Could Germany have successfully invaded Britain by sea without air superiority?

Nov 2009
Outer world
Germany's submarine strategy in WW II was a warmed over simulacrum of its strategy in WW I. There is an essay by an historian of German military history (Geoff Mortimer) who emphasizes that German strategic thinking was always subordinated to tactical excellence. The Germans always seemed impressive in the short run, but their strategic planning was always lacking - possibly because of inadequate resources and hubris.

I don't think the U Boat "menace" was ever going to be a game changer. As discussed elsewhere, the German navy had few operational submarines in summer, 1940 (27). By the spring of 1943, the Battle of the Atlantic was decided in favor of the western Allies. Britain did not starve; the USSR was supplied and the Mediterranean was kept open despite the U Boats. The invasion of North Africa in 1942 was not deterred by the U Boats to any degree.

Just an an additional emphasis to Germany's woeful naval strategy, their major surface units, battleships and Panzerschiffen, were all intended for commerce raiding - a naval strategy that went out with the turn of the 20th century. German naval strategy became in WW II, as it was in WW I, a waste of precious resources with little to show for it.
I agree: especially about Germans excelling at tactics and lacking in grand strategy (perhaps only the Japanese were more delusional). I also agree that, given those conditions, the Germans could not win in the Atlantic. In fact, I said that had Germany played its cards differently, then the outcome may have been different: some of these cards are in hindsight, others are rather "obvious" (or maybe I'm still blinded by hindsight) such as:
1) The most obvious ones: more u-boat. While I understand that Plan Z was scheduled to be completed in 1944, even assuming its completion would leave a few major issues: namely, doctrines (especially for the eventual carriers), tactics (Germans had no experience in this), lack of proper planes (well, they could remedy on this) and experience in their use (this was harder), as well as difficulty to replace the losses. Plus, even with Plan Z, they would be outnumbered by the Allies and, with the US in the game, massively outproduced.
2) They could have accelerated the production of U-Boats by switching to modular productions (as they did in late 1943) and increasing cooperation between shipyards.
For example, they may have invested much more and much earlier had KM's leaders understood the value of U-boats: in 1939 production was of 18 uboats (!), in 1940 50, then 199 (1941), 237 (1942, also with the introduction of Type IX). By diverting resources and, at least, resizing Z Plan they may have increased those numbers a lot.
Just consider that 32 Type VII (the workhorse of the KM) cost 24k tons of steel, which is 50 % less than what Germans used for Bismarck: this means that, envisioning the 10 main factories and the dozen-ish of smaller ones, there would be 2/3 uboats more per factory, namely 30 submarines in a year, when there were 3 submarines produced per month in 1941 (!). All of this employing 1.500 crewmen vs 2.000 of the Bismarck.
Funnily enough, this increased would not even violate the Anglo-German naval treaties signed in 1935: in 1939 the Royal Navy had 82.000 tons of submarines (circa 70 ships, mostly weighing 500-600 tons and 1500-1600), while Germany deployed 2 Type IA, 6 Type IIA, 15 Type IIB, 10 Type VIIA and 16 Type VII (only the latter 26 were ocean-going) for 28.000 tons: so adding 24.000 tons more would mean 54.000 tons of submarines, which would be largely within the limits of the treaties (63 % of the RN tonnage, when the treaty allowed for 100 %).
Keeping all of this in mid, from 1939 to mid-1941, there were on average 30 operative u-boats: considering those "actively in operation" (at sea), on average the KM had 14 (fourteen!) uboats. Using these numbers, the KM sunk 875 ships for 4.16 million tons of Allied shipping, averaging 300,000 tons per month between June 1940 and May 1941, thus outpacing the British monthly production (roughly 200,000 tons per month), meaning also 3.5 sunk-ships per submarine.
From 1939 to mid-1941 (20 months), Germany lost 42 U-Boats, i.e. one for 32 sunk ships.
From May 1941 to June 1942, there were 85 operative U-Boats, averaging 40 at sea at any time, sinking 673 ships for 3.3 million tons sunk, which maintained circa 300,000 tons per month, while seeing a decrease in tons per U-boat (7,000 tons for 1.5 ship) and an increase in losses (45 U-Boats lost, meaning 15 ships sunk for every U-Boat the KM lost).
From June 1942 to May 1943, the KM had 194 operative U-boats, averaging 97 at sea and scoring 1,047 ships sunk amounting to 5.7 million tons sunk, increasing to 475,000 tons per month, but meaning 0.9 ship (5,000 tons) per U-boat, while now American shipyards were in full production with 1 million tons being produced per month. All of this while losing 164 U-Boats (or 6 ships per lost U-boats), of these 120 were sunk by aeroplanes(75%).
From May 1943 the battle was just a massacre, as the ratio of sunk ship/U-Boat went below zero (the KM was so losing more submarines than what ships they could sink).
