Better German naval strategy 1930-1945?

[Admiral Beez]

The Scharnhorsts were somewhat mismatched. Overkill as just a cruiser-killer against treaty cruisers, a bit weak against the Dunkerques they were designed as counters against due to the small guns, and weak enough against a treaty battleship that the RN felt comfortable sending a single one against the Scharnhorst at North Cape.

They were also short ranged for supposed commerce raiding. According to Wikipedia, Scharnhorst's max range was 7,100 nmi 19 knots, and even less for Gneisenau at 6,200 nmi. This is not useful endurance for commerce raiding where you want to disappear into the Atlantic or Indian Oceans to prey on moderately-defended convoys. Only the MTO-focused Littorio class has lower endurance. For comparison on endurance of other pre-war battleship designs: KGV class 15,600 nmi at 10 knots; North Carolina class 17,450 nmi at 15 knots.

The Scharnhorsts need reliable diesel-powered cruising engines combined with turbines for speed and a sizable fuel load, hotel services (food, water production, etc.), and efficient displacement and hull form to give them >18,000 nmi range at >12 knots. It's approx 11k nmi (38 days at 12 knots) from Kiel to Ceylon via the Cape of Good Hope. German or cooperative neutral supply ships can meet her in the Indian Ocean to top up.

 

[z42]

The Scharnhorsts need reliable diesel-powered cruising engines combined with turbines for speed

As mentioned a few pages back in this thread, the problem with such an arrangement is that if you're a raider you need to be ready to run away at maximum speed, in case you encounter superior enemy forces. And for that you need to keep your boilers heated up and ready. So even if your diesels are very fuel efficient, you're still gobbling up boatloads of fuel oil just keeping those boilers ready.

What you really want is a CODAG/CODOG arrangement with diesels for cruising and gas turbines for high speed running, as those can push out maximum power within minutes of turning on. But that's a post-WWII thing.

 

[EwenS]

They were also short ranged for supposed commerce raiding. According to Wikipedia, Scharnhorst's max range was 7,100 nmi 19 knots, and even less for Gneisenau at 6,200 nmi. This is not useful endurance for commerce raiding where you want to disappear into the Atlantic or Indian Oceans to prey on moderately-defended convoys. Only the MTO-focused Littorio class has lower endurance. For comparison on endurance of other pre-war battleship designs: KGV class 15,600 nmi at 10 knots; North Carolina class 17,450 nmi at 15 knots.

The Scharnhorsts need reliable diesel-powered cruising engines combined with turbines for speed and a sizable fuel load, hotel services (food, water production, etc.), and efficient displacement and hull form to give them >18,000 nmi range at >12 knots. It's approx 11k nmi (38 days at 12 knots) from Kiel to Ceylon via the Cape of Good Hope. German or cooperative neutral supply ships can meet her in the Indian Ocean to top up.

Such comparisons with differences in speed are meaningless and in practice these figures bore no relationship to those achieved in service. A few things to point out.

1. Fuel burn increases greatly with speed and not in a straight line (see below re Littorio), and with the displacement of the ship. So a comparison of the Scharnhorsts at 19 knots with other vessels at lesser speeds is meaningless without a lot more information.
2. For example - Washington / North Carolina Friedman gives their service endurance at 13,500 miles @ 15 knots & 9,060 @ 20 knots; South Dakota 15,020 @ 15 knots v 10,610 @ 20 knots; Iowa 14,890 @ 15 knots v 11,700 @20 knots.
3. Ships were designed to be most efficient at particular speeds. This caused some questions to be asked when the USN ships arrived at Scapa Flow in 1942. The then DNC, Sir Stanley Goodall, pointed out that the two navies judged endurance on a different basis. The US ships were, he believed, optimised for best endurance in the range 18-25 knots while the KGVs had been optimised for endurance at full power. I recently came across a table of speed / range data for the KGVs but now can't find it. In reality at operational speeds endurance was a lot less than 15,600 miles. I'll post it if I find it.
3. No one went stooging around the oceans at 10 knots unless there was a problem or unless those ships were tied to close protection of a convoy as happened in the Med.
4. Fleet / Task Force speeds were generally above 15 knots . TF 38/58 generally operated at 15-25 knots. Off the Sakishima Gunto BPF speeds were 17-21 knots. Before racing to intercept Scharnhorst in Dec 1943, DoY had been cruising at 17 knots, later increasing to 24 knots when the weather abated.
5. Having to maintain boilers on standby for quick changes of speed in wartime increased fuel burn & reduced range.
6. What is the fuel capacity of each of the ships involved?

