Better German naval strategy 1930-1945?

Shortround6 said:

We might have to define "particularly scarce".

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Well, compared to the Axis powers at least.

High octane aviation petrol was something of a bottleneck at various points in the war, but fuel oil? That doesn't require anywhere the level of petrochemical heroics as producing high octane gasoline. Of course getting the fuel from the refineries to where it was needed is a potential issue, but the Allies in general had quite plentiful logistics capability, again compared to the Axis. I'm not an expert so maybe I'm missing something, but I haven't read about any serious Allied fuel oil shortages.

Two of them were used in the South Atlantic to search for the Graf Spee. Part of around 20 (?) ships that participated. Surface raiders tied up very large numbers of ships.

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Yes, the surface raiders were arguably a very cost effective weapon. Particularly the auxiliary cruisers were fairly cheap compared to a "real" warship. But as mentioned in this thread before and elsewhere, improved coverage of the oceans be it with long range patrol aircraft, planes launched from ships, radar etc., meant this strategy had a rapidly approaching best-before date. Long range submarines (e.g. Type IX?) might be better at surviving, but will also have a harder time finding targets due to their conning towers sitting low over the water?

We have to be careful with statics. German only spent around 300,000 tons on major surface combatants, defined by me as anything larger Z class destroyers and a few minelayers.
40 Z class destroyers at 2500tons each only gets another 100,000tons
They did spend a lot of tons of steel on the U-boats but that is another topic. Not using U-boats really makes things simple for the British/allies.

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Yes, that matches my napkin calculations. My million ton figure was about 300k tons for the big surface ships, and then 700k tons for the Type VII and IX u-boats.

I'm not really sure what to think of the submarines, should they have spent more or less on them? On one hand they were a pretty cost effective weapon in the sense they sunk a huge amount of tonnage and kept a lot of Allied forces busy, but on the other hand they never really came close to the goal of starving the UK into submission.

Germany was really gambling as Germany depended on imported Iron ore for a lot of it's production. Just before WW II Germany used 22 million tons of iron ore but only 10 million tons were from Germany, the rest was imported. And German iron ore was not high quality stuff and it had to mixed with imported ore to make high quality steel and other elements to make armor quality steel. Germany got 9 million tons of ore in the year before the war from Sweden. Germany had gotten some of it's iron ore from Sweden even in the 1st WW so this was not new and it was well known. What should Germany do to help protect it's supply? and the problems with the Baltic freezing over for 4-6 months depending on year and where in the Baltic were also well known and that is the reason for the port of Narvick which was ice free all year (or mostly) and the reason for the railroad connecting Narvik to Sweden (but not to the rest of Norway). After the fall of France Germany just swiped a lot of French domestic Iron ore. Swedish ore stayed a major source for Germany until some time in 1944.

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Yes, roughly. It seems the Wikipedia article on the subject draws heavily on the 1965 article by Karlbom, which seems isn't the final word on the subject. This article claims that while Swedish ore helped, it wasn't that critical, pointing out what the author thinks are mistakes in the Karlbom article: https://www.tandfonline.com/doi/abs/10.1080/03585522.1973.10407767

As for what the Germans could do, hard to say. Luleå and Narvik were used because they were the only options that could provide sufficient volume. Other approaches and some issues:

• Ship the ore by train down Sweden. Then only a short hop over the southern Baltic would be needed. I suspect there wasn't enough capacity in the Swedish rail network for this, and maybe also not suitable ports in southern Sweden. They did export some limited amount of ore via Gävle and Oxelösund, somewhat north and South of Stockholm.
• Use the Swedish blast furnaces, and instead import steel ingots, which would reduce the volume of cargo. OTOH Sweden, not having any indigenous coal reserves was dependent in importing coal or coke, so in the end whatever would have been gained by importing steel ingots rather than iron ore might have been eaten up by increased volume of coal export to Sweden?

What kind of fleet should the Germans build to help insure the flow of Iron ore from the Swedish mines to Germany? It doesn't have to be deep sea/blue water but until aircraft prove them themselves the Germans have to plan on stopping British small ships from hitting the ore convoys coming down the coast of Norway. Or at least laying mine fields and other harassment.

