in the western ocean, the Fastnet Lighthouse, and any ships passing in and out of the Irish Sea would be within visual contact of it.

RMS Hesperian was beginning a run outward bound from Liverpool to Quebec and Montreal, with a general cargo, also doubling as a hospital ship, and carrying about 800 passengers when she was attacked and sunk by U-20 off the Fastnet.

Her crew attempted to destroy her with explosives the following day, succeeding, however, only in damaging the boat's bow (see picture) but making it effectively inoperative as a warship.

[6][8][9] Novelist Clive Cussler claimed his National Underwater and Marine Agency (NUMA) located the remains of U-20 in 1984, about 400 yards from shore.