For Littorio (from Bagnasco & de Toro's book on the class):- range miles / speed - 4,580/16; 3,920/20; 1,770/30 assuming 4,000 tonnes of fuel.

But in practice only 3,300 tonnes were embarked so the figures became:- 4,290/13 (consumption 10 tonnes per hour); 3,690/18 (consumption 15 tonnes per hour); 3,380/20 (consumption 19.5 tonnes per hour); 2,904/22 (consumption 25 tonnes per hour); 2,275/25 (36 tonnes per hour). Consumption at "sustained combat speed" of 28 knots was 48 tonnes per hour and at max speed of 30 knots reached 65 tonnes per hour. Note how the comsumption rises from 10 tonnes to 65 tonnes per hour as the speed goes from 10 to 30 knota.

By way of comparison the North Carolina carried 6,959 tons (6,313 tonnes) as designed and the KGV 4,091 tons (3,711 tonnes) as built.

 

[Thumpalumpacus]

It's saying that turf wars are inherent in how pretty much any large-scale organization is setup. It's really up to the top management to monitor and intervene and keep down the turf wars to a manageable level so that the organization (in this case, a military as a whole) serves the purpose for which it exists. It seems Germany was doing that part quite poorly. That, as such, is not uncommon in dictatorships, as they often use a divide-and-conquer strategy with turf wars and competition for favors from the Great Leader as a way to control their underlings and keep them loyal.

As for arguments for keeping all aviation in the hands of the Air Force:

• Keeping all aviation in the hands of those who have the most experience in procuring and operating aircraft.
• Economies of scale in things like flight schools, maintenance etc.
• Procurement experience. The Air Force might not have as good an understanding of the Navy's needs as the Navy itself, but they probably understand aircraft better.
• Building bigger strike packages. You might get better results if you have, say, 2N dive bombers hitting an industrial installation today, and the same 2N aircraft bombing shipping tomorrow, than by having those strikes with 1N aircraft and the other half taking a day off as it's not their turf.

These arguments apply better for something like WWII Germany which might need a modest number of land-based maritime patrol and strike aircraft. For something like the USN which by itself is bigger than most countries Air Forces, the economy of scale benefits are marginal at that size, carrier aviation is substantially different from land-based, and the carriers operate in areas which land-based aviation cannot.

So like I said in a previous message, I don't know if the KM would have been better off with control of their own aviation or not. The fundamental problem was the relationship between the KM and LW, and that the high command (ultimately, Hitler himself) did little to fix it. Had they had their own aviation, the problems in getting the LW to do operations that helped the KM would have been replaced by other problems.

My point is that air force officers are more likely to look after air force needs first, and only then say, "Eh? What?" to the admirals. Look at the FAA, or even the US Army's reluctance to devote too many resources to coastal patrol in 1940-1942. Both circumstances greatly hampered the respective navies involved.

And especially with Germany, look the dive-bomber, tactical AF staff and procurement strategies involved, and now that they're procuring for KM needs, I'd be skeptical.

 

[marathag]

The Royal Navy built two hydrogen peroxide fueled submarines after WWII

Explorer class Submarines (1954)

The Explorer class Submarines (1954)...

naval-encyclopedia.com

Nicknames in Service
HMS Explorer = Exploder
HMS Excalibur = Excruciator

 

[WARSPITER]

Nicknames in Service
HMS Explorer = Exploder
HMS Excalibur = Excruciator

Two more were planned. They were to be named Brighter and Whiter.

 

[GrauGeist]

Also, seems much of the German economic rise and rearmament miracle of the 1930'ies was a castle built on sand. Had they not started the war when they did, they would have defaulted on their foreign debts and the German economy would have collapsed

Hitler suspended the war reparations in 1933 and had no intention of ever repaying it.

 

[Thumpalumpacus]

Two more were planned. They were to be named Brighter and Whiter.

I love RN nicknames, they're much wittier than our American nicks for the most part ... "Swayback Maru" and "Dry I" excepted. But for the conversion carriers, "Curious", "Outrageous", and "Uproarious", that's golden stuff there.