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You can travel along islands for a fair bit on the Norwegian coast, which might make it feasible to employ minefields to prevent the British from attaching the ore ships, and also provide a lot of hiding places for things like torpedo boats that can attack British ships trying to come too close. So it might be they don't need a blue water navy to protect those convoys. They might need something though to protect the run from Norway over the Skagerrak to German ports on either the Western or Eastern side of Denmark.

But if they build the PBB's and the sisters to counter the French navy anyway, what they could sacrifice I guess are the Bismarcks and the Hippers. Build a dozen destroyers instead, minelayers, torpedo boats and such, and still save a lot of resources.

I could be looking at this wrong, but probably the most effective change in the German naval strategy should be to give Donitz what he asked for - enough U-boats (300+) - when he asked for them, so that he could keep 100+ hulls busy in anti-shipping operations at sea at any point in time. The other ~200 would be in transit to/from or repairing and resupplying. Instead, Donitz was only able to keep a maximum of around 30-40(I think) on station at one time.

Donitz's reasoning was that 100+ hulls on station would allow sinking ~600,000 tons per month. History shows that this would probably have been an underestimate of what the u-boats could have achieved. I am not sure, but IIRC even with only 30-40 hulls on station at one time they sank over 800,000 tons/month for several months during the "happy time" in 1942.

Whether this would have won the war for Germany or not is obviously open to question. Whether it would have been practical for the Kriegsmarine to operate that many U-boats in terms of suitable manpower and basing requirements is also open to question (I think), at least to a degree.

At the very least, I think it would have delayed and reduced the effects of the US and UK combining forces for a significant period of time.

There have been a number of threads on other sites about the importance of iron ore imports from Sweden, which include a lot of stats and other information. Narvik was not of such great importance as might be presumed. I found this:

"Loads of interesting stats in the tables in this article. Note the reduction in tonnage from Narvik to Germany in table 5a. (Thousand tons)

1939 4027 from a total of 5866 total export from that port
1940 504 from 1266 total export from Narvik
1941 725
1942 1140
1943 1936
1944 1106
From 1941 100% of exports from Narvik were going to Germany

Despite the significant decrease in tonnage moved via Narvik Table 5d shows how it increased in importance as the war went on accounting for 6% in 1940 but 30% in 1944. But by 1944 exports to Germany were only about one third of their 1939 levels.
https://www.tandfonline.com/doi/pdf/10.1080/03585522.1965.10414365

By 1944 allied political pressure saw a shift in Swedish attitudes to continuing to supply Germany. Add to that that from Sept 1944 when Finland switched sides, Soviet submarines could begin to roam more freely around the Baltic"

How much time, money and blood was spent trying to damage/degrade a number of German industries by even 20-50%?

The German Steel making industry was a complex subject. The Swedish ore may have been a simple solution.

Planning in 1935-38 for the navy you need 1939-41 to ensure delivery of Swedish ore requires a crystal ball. Especially trying to figure in weather, like the winter of 1940.
I have read the article linked to in an early post, it does point out a lot of the differences. It also leaves one factor out in the coal supply. In the 1930s Italy got a large amount of their coal from England. After the summer of 1940 Germany had to send large quantities of coal by rail to Italy. Which affects both the coal supply and the transport supply/capacity.
Iron ore varies as noted in some of the articles, not only in chemical composition but in the amount of actual iron per ton of ore.
There is also a vast difference between all blast furnaces shutting down and some shutting down. When you are at war with all/most of your neighbors having some of your blast furnaces shut down is not a good thing. Using work arounds only solves part of the problem. A bit like using low grade oil, you have to use extra processes/steps that require more raw materials/energy which means that something else isn't getting something it needs (coal, electricity, transport capacity).
The Swedish ore flow was somewhat seasonal. And large scale transport by rail to the southern Baltic ports (and some ice breakers?) requires a lot of coal going to Sweden.
Germany had a lot of coal, but it needs miners and trains/coal cars and river barges (and unfrozen canals/rivers) so what else/who else is getting the coal? The synthetic gasoline plants?

ThomasP said:

I am not sure, but IIRC even with only 30-40 hulls on station at one time they sank over 800,000 tons/month for several months during the "happy time" in 1942.

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To be fair, a good portion of that was off the East Coast and later in the Caribbean and Gulf of Mexico in the period where the USN lacked the will and/or the numbers of escorts to establish a convoy system. Granted that adding 150% to ongoing patrolling subs would have greatly aided Kriegsmarine efforts at any time before midwar.

