American Merchant Marine Casualties says 9,521 dead, a 1 in 26 fatality rate, higher than any other US service.
The UK history Merchant shipping and the demands of war by Behrens notes some 25,862 "British" men were lost when their ships were attacked by the enemy, this includes civilian crews and the military personnel on board manning the guns. British is basically defined as crew on board the ships from the traditional sources of such men, Oriental sailors, Lascars, made up maybe 18% of the crews. Merchant sailors tended to sign on for a voyage, there was not the equivalent of a military personnel system tracking them when not at sea.
The above figures exclude men lost when the ship that rescued them was attacked or if the seamen were officially passengers. Then the total comes to around 26,833 merchant sailors. Then add the men lost to non direct combat causes, like marine accidents or friendly mines which raises the total to 31,908. However these figures exclude at least some men who were discharged because of their injuries and who subsequently died, effectively died of wounds, or who were "permanently damaged" to the point they could not obtain life insurance or would be insurable only under special terms. The history estimates of this number comes to around 8,000 for men discharged in 1943 and 1944. The history comes up with an estimated figure of 43,886 deaths or "permanent damage" from all causes when estimating the total lost to the end of 1944, then comes the 1945 deaths, about 317 direct deaths plus those ashore etc. So maybe 45,000 deaths and permanent damage, of which at least 32,000 were deaths. Or about half the total air raid casualties Britain took during the war. Like so many statistics it is what you count and when that determines the totals.
I understand that the machinery to cut the gears for steam turbine installations took a year to make. Similarly ships lost in 1939 meant 6 wartime years where they otherwise could have made voyages. Merchant ships did not become obsolete or worn out nearly as quickly as aircraft. Convoys cut shipping efficiency, military cargo cut shipping efficiency, military operations upped shipping requirements, the pre war fleets needed to be expanded.
In the period March to May 1944 weekly imports of oil products into the UK climbed to 402,000 tons, UK oil imports in 1938 had been an average of 223,000 tons per week, or over 9 million tons less per year. Given the round trip times in mid 1944 were of the order of 5 to 7 weeks you can see the size of the tanker fleet needed, then add the usual overheads for ships needing repairs.
"British" tanker losses to end November 1941 were around 2.1 million tons, balanced by under 3.1 million tons of gains, mostly the tanker fleets of countries the Germans had invaded. (850,000 tons was new construction.). So the fleet was around 4.2 million tons. The US non great lakes tanker fleet was 2.76 million tons in 1939, plus there was another 470,000 tons under the Panamanian flag. In the December 1941 to May 1942 period the allies lost 1.85 million tons of tankers, and another 750,000 June to August mainly off the US coast and in the Caribbean. Allied tanker tonnage went backwards to at least December 1942 and it took until the end of 1943 to get back to the end of 1941 position. After that the gains were more than a million tons per quarter more than the losses. Even so to help ease the shortage tankers were allowed out of convoy when crossing the Atlantic in 1944.
By destination in May 1944 the allied tanker fleet in Gross Register Tons, ships over 1,600 GRT, was distributed as follows,
2.2 million with the navies,
1.7 million under repair, refit etc.,
3.3 million supplying England, Iceland and the USSR,
1.4 million Western Mediterranean, Azores and West Africa,
2 million Pacific including the Vladivostok run,
2 million US and Canadian Eastern Seaboard,
2 million other Western Hemisphere,
1.8 million "Indian Ocean", including Libya, East and South Africa, India, Ceylon, Australia, New Zealand,
total 16.9 million
It was only the surrender of Germany in mid 1945 that saved the allies from having to decide which theatre to slow down because they did not have enough tanker capacity to service both. The Western Allies were hampered by merchant shipping all war and invasion shipping to around end 1944, the US decision to order large numbers of LST then cancel many of them in favour of DE then cancel many of the DE in favour of LST had a real cost.
