RWE,
The below figures are from a variety of sites. Keep in mind that the U.S. had a total of about 6000 merchant vessels (including those existing in 1939 and those builtduringthee war); assume that that Great Britain and Canada together had the same amount (I have not been able to find figures for them). That would have 20-25% of the shipping being sunk. you reference the “happy days”, which is generally considered to be prior to the U.S. entry (different sources define that period differently). That period was not the “Allied low point” in the Battle of the Atlantic – that point was in mid-late 1942. So yes, the Germans came uncomfortably close to winning the war with their submarines.

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“Nazi Germany estimated that they needed to sink 150 merchant ships each month to starve us out.”

The below figures are for Allied ships sunk in the Atlantic.

1939 : 222 ships sunk (114 by submarine)
1940 : 1059 ships sunk (471 by submarine)
1941 : 1328 ships sunk (432 by submarine)
1942 : 1661 ships sunk (1159 by submarine)
1943 : 597 ships sunk (463 by submarine)
1944 : 247 ships sunk (132 by submarine)
1945 : 105 ships sunk (56 by submarine)

“Allied Merchant Navy shipping losses reached a peak in 1942, exceeding those recorded for any other year during the Second World War. That year total losses in all spheres of the war amounted to 1,664 merchant ships, totalling 7,790,697 gross registered tons. The majority of these losses were in the Atlantic Ocean, where U-boats sank 5,471,222 tons (1,006 ships) of allied shipping, with the British Merchant Navy bearing the brunt of these losses.”