Enough to knock out the UK? Maybe, maybe not, it's hard to say, between the cryptology war, the technology war, and an increasingly aggressive neutral America. Does "shoot-on-sight" come earlier if Britons are going hungry?

I used to think that spamming U-boats would be a good answer, and it very definitely has the benefits you list, but whether that's enough if USA gets involved earlier and starts spamming cargo hulls and DEs and CVEs, I think that's too nebulous to say.

ThomasP said:

I could be looking at this wrong, but probably the most effective change in the German naval strategy should be to give Donitz what he asked for - enough U-boats (300+) - when he asked for them, so that he could keep 100+ hulls busy in anti-shipping operations at sea at any point in time. The other ~200 would be in transit to/from or repairing and resupplying. Instead, Donitz was only able to keep a maximum of around 30-40(I think) on station at one time.

Donitz's reasoning was that 100+ hulls on station would allow sinking ~600,000 tons per month. History shows that this would probably have been an underestimate of what the u-boats could have achieved. I am not sure, but IIRC even with only 30-40 hulls on station at one time they sank over 800,000 tons/month for several months during the "happy time" in 1942.

Whether this would have won the war for Germany or not is obviously open to question. Whether it would have been practical for the Kriegsmarine to operate that many U-boats in terms of suitable manpower and basing requirements is also open to question (I think), at least to a degree.

At the very least, I think it would have delayed and reduced the effects of the US and UK combining forces for a significant period of time.

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Thumpalumpacus said:

To be fair, a good portion of that was off the East Coast and later in the Caribbean and Gulf of Mexico in the period where the USN lacked the will and/or the numbers of escorts to establish a convoy system. Granted that adding 150% to ongoing patrolling subs would have greatly aided Kriegsmarine efforts at any time before midwar.

Enough to knock out the UK? Maybe, maybe not, it's hard to say, between the cryptology war, the technology war, and an increasingly aggressive neutral America. Does "shoot-on-sight" come earlier if Britons are going hungry?

I used to think that spamming U-boats would be a good answer, and it very definitely has the benefits you list, but whether that's enough if USA gets involved earlier and starts spamming cargo hulls and DEs and CVEs, I think that's too nebulous to say.

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U-boat combat strength




Monthly Allied Shipping losses, annotated to show the various campaigns and other highlights


Note the effect of opening up the French U-boat bases from Aug 1940.

Right, there were more factors than simply numbers of U-boats.

IIRC during the battle of the Atlantic, about 10% of convoys were attacked, and of those, about 10% were hit (ships sunk?). So overall that would be a loss rate of 1%. Annoying, expensive, and tragic, but I imagine quite far from forcing the UK out of the war.

So what kind of loss rate would the subs have to inflict in order to achieve that? 5%, or even 10%? But expanding the sub fleet by a factor of 5-10x sounds far from realistic. So what to do?

Perhaps increase the Luftwaffe's maritime types as an assist?

z42 said:

IIRC during the battle of the Atlantic, about 10% of convoys were attacked, and of those, about 10% were hit (ships sunk?). So overall that would be a loss rate of 1%. Annoying, expensive, and tragic, but I imagine quite far from forcing the UK out of the war.

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When?

For the whole war or for certain parts of it?

See the charts in post # 166. There were periods of time (one month or several months) where losses exceeded the Allies capability to build replacement ships/tonnage.

Fortunately for the allies the corner was turned in spring of 1943 but 1942 was pretty dismal. Only 2 months when it was under 400,000 tons a month sunk.
Peak production of Liberty ships was in 1943.

U-boats were defeated by a number of paths.
Intelligence (code breaking, radio intercepts)
Different tactics by convoys
Better weapons and sensors
Increased numbers of escorts and anti-sub planes/ships.
Other?

Now we can get into the butterfly effects.
No U-boat campaign (or much reduced) means more "stuff" arriving in England and later Russia.
It means more "stuff" available in North Africa which means more "stuff" in Burma/Singapore at the end of 1941 (trickle down but there).
Stuff is aircraft, weapons, men, fuel, food, and so on.
A ship lost is more than a ship lost. It is the loss of the transport capability that ship could perform for years to come. A ship sunk in 1940 is the loss of maybe 6 round trips per year across the Atlantic?
The Allies lost around 2825 merchant ships to the U-Boats. The American built 2710 Liberty ships only in 18 yards ( British commonwealth built additional merchant ships) and Liberty ships were larger than most pre war freighters. There were hundreds if not thousands of none Liberty merchant ships built.