3) Understanding that they could not match the Royal Navy: they did not have the resources (at least not without withholding crucial resources for other branches) nor the time.
4) Understanding that Germany did not need a huge fleet: Germany did not need to control the sea, they just had to deny it. In this perspective, u-boat are the only logical choice. By diverting the resources wasted on the Bismarck and Tirpitz to U-boats, the KM may have inflicted much greater damages.
Yes, those ships tied down many British naval assets and threatened Arctic convoys but a fleet in being is not enough to subdue an enemy that cannot be defeated on land, while submarines could actually inflict disproportionately high losses, at a fraction of their cost (well, crewmen were particularly well trained, so...).
Nov 2009
Outer world
5) There were many technologies that a less conservative Kriegsmarine may have invested in and adopted much earlier: earliest prototypes of Schnorckel were being tested in 1934, the Radar was being hypothesised since 1904 and many other technical expedients required lots of time (it took THREE YEARS to devise a anti-radar rubber coating). Doctrine-wise the Kriegsmarine hardly evolved from the WWI , similarly the U-boats themselves had made little progression from WWI while the ASW means had immensely evolved. In addition, certain tactics and expedients were evidently only temporary (tinfoil balloons, rumour generators, fake targets, radar-deceiving system) or suicidal (transforming U-boats in AA fortress was insanely idiotic). Final example, the natural aim of the submarine branch (Unterseebotenwaffe) was the Elektroboote. Quoting Eberhard Rössler (possible the most important historian on German U-Boats):
""The November meeting (1942) would have ended on a disappointing note for the U-Boat Arm had it not been for two submarine engineers named Schurer and Brocking. Listening to Walter's lecture, they came up with a simplier solution to Donitz's probelm. If the Admiral wanted higher underwater speeds and longer ranges, then why not just add another hull full of batteries underneath an ordinary submarine ? The additional electrical capacity should provide the necessary power to meet requirements. A quick calculation showed the suggestion was indeed feasible. Thus at a stroke pre-world War I technology was adapted to the concept of the electro-submarine, which later became U-Boat Type XXI. In retrospect it appears strange that no one had thought of this earlier"
6) Communications were a crux: by intensely using radio communication, submarines were exposed to HDF-DF interceptions, which meant that Wolf tactics became increasingly harder to implement. I don't know much about Enigma so I don't know if the Germans could have actually imagined that the Britons had cracked their code.
In fact, quoting report SRH-023 of the US Navy:
U-boat war from beginning of 1943 to end of war.
The U-boat was at a distinct disadvantage throughout this period because it had lost its "invisibility."
Its movements, were known, whether by decryption intelligence or D/F, or radar, or sonic devices -- or by a combination of all these Allied location devices. It was the U-boat that was more and more "surprised." [...]
Throughout the search for causes of the U-boats loss of "invisibility" and "its one advantage in, combat: surprise," there was one Allied location measure which remained unknown to the Germans. Behind D/F and radar there was a further locating device, decryption intelligence, whose existence the Germans may have feared on occasions but in which they apparently never really believed. At least those responsible for final decisions never acted as though they believed in it; the changes which might have countered the Allied cryptanalytic attack were never introduced. [...]
Every kind of U-boat radiation was studied: heat and electronic which might be picked up in special allied receivers, U-boat transmitter radiations in tuning, U-boat receiver radiations, and their, own radio transmissions D/F'd by Allied intercept net. In truth, the critical radiations were the radio transmissions and the receiver was the Allied intercept net. But the Allies were not simply D/F'ing the unit transmissions, they were reading the messages -- U-boat Command's as well as those from units. U-boat Command's messages were more revealing than those from individual U-boats. [...]
During the first 5 months of 1943, the primary use of radio intelligence was in convoy diversion, for at this stage sufficient anti-submarine and escort forces were not available for an immediate and direct offensive ... The defensive use of ULTRA intelligence continued throughout the war, but with the increase of anti-submarine forces the direct offensive use became more and more characteristic. One of the first instances of the US Navy offensive use of ULTRA was the attack delivered by the USS Bogue on U-boat group Trutz early in June 1943. From that time on U.S. Navy CVE forces carried out frequent attacks, giving special attention to U-boat refueling rendezvous in the Middle Atlantic. [...]

