"Winston Churchill once wrote that the only thing that ever 'really frightened him' during the Second World War was the threat presented by Germany's U-boats." (Canada Post)
German U-boats in Loch Ryan circa 1945 courtesy http://www.dailymail.co.uk/news/article-3072281/Wartime-navy-captain-blamed-letting-Nazi-U-boat-away-failed-attack-hailed-hero-oceanographer-Titanic-locates-wreck-discovers-DID-hit-all.html.
England and France owe a great debt to Canada and the United States for the supplies that they sent overseas during the Second World War. Lend Lease, an "Act to Promote the defense of the United States", provided the Allies with warships, warplanes, trucks, material, oil and food from 1941 to 1945. While Canada did not have a Lend Lease program, it did give Britain "gifts totalling $3.5 million during the war" (https://en.wikipedia.org/wiki/Lend-Lease).
Lend Lease Convoy circa 1941 courtesy https://seanmunger.com/2015/09/03/the-winds-of-war-live-blogged-8-honeymoon-in-lisbon-wine-in-boston/.
The problem for Canada and the United States was not producing the goods, but transporting them across the Atlantic. "Winston Churchill once wrote that the only thing that ever 'really frightened him' during the Second World War was the threat presented by Germany's U-boats." (http://www.communitywire.ca/en/2005-04-28/longest-battle-second-world-war-honoured-new-canadian-postage-stamp) The Battle of the Atlantic, according to Winston Churchill, "was the dominating factor all through the war. Never for one moment could we forget that everything happening elsewhere, on land, at sea or in the air, depended ultimately on its outcome." It remains the longest continuous battle of the Second World War, lasting from 1939 to 1945 (https://en.wikipedia.org/wiki/Battle_of_the_AtlanticT).
Allied tanker Dixie Arrow torpedoed in 1942 courtesy https://en.wikipedia.org/wiki/Battle_of_the_Atlantic.
Canadian forces played a crucial role in protecting Allied merchant ships as they crossed the Atlantic in convoys. Marauding German U-boats were often lurking beneath the depths of the ocean, ready to strike. By V-E Day, 22 Canadian ships had been lost and more than 4,000 Canadians had died, including 2,000 sailors,750 airmen and 650 seamen from the merchant fleet.
Canadian artist Sarty assembled several images to create a stamp design honouring The Battle of the Atlantic including a convoy of vessels, a torpedoed supply ship, a Canadian navy corvette and a German U-boat.