What could the Allies have done with the capacity if they had built 1000 fewer Liberty ships? Or 2000?

one account claims that the Allies spent 26.4 billion to "fight" the U-boats and the estimates could be way off but several thousand patrol planes, hundreds if not thousands of carrier based planes in later years?
The numbers of destroyers, Destroyer escorts, Frigates, Corvettes, and others, the numbers of escort carriers and so on.
US and Canada build tanks instead like the German advocates want to do with saved tonnage from the Kriegsmairne?
I am using German advocates only to refer to the point of view advocating for a different course of action during the war. Not saying that anyone on this board is/was advocating for any German/Nazi policies/politics. etc.

The U-boats were the only hope that the German navy had to really influence things but 5-6 years is a long time and changes in tactics, weapons, technology, supply can change the original estimates considerably. What was a good strategy in 1939/40 may be a bad one in 1944 but if you have a lot invested in it how do you get out?

Shortround6 said:

When?

For the whole war or for certain parts of it?

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It was for the whole war. And yes, as the charts show, there were periods where losses were higher, and others when they were lower. As one would expect.

US and Canada build tanks instead like the German advocates want to do with saved tonnage from the Kriegsmairne?
I am using German advocates only to refer to the point of view advocating for a different course of action during the war. Not saying that anyone on this board is/was advocating for any German/Nazi policies/politics. etc.

The U-boats were the only hope that the German navy had to really influence things but 5-6 years is a long time and changes in tactics, weapons, technology, supply can change the original estimates considerably. What was a good strategy in 1939/40 may be a bad one in 1944 but if you have a lot invested in it how do you get out?

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I wouldn't call myself a "German advocate". Germany is maybe more interesting for what-if scenarios because they were the underdog, and bungled many major strategic questions. Allies? Well, they could certainly have done better in a myriad ways, but in the grand scheme of things they did mostly the right decisions. "Win even harder" as a scenario isn't maybe that exciting.

As for building tanks instead of submarines, my point was that they were in fact quite far away from winning the battle of the Atlantic. Had they wanted to win it, they would have needed to do something dramatically different. If making 10x more submarines isn't feasible, then they would have needed to make better submarines (say, better submerged performance, snorkels, acoustic homing torpedoes, burst transmission radios, as I mentioned earlier). And also improve recon so they don't miss so many opportunities (launch a constellation of SAR satellites ?).

And of course, it isn't a given that investing more into submarine related tech wouldn't have caused the Allies to out-innovate them and do the same with ASW. Homing torpedoes, for instance, might be a great improvement for submarines, but Allied convoy escorts with improved ASDIC's and capable of launching homing torpedoes against submarines might again tilt the balance to the Allies?

z42 said:

And of course, it isn't a given that investing more into submarine related tech wouldn't have caused the Allies to out-innovate them and do the same with ASW. Homing torpedoes, for instance, might be a great improvement for submarines, but Allied convoy escorts with improved ASDIC's and capable of launching homing torpedoes against submarines might again tilt the balance to the Allies?

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Or for a slightly left field response by the Allies, ASW helicopters. Hear me out, this might actually not be as totally bonkers as it sounds:

• Choppers, check. Used by the US and Germany in small numbers during WWII. Early cold war ASW choppas like the Sikorsky H-19/Westland Whirlwind used radial engines (R-1340) that were fairly pedestrian compared to what many WWII aircraft had.
• Dippable sonars. Sonobuoys were used during WWII. So just add a cable and a winch.
• Homing torpedoes, again used during WWII.

So if in response to Germany fielding large numbers of more advanced subs, the Allies start churning out escort frigates with a helipad at the back and a hangar for, say, two ASW choppers. Sad day for the u-boats.

The helicopters available in WW2 and immediately thereafter had minimal load lifting capability. Check out the Sikorsky R-4 & R-6 (British Hoverfly I/II). The S-51/R-5 showed a bit, but not much, of an improvement becoming a search and rescue chopper.

The R-4s lifting capacity was its 2 crew and a litter for a casualty. There were trials of it in the ASW role from a ship platform but it could do little more than spot a U-boat.