 

[don4331]

They were also short ranged for supposed commerce raiding. According to Wikipedia, Scharnhorst's max range was 7,100 nmi 19 knots, and even less for Gneisenau at 6,200 nmi. This is not useful endurance for commerce raiding where you want to disappear into the Atlantic or Indian Oceans to prey on moderately-defended convoys. Only the MTO-focused Littorio class has lower endurance. For comparison on endurance of other pre-war battleship designs: KGV class 15,600 nmi at 10 knots; North Carolina class 17,450 nmi at 15 knots.

The Scharnhorsts need reliable diesel-powered cruising engines combined with turbines for speed and a sizable fuel load, hotel services (food, water production, etc.), and efficient displacement and hull form to give them >18,000 nmi range at >12 knots. It's approx 11k nmi (38 days at 12 knots) from Kiel to Ceylon via the Cape of Good Hope. German or cooperative neutral supply ships can meet her in the Indian Ocean to top up.

Problem 1: Diesel engines (the 30s MAN ones) burn diesel. The boilers burn fuel oil. Neither may use the other fuel. How much of each to do you load into your CODAS ship? What happens if you get it wrong?? e.g. cruiser Karlsruhe could travel 18,000nm@10kts if all 1,350 tons were diesel, but max speed would be 10.5kts versus 7,200nm@17kts if all fuel oil. But range drops to only 2,760nm@15kts under wartime conditions*
Problem 2: Feathering propellers of 30s have a maximum capacity of ~12k hp. So, your high speed turbine propellers must be disconnected when cruising or the 'cold' blades will destroy the turbine. Similarly, the diesels must be disconnected before high speed running or they will be over rev'd and engine will be spitting rods. And the disconnecting/reconnecting needs to be done at slow speed e.g. <10kn. It also takes >20 minutes to disconnect the diesels/connect and warm up the turbines. So, during the war, ships are running only on turbines. (Scharnhorst and Gneisenau actually disconnected the outer shafts when cruising to improve efficiency and basically stopped to reconnect them. With a lot of praying that nothing bad comes over the horizon while its taking place).

*RN figures show similar drops in range when prepared for combat. Range numbers in Wiki are for peacetime, clean hulls, not war conditions/6 months out of dock in tropical water.

Scharnhorst class has a terrible gestation. Raeder ordered the 2 D class ships, hulls were laid down in Feb. '34.

Now when we understand ship construction and the project planning of same, we know that the laying down ceremony is a formality.​

There are already 100s (1,000s) of tons of steel assembled, ready to install the day after the dignitaries have left and real work commences (there are photos of the bow section ready to be installed waiting slip side).​

For 2 ships * 2 turrets * 3 guns/turret, plus a couple proof barrels need to be ready for when the trial assembly of the turret is done, after which it is disassembled, transported to the fitting out dock and assembled into the hull. And you want this all ready by the time the hull is launched (Plan was ~15 month build time from laying down to launch based on historic Scharnhorst). Knowing the guns were actually ordered in Nov. 33, and assuming Krupp can manufacture 1/month, the 12 barrels would be ready by Nov. '34, assembled in turrets and tested by Mar' 35 then disassembled and delivered pier side for the May, '35. By the Jun, '35 launch, the initial reassembly would be complete and they could be installed into the awaiting hull. The plan comes together on schedule​

​

For the D class, there were 2 options - using 8 - 12 cylinder 42/58 diesels as in previous Panzerschiffe, which would provide 80k hp versus medium temperature/pressure boilers/turbines which would provide 125k hp. The extra ~50% power makes the ship ~3kts faster (32 vs 29kts). And the steam/turbine team was offering cruising turbines which would provide the same range as diesels. So, steam power was selected.​

At the time, it wasn't considered that the 2 -12k hp cruising turbines would put ~25k load on the main turbines (Vulcan clutches work equally driven as driving/ cruising turbines must be kept from over-speeding aka braked) making the power difference much smaller. And during cruising, the main turbines needed some steam to keep them warm, so they don't eat themselves, so the range wasn't as good as predicted. As a result the historic Scharnhorst class ditches the cruising turbines and take 25% range hit of using main engines all the time.​

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KM also doesn't know getting fuel oil in WWII is going to be as problematic as steaming coal was in WWI.​

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Now, Raeder has been campaigning coaxing Uncle Adolf for more firepower i.e. either 33cm or a 3rd turret in Panzerschiffe D, finally selling it on KM ship will have 9 guns to Dunkerque's 8 (RN campaigning for 12" main guns for future capital ships has make 33cm guns undesirable). So, Uncle Adolf asks a couple simple questions:

