HMS Sennen (Y21): The Coast Guard Cutter That Once Patrolled for Icebergs, Then Hunted U-Boats

Built to Watch for Icebergs

HMS Sennen began her career as USCGC Champlain, a Lake-class cutter built for the United States Coast Guard, launched in June 1928 and commissioned in March 1929, homeported at Stapleton, New York.

She spent twelve years in American service, including a season on the International Ice Patrol in 1933, the vital watch established in the aftermath of the Titanic disaster to warn shipping of dangerous icebergs in the North Atlantic shipping lanes.

She was built to chase bootleggers. The 250-foot Lake class came about because Prohibition enforcement had swallowed the United States Coast Guard whole: the service had borrowed a fleet of elderly destroyers from the Navy to run down rum-runners off the east coast, the destroyers turned out to be miserable sea boats for the job, and the Navy wanted them back. So the Coast Guard designed its own ship instead, ten of them, authorised and commissioned between 1927 and 1931, with a turbo-electric plant worked up by one of its own engineering officers that ran the entire vessel from a single main turbine. They were good, roomy, long-legged ships, and before Champlain was five years old the thing she had been designed to fight had been repealed out from under her.

That is how she ended up on the International Ice Patrol instead, which is the other half of the joke. A cutter drawn to intercept liquor smugglers in the Florida Straits spent a season in the North Atlantic looking for icebergs, in a patrol that existed only because of the Titanic, and then went to war against submarines. Very few ships get three unrelated careers before they are fifteen years old.

A New Name and a New War

Under the Lend-Lease programme, Champlain was transferred to the Royal Navy, renamed HMS Sennen and commissioned into British service on 12 May 1941, becoming a Banff-class sloop and joining the growing fleet of American-built escort vessels helping Britain make good its shortage of ships for the Battle of the Atlantic.

The transfer was not a trickle, it was the whole class. All ten Lake-class cutters went to the Royal Navy in the spring of 1941, and they were renamed after British coastal places to become the Banff class: Champlain became Sennen after the cove in Cornwall, Cayuga became Totland, Chelan became Lulworth, Itasca became Gorleston, Mendota became Culver, Pontchartrain became Hartland and Saranac became Banff. That left the United States Coast Guard with seventeen gunboats to cover the whole of its own North Atlantic escort commitment, at a point when the United States was not yet in the war and had no obligation to hand over anything at all. It was one of the more substantial things America did for Britain before Pearl Harbor and it is almost entirely forgotten.

Three of her sisters have their own accounts here. HMS Totland (Y88) sank a U-boat off West Africa, HMS Lulworth (Y60) sank an Italian one, and HMS Fishguard (Y59) took the crew of one prisoner. Two of the ten did not come through. Hartland and Walney were sent in to force the harbour boom at Oran on the morning of 8 November 1942, in daylight, against a French fleet that had not agreed to surrender, and both were shot to pieces at point-blank range with terrible loss of life. Sennen's war could have gone that way. It is worth remembering when reading the words "convoy escort duty".

Helping Sink U-954

Sennen went on to serve on anti-submarine escort duty, and on 19 May 1943 achieved her most significant wartime success, assisting HMS Jed in sinking the German submarine U-954 with depth charges southeast of Cape Farewell, Greenland, while escorting Convoy SC 130, a confirmed kill that stands as clear evidence of her genuine contribution to the long, hard-fought campaign against the U-boat threat.

All forty-seven of U-954's crew were lost, among them Peter Doenitz, the youngest son of the German navy's own commander-in-chief, Admiral Karl Doenitz, who was serving aboard as a watch officer. Convoy SC 130 itself lost no merchant ships at all that week, a rare clean sweep against a wolfpack of some thirty U-boats that also cost the Germans four other submarines.

What that clause about SC 130 is carrying deserves spelling out. May 1943 is the month the Battle of the Atlantic was decided, and SC 130 is the convoy where it became obvious. Thirty-odd U-boats were put onto a slow convoy of merchant ships and, over three days, the escorts and the aircraft killed five of them and the convoy did not lose a single ship. Not one. That had never happened before against a concentration of that size, and within a fortnight Donitz had pulled his boats out of the North Atlantic altogether. Sennen and Jed sinking U-954 on 19 May is one fifth of the reason.

And the death of Peter Donitz aboard her is not a footnote either. The admiral commanding the German submarine service lost his younger son in the convoy battle that broke his own campaign, in a boat sunk by a twenty-year-old American Prohibition cutter flying a British flag. He lost his other son the following year. There is no satisfaction to be had in that and it is worth recording anyway.

For almost fifty years the Admiralty had the victim wrong. Wartime records credited Sennen and Jed's attack to a different boat entirely, U-209, and it took a reassessment by the Ministry of Defence's own Naval Historical Branch in October 1991 to set things straight: U-209 had in fact already been lost nearly a fortnight earlier, on or about 7 May, in a diving accident after a Canadian Canso aircraft badly damaged her, and had simply gone silent rather than been sunk in combat. The submarine Sennen and Jed actually destroyed on 19 May was U-954, a small but genuine piece of naval record-keeping only finally corrected the better part of half a century after the fact.

East to the Indian Ocean

Sennen's war did not end in the Atlantic. By 1944 she had been transferred, together with four sister Banff-class sloops, HMS Banff, HMS Landguard, HMS Lulworth and HMS Totland, to the East Indies Fleet, escorting troop convoys out of the Royal Navy's Kilindini base in Kenya in support of the Allied advance towards Burma and Malaya. On 5 February 1944 she sailed as part of the escort for the troop convoy KR 8, alongside the heavy cruiser HMS Hawkins, before parting company off the East African coast four days later to return to Kilindini.

She remained on Indian Ocean and Far Eastern convoy duty for the rest of the war, only crossing back to home waters, and back to the American flag she had first sailed under, after Japan's surrender brought the whole conflict to a close.

Home to America

With the war finally over, Sennen was returned to the United States Coast Guard in March 1946, her original designation of WPG-319 restored, closing out her British service and returning to the flag under which she had first been built nearly two decades earlier. After a brief period in reserve, she was sold to Hughes Brothers Inc. of New York in 1948.

A Cutter With Two Flags and Two Genuine Purposes

HMS Sennen's career neatly bridges two very different kinds of maritime service, from peacetime iceberg watch duty protecting transatlantic shipping from a natural hazard, to wartime convoy escort work protecting the same shipping lanes from a very deliberate human one.

Her part in sinking U-954 stands as a genuine wartime achievement earned under a borrowed flag before she returned home to the country and the service that had originally built her, having fought her war on two different oceans and, as it turned out, sunk the right U-boat under the wrong name for the best part of fifty years.