When the USN version of the Sikorsky S-55/H-19 was produced from 1950, the HO4S, it could carry EITHER a dipping sonar OR the homing torpedoes to kill any sub detected, and was considered underpowered. Attention then turned to the twin rotor Piasecki HUP-1, which also proved underpowered. Only with the HO4S-3 variant with an uprated 700hp R-1300 engine, did they begin to prove adequate for the task.

Even the successor S-58/ HSS-1 Seabat, which entered service in 1955 had to operate as a hunter/killer pairing.

It was the Westland Wessex HAS.1, developed from the S-58, and with turbine power that finally combined both roles in a single airframe from about 1960. The big improvement in the helicopter as an ASW weapon only came in the 1960s with the service debut of the S-61 HSS-2 Sea King.

WW2 is just too early for an effective ASW helicopter.

z42 said:

It was for the whole war. And yes, as the charts show, there were periods where losses were higher, and others when they were lower. As one would expect.

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As the charts show, there were periods of time when the U-boat was was going well for the Germans, not so well for the allies and could be considered a viable strategy. If you average it out over the entire war with the rather dismal results of 2nd half of 1943 and all of 1944 (nothing in 1945 was going to save the Germans) then it doesn't look good at all...but.....

z42 said:

If making 10x more submarines isn't feasible, then they would have needed to make better submarines (say, better submerged performance, snorkels, acoustic homing torpedoes, burst transmission radios, as I mentioned earlier). And also improve recon so they don't miss so many opportunities

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The Germans lost the technology race. In bits and pieces and sometimes in giant leaps. When everybody was using eyeballs and binoculars running boats on the surface at night worked pretty well. Radar tilted the whole playing field. Using a radar detector didn't level the field, it just make it a bit less sloped. Improved radars tilted it even further.

British subs had slightly better under water performance than Germans but some of that was due to being a bit more streamline, like fewer AA guns and other protuberances. On the flip side the British boats were often 1-2kts slower on the surface. For better underwater performance you need better streamlining and you need larger heavier batteries. Lead acid batteries had a rather discouraging power curve for high speed. You get a lot less power out of the battery at high draw than you do at low draw. Batteries that will drive the boat at 4 kts for 15-20 hours are only good for 1-2 hours at 8-9 kts. Some of the post war 'fast' electrics were actually modified by cutting the hull in half and adding extra sections to hold a 2nd battery. They also experimented with different batteries. Homing torpedoes also had some rather big limits in the early models, The sensors were not sensitive enough which limited range on slow (quite ) targets and also limited the speed of the torpedo to speeds of 24-25kts for the German T5 torpedo. higher speeds meant the torpedo could not hear the target. Also the normal wet heater torpedo was just too noisy at any speed. Germans started development in 1936. Expense often meant that the U-boats used their 2 or 4 homing torpedoes against the escorts to try to open the convoy to attacks from normal torpedoes.

There were certain times when more FW 200s or He 177s would have given good results, late 1943 was not it. Planes from the increasing numbers of escort carriers would have countered them fairly well.

Snorkels seem to have been a bit over looked, but they also had some problems. Germans captured two in 1940 from the Dutch, several Dutch subs got away with them.
Not Invented Here or not ready for prime time? US didn't test one until 1946/47. Early German ones limited speed to about 6kts but did allow charging under water. But Snort could some times be seen at over 2 miles in daylight and did show up on radar, much less than a surfaced boat. There were problems with the engines sucking the air out of the boat when the valve slammed shut due to a wave.
For the Germans when they finally figured out they really needed to use them the Allies were on the 2nd or 3rd generation radar which worked a lot better on small targets than the earlier radars.

z42 said:

Choppers, check. Used by the US and Germany in small numbers during WWII.

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As outlined very well by EwenS it was a long development cycle to reach really effective ASW helicopters.
In part they had to develop better rotors/blades and transmissions. The engines were often not the hard part. Some of the early helicopters (S-51s) actually used rotor blades of steel, laminated spruce and doped fabric. Later ones got all metal blades. Jumping from a 180-200hp machine to 1000hp or more takes a bit work. The light weight rotor construction won't work. you have to figure out how to cool the engine at high power settings at low airspeed. You need a much different reduction gear set up and you need the whole rotor hub control set up and you have to know it won't fail. A lot harder to auto rotate than it is to glide and auto rotation only works when all the pieces stay attached.