What is distance between superimposed turrets - based on ugly twins its 19m.​

How much longer does the hull need to be for the 12 cylinder engine...well actually as the beam has increased by 4m and the auxiliary engines are 6M32/44 vs 5M42/58, the port and starboard powerplants fit side by side, not one in front of the other, so with the juggling of the electrical generation engines, the engine compartment is shorter! And should steam be chosen instead, the same power or more could be accommodated in this space.​

But KM wants turrets not mounts and additional HAA, in the extra space so you can't shorten hull around powerplant compartment.​

So, 181m for AGS + 19m = 200m; plan for Panzerschiffe D is 207m; AGS is 11k tons, Panzerschiffe D is 18k tons for 50% additional main armament; displacement seems inline. Uncle Adolf doesn't see any issue with 3 turrets and approves. (OK, there's doing to be slight delay to rearrange forward barbette(s) but the naval architect gets paid to fix that PDQ). **​

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Based on the above Raeder stops construction on the D class in July, '35, with the hull for Panzerschiffe D >40% complete, with another 20+% material worked/on order. Krupp has finished 8+ 28cm barrels with another couple in process and Deschimag/Brown Boleri have boilers ~50% complete. Krupp/Deschimag/Brown Boleri as private firms (and in Brown Boleri foreign country) have airtight contracts in place that they will deliver and be paid for what was ordered.

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And the Panzerschiffe redesigned into Schlachtschiff (not really what was authorized) needing to reused as much of the material as possible - specifically the 28cm guns and the boilers/turbines. Increasing armour from 220m belt/70 mm deck to 320mm/105mm respectively causes the weight to grow from 18k tons to 26k tons. To maintain speed, a 3rd boiler/turbines set is added. This increase length from 207m to 231m, which adds weight, but worse, it means the keel must be deeper so the hull doesn't fold in 2. Which raises the boiler and the armoured deck over them. As the whole process is rushed - the ships being a 18 months behind original schedule - no one notices this has put a gap in the armour until it is too late (an Achilles heal worse than Hood; sort of similar to the mismatch between barbettes and turrets for Iowa class where everyone one wasn't communicating.).​

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So, if you're going to build additional Scharnhorst class and/or up gunned versions i.e. 3 x 2 - 38cm, there is going to be huge push from the design office to clean up the mess. And as a result they will be entirely different class.
Aside: 12 - 42/58 diesels (120k hp) would give just over 28kts based on the speed/power curve I have for Scharnhorst in Whitley German Capital ships; range would be roughly double as diesels are that efficient at cruise.

**It would have been interesting to see what a 3 turret Panzerschiffe D would have turned out like...probably as messed up the historic Scharnhorst class.

My 64 Reichsmarks question: We know Raeder was planning Panzerschiffe F (at least) under Versailles to be a treaty cruiser (8" guns/10k tons). How is Panzerschiffe D, even in original form, not superior?

 

[Thumpalumpacus]

Hitler suspended the war reparations in 1933 and had no intention of ever repaying it.

As much as you're right about Hitler's not repaying reparations, von Neurath warned Hitler in 1938 that the economy was overheating and that foreign reserves were running out -- that the military buildup could not last much longer.

 

[Thumpalumpacus]

France's quick surrender must have emboldened them. Poland bravely fought the might of both Germany and the USSR for 26 days. Meanwhile the seemingly world military power of France lasted only nine days more. Tell Mussolini to remain neutral so to avoid distractions in North Africa and the Mediterranean, and don't declare war on the USA in Dec 1941 (the staff at the German embassy in Washington who understood US resolve and capability must have thought Hitler was insane). Then, go for it all against the USSR.

With Britain undefeated, you've not only got their blockade, you've just cut off your own source for grain, oil, and links to the Far East for rubber.

Attacking Russia with the UK still standing was Hitler's stupidest decision. He committed at that point to a two-front war.

 

[SaparotRob]

I love RN nicknames, they're much wittier than our American nicks for the most part ... "Swayback Maru" and "Dry I" excepted. But for the conversion carriers, "Curious", "Outrageous", and "Uproarious", that's golden stuff there.

But what could match the poetic sweetness of "Whistling Sh!# Can"?

 

[Thumpalumpacus]

But what could match the poetic sweetness of "Whistling Sh!# Can"?

Wild guess, USS Wichita?