The HF-DF system was nothing revolutionary; hence the vital crux of the KM and of Rössler point: the KM should have aimed at transitioning fast from surface tactics to actual submarine tactics (i.e. Schnorckel and Elektroboote).
7) Cooperation with the Luftwaffe was improvised (like the whole FW200 programme) and often hampered by inter-branch rivalry, while both the KM and the Luftwaffe failed to understand the vital importance of aero-naval cooperation: from arranging reconnaissance and shipping attacks to having a quota of the Luftwaffe consistently engaged (especially pre-Barbarossa) to attack British harbours and delaying convoys and their unloading.
Likes: Edratman
Jul 2009

Thanks for the extensive comments. Reprising the fleet-in-being debate, in 1939, the better strategy for the K.M. would have been a fast strike force that could sortie in the North Sea and harass the Royal Navy when and as much as it could. Just as a suggestion, the battle cruisers (or fast BBs) Scharnhorst and Gneisenau, several cruisers to 'scout' for the heavy units (they only had a handful of cruisers, so the heavy cruisers would have been useful in a strike force due to their heavier armament), and torpedo craft - sufficient destroyers and ocean-going torpedo boats to enhance offensive capabilities and to screen for the two BBs.

The French in 1939 had a similar force at Brest to act as a fast hunting group intended to counter the German pocket battleships and other raiders. It consisted of the two fast battleships Dunkerque and Starsbourg, three light cruisers and eight large destroyers. A similar fast strike force at Kiel, able to enter the North Sea from two directions (and also operate in the Baltic) would have presented a dangerous threat to both British lines of supply and communication on the Continent, as well as to merchant shipping in the North Sea. As a surface fleet, hit and run was the best the K.M. could do.

This is thinking of 1939. What had yet to be appreciated, and not well planned for, was the effect air power was to have in the next year or so. The K.M. was virtually without air cover other than in the North Sea and the Baltic. The geography and operational ranges of contemporary aircraft should have been more evident to the planners.

Whatever the technological and tactical developments, the weakness of the K.M. seems to me to dictate that they had to conserve and concentrate their available surface units and remain as much of a threat as possible, denying total control of the North Sea to the R.N.. The Germans wasted resources on only two large battleships that had no reasonable utility and that were of little real consequence, they 'exiled' the other two smaller capital ships to a location in France that resulted mostly in their becoming vulnerable to damage. The mostly useless pocket battleships would IMO have been more useful in providing gunfire support for the army on the Baltic coast.

German naval strategy would have been a failure in any event, but the surface force deployments of 1939 seem to be harebrained - and futile - at the least. My opinion.
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Ad Honorem
Nov 2010
Stockport Cheshire UK
2) They could have accelerated the production of U-Boats by switching to modular productions (as they did in late 1943)
Of the 114 boats built by modular construction from 1943 until the end of the war only 4 were considered seaworthy by it's end. The other 110 were sitting around the harbours of Germany while the workforce tried to stop them leaking because the parts didn't fit together properly.
It was a complete and utter disaster for the U-Boat arm of the German Navy
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Likes: Edratman


Ad Honorem
Oct 2010
I agree: especially about Germans excelling at tactics and lacking in grand strategy (perhaps only the Japanese were more delusional). I also agree that, given those conditions, the Germans could not win in the Atlantic. In fact, I said that had Germany played its cards differently, then the outcome may have been different: some of these cards are in hindsight, others are rather "obvious" (or maybe I'm still blinded by hindsight) such as:
And teh Allies had quite a few options to play their cards better. Like using not letting bomber command take all the VLR aircraft. Merchant carriers could have been implemented earlier. etc.
Feb 2011
Back to the OP- I would like to quote two relevant texts- Operation Sea Lion by Leo McKinstry - and the text on the Sandhurst War Game of 1974, Telegraph Magazine by Paddy Griffith Sandhurst game by Paddy Griffith.pdf and Operation Sea Lion (wargame) - Wikipedia.
Summary of the game by Richard Cox

Neither side had much clear information about the other. German attempts to install agents were all turned and the 'fifth column' proved a myth. The Germans tended to overestimate British military strength. This and unpreparedness averted any invasion in July/August, when it might have proven most lethal. The British had a good idea of the transport fleet and forces to be used, but bombing of the transports was not hugely effective and the British had a long coastline to defend but no idea when or where (or if) an invasion would come.