It is also worth noting that torpedo production was a complicated matter with German torpedoes taking from 1,000 to over 3,000
man hours to produce. Not as easy to knock out of the shop as artillery shells etc.

There were also huge problems with German torpedoes in the first two years of the war. Part of the reason scores went up in 1942.
HMS Nelson was hit by three early in the war and the percussion caps disintegrated on impact. There was also a problem with the
torpedoes diving as they went and passing under the target vessel.

Doenitz was told about radar pre war, including its probable effect on night surface attacks.

Steam and Motor tonnage, 100GRT or larger, 1937, British empire 20,398,000 GRT, then over about a year the axis powers attacked Belgium 420,000 GRT, Denmark 1,118,000 GRT, Greece 1,855,000 GRT, Holland 2,631,000 GRT and Norway 4,437,000 GRT adding a nominal 50% to "British" shipping, all up allied and neutral shipping losses to end 1940 slightly under 5,000,000 GRT, while Britain built 840,000 GRT of merchant ships in 1940, the US 449,000 GRT.

According to Lloyds neutrals lost 252,135 GRT in 1939, allies 529,612 GRT, neutrals lost 678,368 GRT in 1940, allies 3,314,830 GRT, apart from the losses the maritime cause damage rate went up, Britain generally had 1,500,000 to 2,000,000 tons under repair in 1941, about half while working cargo, half immobilised.

The US Maritime Commission reports building 5,777 ships 1939 to 1945, 52,292,000 Deadweight / 39,919,000 Gross Register Tons, Lloyds has around 22,000,000 GRT of allied and neutral ships lost to war causes then comes maritime cause losses and around 1,400,000 GRT captured by axis powers.

Clay Blair notes September 1939 to December 1941 inbound convoys to UK lost 291 merchant ships to enemy action out of 12,057 in convoy, for January to August 1942, it was 30 out of 3,253. Blair notes around 900 Atlantic convoys both ways to end 1941. Switching back to Lloyds 1939 to 1941 saw 959 allied and neutral merchant ships sunk by submarines, despite the many stories of wolf pack versus convoy, and such battles did happen in this period, Blair lists 19 convoys that lost at least 6 ships and 187 between them, the majority of merchant ship losses were out of convoy. Helped by the Germans reading the Merchant Ship Code.

The 1942 balance sheet was around 8,000,000 GRT allied and neutral ships lost or capture, plus maritime cause losses, Britain built 1,301,000 GRT, the US Maritime Commission 5,411,000 GRT, plus contributions from other allies. Tanker losses September 1939 to end November 1941 were 2,105,000 deadweight tons, then 1,859,000 DWT December 1941 to May 1942, another 754,000 DWT June to August 1942. The allied tanker fleet did not regain its November 1941 strength until around November 1943, the fleet spent most of 1942 to February 1943 down a nett 1.7 million GRT.

Or to put in another way in 1942 UK fuel imports were lower than 1938, down to 10.26 million tons, a drop of nearly 3 million tons from 1941, versus 11.61 million tons of consumption in 1942. In 1943 imports were around 2 million tons more than consumption. The allied tanker fleet grew 4.2 million DWT May 1943 to May 1944 plus 0.6 million less DWT was under repair, so a gain of 4.8 million in use versus 10.4 million in use in May 1943, and the allies still considered themselves short of tankers. One hidden factor was the new tankers were faster and could load and discharge faster.

The British Bombing Survey unit figures for 1939 show Germany importing 10,937,500 metric tons from Sweden (10,069,800 tons of Iron Ore) and 1,658,000 tons from Norway (1,071,800 tons of iron ore). In return Germany exported 2,656,600 metric tons to Sweden, (1,679,300 tons of coal/coke), and 461,100 tons to Norway, (169,700 tons of coal/coke).

Given different countries had access to different types of ores their steel industries had different mixes of the various steel making processes, the Swedish ores were for the Thomas process, which required high phosphorus content and were for generally lower quality steel, the open hearth or Siemens Martin process was the major German method.

Swedish iron ore was important but made less important by the capture of the French, Belgian and other ore fields and steel mills. Though given the shortage of steel the Germans felt they had the more the occupied countries could continue with their production the better Germany would be, so the Swedish ores were useful.

In early 1944 Sweden cut Germany's quota of iron ore to 7 million tons.