 

[z42]

Problem 1: Diesel engines (the 30s MAN ones) burn diesel. The boilers burn fuel oil. Neither may use the other fuel.

Why wouldn't the boilers be able to use diesel? I would imagine they are quite flexible? Fuel oil would be preferred due to cost, but I don't see why diesel wouldn't work in a pinch?

As for diesel and heavy fuel oil, post-war low and medium speed diesel are able to use it. In the interwar era there were low speed diesels running on HFO, but I'm not aware of any medium speed diesels using it. Not sure what innovation was needed to enable that, maybe more efficient filters?

My 64 Reichsmarks question: We know Raeder was planning Panzerschiffe F (at least) under Versailles to be a treaty cruiser (8" guns/10k tons). How is Panzerschiffe D, even in original form, not superior?

Presumably you're angling for something different, but the obvious answers would be

• Per AGNA the D class would eat into capital ship tonnage, and they wanted to reserve that tonnage for "real" battleships?
• In the same sense a fast battleship is superior to a cruiser. But also more expensive to build and run. Of course, the Hippers ran way past the treaty tonnage limit, so the D class would presumably not be that much more expensive.

 

[ThomasP]

There is no particular difficulty using diesel in ship's boilers. It was actually tested in larger ship arrangements (before WWII), and done occasionally in smaller combatants. My understanding is that the only needed modifications could be done relatively easily, by the ship's crew. I am not sure if I am using the correct terms here (it has been a long time since I read about this) but the fuel sprayers (nozzles?) in the boilers had to be changed out, and the fuel flow rate had to be adjusted. IIRC that is all as far as using diesel in boilers that usually used bunker fuel. I do not recall any mention of different filtering being required - I think the sprayers (nozzle) openings were not small enough for that to be necessary.

 

[Admiral Beez]

Attacking Russia with the UK still standing was Hitler's stupidest decision. He committed at that point to a two-front war.

But there was no way Germany could defeat the UK militarily. But there is a way to take Britain out of the war and free up all forces for Barbarossa. 22 June 1940 Armistice is signed with France, but with a twist of Otto Von Bismarck's making. Germany demands that France become neutral in exchange for a complete German withdrawal from French territory. No Vichy or puppet government, France can govern how they'd like as long as the country remains neutral. France agrees, and German forces march out of the country on 1 July 1940. A week later Hitler orders a halt to all Luftwaffe strikes on the UK as well as ordering the U-Boat and surface fleets to return to base, followed by Germany offering an armistice with Britain. To sweeten the deal, Germany makes a resource supply deal with the government in Oslo and withdraws all German forces from Norway. With a now neutral and free France in Europe, the British people will be wondering why they're still fighting Germany, and Churchill's hold on government becomes shaky. By the autumn of 1940 Churchill's governing coalition falls and Labour led by Clement Attlee takes over, accepting the Armistice with Germany.

Now Germany still have a major economic problem, that without stealing the resources and wealth of France, Norway, etc. the country will soon go broke. But they've also removed the two-front war and can go for broke against the USSR. Again, a smart Germany may use wealth torn out of the USSR and Poland to purchase armaments from France. Now, what about the Danes, Dutch and Belgians? Once the USSR is defeated and its lands subjugated by the Germans, Berlin can then turn around and go to war against France, et al again.

 

[Shortround6]

As for diesel and heavy fuel oil, post-war low and medium speed diesel are able to use it. In the interwar era there were low speed diesels running on HFO, but I'm not aware of any medium speed diesels using it. Not sure what innovation was needed to enable that, maybe more efficient filters?

Bunker C is about one grade thinner than asphalt. Slight exaggeration

It needs to be heated, usually with steam pipes in order to get it to flow, especially in ships operating in cold water.

What they could do with it post WW II is not what they could do with it in the 1930s. Yes with good fuel heating and good filters you run diesels on it.
But at what cost (space and maintenance) in the 1930s.
In the history of marine engineering they often used commercial ships for testing. Once they got something to work in the lab/shop/test bed they stuck it in one or more small ships to see how well it actually worked in service. Ferries and coastal passenger ships were popular. If they broke they weren't too far from shore and/or were not far from regular maintenance.
German navy historically thought their machinery builders could do better than they actually could or at least they hoped they could.
With the needed power rising at exponential rates successful commercial use was no guarantee of successful high speed military use in early adaptations.
First commercial ship with steam turbine.

a little under twice the tonnage of the HMS Viper and 1/3 the power, 1901.
It was used for speed, fuel consumption and other performance evaluations like weight and space of engine rooms compared to other steam plants. It also stayed in service until 1951/52. Within 5 years the Cunard line specified turbines on the Lusitania and Mauretania (took about 5 years to complete).
The Steam turbine worked out rather well, but a high powered triple expansion steam engine had some drawbacks, especially for high speed use.
Pushing the boundaries does not always work so well even though the goal does get reached.