Analysis of the Battle of the Atlantic faces the reality many things kept changing at the same time. More and better radars, direction finding, code breaking, sonar, anti submarine weapons and escorts all played their part, D/F on ships was tactically quite useful, more so if there was a fast escort present to send on a search. In the period July 1944 to end of war RN depth charge attack method success rate was 6.34%, Hedgehog 29.69%, Double Squid 40.74%, Squid 30%.
The wartime U-boat fleet hits its smallest size in April 1940, 49 down from 57 in August 1939, despite adding 15 new U-boats to the fleet. While many of the merchant ships sunk were neutrals. Then came the capture of French bases, the removal of the French navy and the need for Britain to maintain more light forces around Britain. Then the British were successful in evasive routing of convoys in the second half of 1941 thanks to cracking the U-boat Enigma. In the first half of 1941 the U-boats sank an average of around 500 tons per patrol day in the North Atlantic, down to 100 tons in the second half. However the increase in escorts, a reduction in ships sailing independently while increasing the minimum speed of any independent ships were the big reasons for lowering losses. Yet also add U-boats sent to the Mediterranean and Arctic reducing those available in the Atlantic.
Despite the classic convoy battle ideas the majority of sinkings were taking place outside convoys. According to the RN official history from January 1942 to May 1943 inclusive 537 ships were sunk in convoys (411 to subs), versus 1067 ships sailing independently (934 to subs). Even in the first 5 months of 1943 the convoys lost 193 ships versus 105 sailing independently.
The U-boat fleet changed its coding system at the end of 1941 which was probably fortunate for the allies, if the U-boats had stayed on the Atlantic routes the sudden increase in sightings would have been a good clue the old code had been read. The allies took until the end of 1942 to crack the new code (4th wheel).
Lloyd's War Losses
Percentage of total shipping sunk that was neutral shipping.
percentage of ships / percentage of GRT
1939 38.15 / 32.25
1940 20.00 / 16.99
1941 5.52 / 5.40
1942 5.24 / 3.51
1943 9.48 / 2.54
1944 13.08 / 2.22
1945 4.69 / 0.29
Grand 10.66 / 7.14
Cause of loss Submarines number / GRT, 2,755 / 14,392,398, mines 781 / 1,740,919, Aircraft 1,063 / 3,213,085, surface ship or raider 223 / 1,121,296, E-boat/MTB 118 / 263,916, scuttled 117 / 143,264, other 56 / 123,197 and unknown 298 / 316,573, Total 5,411 ships 21,314,648 GRT. Of this 577 ships 1,522,432 GRT were from neutral nations. Lloyd's reports a further 753 ships 1,416,185 GRT as captured. I am not sure of the lower tonnage limits for the Lloyd's figures, but as an example they include a ship of 17 tons.
As well Lloyds list another 387 ships of 951,078 GRT as unknown reasons for loss. Cross referencing Axis Submarine Successes by Jurgen Rohwer accounts for half of these and most of the tonnage. As well other ships have notes like "German radio claimed ship sunk by aircraft". Rohwer is clearly using the Lloyds list and he has different degrees of certainty when assigning the loss. From definite report with definite identification from the submarine to submarine and ship in same area, submarine did not come back to report. So there are some "ifs" in his assignments. I make the submarines could account for 194 of these ships, 746,783 GRT, so by far most of this tonnage was lost to war causes.
The RN calculated in 1942 that each U-boat sunk saved 4.8 merchant ships because it took the Germans 6 months to train a new U-boat crew. In the second half of 1942 long range anti submarine aircraft operations were calculated to save the following number of merchant ships per 100 sorties.
Continuous Convoy Escort, 3
Bombing U-boat bases, 1
Bay of Biscay patrols, 3
Escort of convoys known to be under attack, 30.
In WWII a general cargo ship estimated cost 7 or 8 units per ton, a large gun armed warship 100 and a U-boat 150, the economic ratio was about 5 merchant ships to the submarine. A lightly escorted convoy had little chance to sink U-boats, a heavily escorted one did. A convoy with 10 escorts was needed for an average break even economic loss, not counting cargoes, going from 10 to 14 escorts doubled the chances of sinking a U-boat, going to 18 escorts tripled it.