Sweden deliberately declared the port of Lulea, the main iron ore port, to still be ice bound in May 1944, even though it was clear enough. In June the port was opened. The Germans had accepted the Swedish reports of bad icing. It also appears by mid 1944 that the trains to Narvik were also cut to a half or third the 1943 level.

July 1944 was the last month of "normal" Sweden Germany trade.
August 1944 Sweden banned its ships from being used for trade with Germany.
19 September Finland signed an armistice.
27 September 1944 Sweden prohibited German controlled shipping from using its ports.

According to Lloyds Sweden lost 201 ships of 478,690 GRT during the war, it had 1,494,000 GRT in 1937. Extending the U-boat war extended allied resources, it also had downsides, like Brazil declaring War on Germany in September 1942, after losing 17 merchant ships 70,529 GRT February to August, and another 3 ships of 9,521 GRT in September.

The French bauxite mines made a difference to the economics of the Luftwaffe as Germany rigged trade with France.

marathag said:

data from Wiki

General characteristics Type
Type:	Type VIIB U-boat	Diesel-electric submarine P Class SS 176-181
Test depth:	230 m (750 ft). Calculated crush depth: 250–295 m (820–968 ft)	250 ft (80 m)[3]

It's not apparent from the stats, but the US Boats were far more habitable than any German sub, had better storage for food, plus air conditioning, for comfort and removing excess humidity, so fewer electrical shorts from condensation.
Keeping humidity controlled also allowed the Torpedo Data Computer to be fitted, an analog electromechanical calculator to make targeting far easier, and during the War were retrofitted with air and surface search Radar sets.

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This was a major advantage of German U-boats - it allowed subs to much more-reliably take advantage of differing temperature water layers to break up Allied sonar signals, and to attenuate sub-generated noise below what ASDIC could usefully detect.

When the first reaction of any sub crew after sending torpedoes towards enemy ships escorted by warships is "take her down, max depth", being able to put more than 3 times the water between attackers and yourself, causing them to lose track of where you are, is as or more important than crew comfort.

EwenS said:

The Eyes and Ears of the Convoy: Development of the Helicopter as an Anti-Submarine Weapon – Igor I Sikorsky Historical Archives

sikorskyarchives.com

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Thanks, that was interesting. But it does reinforce the point I was making, namely had the Germans been a lot more successful in sending shipping to the bottom, the Allies could have made it a priority to develop ASW helicopters a lot faster. Quoting:

Had the submarine menace increased rather than declined in 1942, more resources might have been poured by the United States into the development of the helicopter as an anti-submarine-warfare weapon.

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As it was, there was no immediate need for such an expensive rapid development project.

Shortround6 said:

As the charts show, there were periods of time when the U-boat was was going well for the Germans, not so well for the allies and could be considered a viable strategy. If you average it out over the entire war with the rather dismal results of 2nd half of 1943 and all of 1944 (nothing in 1945 was going to save the Germans) then it doesn't look good at all...but.....

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Unsure what the argument here is? In order to force the UK out of the war, the subs would have to on average cause unsustainable losses.

That they maybe achieved that for some brief moments doesn't win them the battle of the Atlantic. At best it might prove that without appropriate Allied countermeasures they had the capability to do it.

The Germans lost the technology race.

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Absolutely. In general, not only for subs vs ASW, with some certainly notable exceptions, the Allies out-innovated the Germans.

There were certain times when more FW 200s or He 177s would have given good results, late 1943 was not it. Planes from the increasing numbers of escort carriers would have countered them fairly well.

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I agree, and that does present the Germans with quite a conundrum how to efficiently find the convoys.

As outlined very well by EwenS it was a long development cycle to reach really effective ASW helicopters.
In part they had to develop better rotors/blades and transmissions. The engines were often not the hard part. Some of the early helicopters (S-51s) actually used rotor blades of steel, laminated spruce and doped fabric. Later ones got all metal blades. Jumping from a 180-200hp machine to 1000hp or more takes a bit work. The light weight rotor construction won't work. you have to figure out how to cool the engine at high power settings at low airspeed. You need a much different reduction gear set up and you need the whole rotor hub control set up and you have to know it won't fail. A lot harder to auto rotate than it is to glide and auto rotation only works when all the pieces stay attached.

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That's certainly a good point. And indeed they had many great radials to choose from. Say, if the R-1340 is too weak for a full-featured ASW chopper, there's, say, the R-1820 & R-1830.