 

[don4331]

Why wouldn't the boilers be able to use diesel? I would imagine they are quite flexible? Fuel oil would be preferred due to cost, but I don't see why diesel wouldn't work in a pinch?

As for diesel and heavy fuel oil, post-war low and medium speed diesel are able to use it. In the interwar era there were low speed diesels running on HFO, but I'm not aware of any medium speed diesels using it. Not sure what innovation was needed to enable that, maybe more efficient filters?

There is no particular difficulty using diesel in ship's boilers. It was actually tested in larger ship arrangements (before WWII), and done occasionally in smaller combatants. My understanding is that the only needed modifications could be done relatively easily, by the ship's crew. I am not sure if I am using the correct terms here (it has been a long time since I read about this) but the fuel sprayers (nozzles?) in the boilers had to be changed out, and the fuel flow rate had to be adjusted. IIRC that is all as far as using diesel in boilers that usually used bunker fuel. I do not recall any mention of different filtering being required - I think the sprayers (nozzle) openings were not small enough for that to be necessary.

If you plan to have a flex fuel (HFO and Diesel) in your boiler, it is relatively easy.

As T ThomasP says at the end of the line, the nozzles needed to be changed as the fuel/air ratio is different do to the different carbon/hydrogen ratio.​

But before that you need to modify the fuel flow. As HFO is extremely viscous at room (or N. Atlantic sea water) temperature, it is pumped through a heater to allow it to flow better. If you heat diesel under pressure it starts separating into even lighter petroleum products...and as the Japanese found when they tried to burn diesel in unmodified boiler, they explode!​

There's also issues with the seals that contain HFO on your pumps may not prevent diesel from leaking*...and getting everything coated in diesel is asking for an engine room fire.​

​

*Much like the seals which kept water in the ICE engines allowed ethylene glycol (anti-freeze) to leak out. And ethylene glycol on the exhaust manifold was a fire waiting to happen.

The post-war medium speed diesels run an order of magnitude higher injection pressure which allows them to run HFO. The materials to allow those pressures didn't exist in pre-WWII times.

 

[don4331]

Presumably you're angling for something different, but the obvious answers would be

• Per AGNA the D class would eat into capital ship tonnage, and they wanted to reserve that tonnage for "real" battleships?
• In the same sense a fast battleship is superior to a cruiser. But also more expensive to build and run. Of course, the Hippers ran way past the treaty tonnage limit, so the D class would presumably not be that much more expensive.

But when D class is being laid down, for all intents and purposes, even when Scharnhorst is laid down there isn't an AGNA. There might never be an AGNA in a better naval strategy and/or the class restrictions might be done much differently e.g. much lower carrier limit (say 25%) for higher light cruiser limit (say 45%).

And I'm still sure, I can replace the triple 28cm turrets with dual-twin 20.3cm ones on the Panzerschiffe to get the reclassified as treaty cruiser if capital ship tonnage becomes a problem.​

Raeder has spent over a 100 million Reichsmarks, and more/less a year's production on Panzerschiffe D. P!$$ing it away is something Germany can't afford.​

 

[don4331]

With Britain undefeated, you've not only got their blockade, you've just cut off your own source for grain, oil, and links to the Far East for rubber.

Attacking Russia with the UK still standing was Hitler's stupidest decision. He committed at that point to a two-front war.

With 20/20 hindsight, we know going to war against Stalin was stupid decision, but ~25 years earlier, the Kaiser went to war against the Czar and 18 months later had a peace treaty in hand, with Russian paying huge penalty.
The issue in WWI was the French, in 4 years basically nothing had been decided.

So, in WWII, when the French cave in a couple months why wouldn't you think that the Soviets could be overcome quickly as well?

If Stalin surrenders Ukraine and Caucasus oil fields, Germany has the oil and grain it needs. If you have oil, synthetic rubber can be manufactured.​

​

At this point, the blockade is just a economic burden on UK and Hitler may open negotiations on his terms.