By September the British had disposed of General Ironside as commander, who had a very defensive mentality with multiple defensive lines envisaged -and replaced him with Alan Brooke, who favoured mobility and counterattack. British land forces were much stronger by September but still deficient in tanks, anti tank guns and artillery and spread over a wide area. Alan Brooke had mobile reserves however.

The German transports were (as already mentioned) slow and inefficient at unloading. They had developed some submersible and amphibious tanks. They had not developed fast boats for landing of elite forces, though this had been mooted. Their forces were on the whole more experienced in combat.

Both sides utilised minefields. The Germans hoped that the Luftwaffe might neutralise British naval superiority during the initial landings and buildup. It has already been mentioned that the Luftwaffe was not particularly effective in an anti shipping role, though in the course of withdrawal of the second BEF they did inflict the most destructive sinking of a single ship in British naval history - upwards of 4,000 men on the loss of the Lancastria, three times the loss on the Titanic. The sinking was hushed up.

There was a British propaganda campaign to undermine German morale based upon a claimed ability to 'set the sea on fire' using plentiful reserves of petrol. This began to have some effect. The Luftwaffe morale also began to suffer due to losses- losing 2-1 in ratio of planes downed meant a multiple of many times this in terms of lost personnel. Downed British pilots, unless badly injured or killed, could come back. Downed German aircrew were lost altogether as killed, wounded or prisoners.

The Germans planned a descent upon the South east coast by army and paratroops, also using decoy invasions possibly upon East Anglia, Iceland or Ireland to try to prevent British relocation of forces.

Churchill retained the final word, but it is very likely that in the event of an invasion he would have resorted to use of gas weapons against the invaders.

The War Game in 1974 included Galland and Ruge on its panel of umpires,. It utilised plans as known at the time, though it was not then realised the Germans would attempt landing of much support material on day one also- possibly overoptimistic. The game assumed the Luftwaffe did not have air superiority but did not attack London and that upwards of 90,000 men got ashore on Day 1.

The decisive action was British naval intervention subsequently which ultimately wrecked the support fleet. Unable to supply or reinforce the troops ashore, the invasion failed and only 15,400 were deemed to have returned to German territory.
Likes: Edratman


Ad Honoris
Jul 2013
San Antonio, Tx
The German’s won through concentration of force and speed, in their invasion it would be them who were trying to defend everything and moving very slowly, while the RN would be concentrating their superior forces and moving fast.
Also the RN doesn’t need to sink vast amounts of barges they merely need to cause chaos amongst the invasion fleet, scattering them, forcing tugs to abandon their towed barges to escape, or cause them to turn back, all this would doom the invasion.
The declassified summation and analysis of Operation Sea Lion referenced above appears to show that the Germans could never make up their collective minds about the possibilities of success for the cross-channel invasion. The operational fantasies, and the obvious real logistical issues had larger roles in the abandonment of Sea Lion than did admirals, generals and Herr Hitler.

All the resources allocated to Sea Lion would have detracted from the huge requirements for the inevitable war against Russia. And the time line was a conflict of objectives in itself.

The battle of Britain began in July, 1940. Plans were already being developed for what became Barbarossa in July, 1940. Evidently Hitler (who only listened to expert military opinion when it validated his preconceptions) had convinced himself that Britain would sue for a peace advantageous to Germany once the Soviet Union was defeated. With his intelligent but unbalanced mentality, he tended to look upon modern warfare as an exercise of arrows drawn on maps.

The Bolsheviks were always the intended major enemy. It is actually doubtful if even Herr Hitler thought a successful invasion was possible. Adm Raeder and others talking him out of it - given Hitler's mentality - may go to that argument. In summer, 1940 Britain was not a threat to Germany. The B.o.B. was a waste of German resources to no purpose.
I think it says a great deal that the Germans needed to transport very large numbers of horses across the channel. I assume they needed a very large quantity of fodder as well. Even the British rarely or never used horses. Germany's much-feared panzer forces were the point of the spear, but the spear wasn’t that big or that deep.

The Germans showed the world the potential of aggressive and well-organized and thought-through armored warfare. The rest of the undefeated world took notice - nothing stands still - and learned its lessons and riposted the Germans with large mechanized armies eventually just crushing them.