Further web-searching "SS Cyrene" found this excellent Scarborough website giving more details of this sad night:

"The success of the convoy system in deep water had forced the U-Boats to hunt elsewhere; in the coastal waters around Britain and during the early months of 1918 the convoying had begun in the shallower sea-lanes around the coast. Nevertheless, despite these measures, midnight ships had still continued to be sunk by enemy submarines, albeit on a much larger scale. ‘The Scarborough Mercury’ of Friday the 12TH of April 1918 had reported;

‘Bad week for U-Boats - The return issued on Wednesday night by the Secretary of the Admiralty, for the week ending Saturday midnight, April 6TH, shows that the total arrivals and sailings were 5,029, an increase of 234 compared with the previous week. The total losses last week were 4 large and two small British merchant ships as against 5 large, and 6 small merchant ships and 6 fishing vessels in the previous week. The total arrivals and sailings is the largest in any week this year’…

One of the four large British merchant ships to be sunk that week had been a grubby 2,904 tons collier that had been carrying a cargo of Northumbrian coal from Newcastle to Blaye, a port located on the Gironde Estuary near Bordeaux, France. Built at Sunderland during 1888, the Steamship Cyrene, owned by William Coupland & Co., and commanded by Captain C.W. Lawrenson, had been steaming independently southwards at about five knots, and had been some fifteen miles to the north of Bardsay Island by the early hours of Friday the fifth of April when she had been spotted by Oberleutnant Kurt Siewert through the periscope of his coastal minelaying submarine, UC-31. Unfortunately the view that had confronted Siewert through the lense of his periscope is not recorded. The scene may have been the same as that recalled by another U-Boat captain during the same war;

‘The steamer appeared close to us and looked colossal. I saw the captain walking on his bridge, a small whistle in his mouth. I saw the crew cleaning the deck forward…stand by for firing a torpedo! I called down to the control room… Fire!. A slight tremor went through the boat—the torpedo had gone… The death bringing shot was a true one, and the torpedo ran towards the doomed ship at high speed. I could follow its course exactly by the light streak of bubbles which was left in its wake’..

I saw that the bubble track of the torpedo had been discovered on the bridge of the steamer, as frightened arms pointed towards the water…Then a frightful explosion followed, and we were all thrown against one another by the concussion, and then, like Vulcan, huge and majestic, a column of water two hundred metres high and fifty metres broad, terrible in its beauty and power, shot up to the heavens’…...

Carrying a full cargo of coal, and with her back broken, the Cyrene had begun to founder almost immediately. Nevertheless, despite the appalling list of the stricken ship one of the two lifeboats had been launched and a number of Cyrene’s crew had managed to get away before the vessel had sank, taking her captain and a number of crew members with her.

Having sank within sight of the Caenarfonshire coast, one may well have imagined the surviving members of the crew had made it to safety, this appears to have not been the case, and although the weather that cloudy and shower-filled day had been described as ‘mild with a ‘light to moderate south easterly wind of around force five or six’, all except three of the handful of survivors had perished that night. Obviously cold and wet as a result of the sinking one can only assume that the unfortunate men had died, like so many shipwrecked seamen, as a result of exposure to the elements. Twenty-four men including the Master died that night."

http://www.scarboroughsmaritimeheritage.org.uk/greatwar/s24-lost-at-sea-1918.php

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NB: In those days "large" ships was defined as "over 1,600 tons". I've sailed to France and Northern Spain many times and have occasionally suffered illness from "rough" summer seas. The worst was on a 9,000 ton car ferry in August c1992, being only a little bigger than the Cyrene it caught the waves even in the shallow seas off Dunkirk, not fun at all for the majority aboard. When I then consider that modern ferries on the more exposed routes to Western France similar to that of the Cyrene can still feel unpleasantly rough despite being fully-stabilised and upwards of 40,000 tons, then I can only admire and thank the tough bravery of such men as William Parsons. AND he was under the constant fear of attack from an unseen enemy !. I wonder why he was at sea, were his fore-fathers sailors ? Cinderford does (did?) have a "Soldiers & Sailors Club" but I'm not aware of any great naval heritage in the town. Searching Cyrene in the Welsh newspapers website gives several hits among the Ships Arrivals & Departures lists for Cardiff etc 20 odd years before she was lost; I guess she had always been a collier so it's not unlikely for a lad from a coalmining town within easy reach of the Welsh coal docks to eventually serve on such a coastal ship with no real pretentions of oceangoing seafaring.

There is also a page devoted to the Cyrene on this shipwreck site.
http://www.wrecksite.eu/wreck.aspx?11546

More details about submarine UC-31 and it's War Service sinking 38 Allied ships are here.
https://en.wikipedia.org/wiki/SM_UC-31
http://uboat.net/wwi/boats/?boat=UC+31

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If any readers are considering visiting the Tower of London, may I suggest they also visit the adjacent Trinity House Memorial which commemorates men and women of the Merchant Navy and Fishing Fleets lost in both World Wars with no known grave. During the First World War 3,305 merchant ships had been lost with a total of 17,000 lives, almost 12,000 of these are still at sea.

http://www.ww1cemeteries.com/british_cemeteries_memorials_ext/tower_hill_mem.htm

The above site reminds me to search the Commonwealth War Graves Commission website, under Navy not Merchant Service, which advises that Able Seaman William Parsons is commemorated on the Plymouth Naval Memorial in Devon.

http://www.cwgc.org/find-war-dead/casualty/3046554/PARSONS,%20WILLIAM%20HENRY