This was originally meant to be a "what's gotten my attention this week" post, however it started sprawling into a really long post, so I turned it into an article. It all started when I saw a YouTuber talking about the Churchill Gun Carrier. And he said something like "If the Germans had invaded we'd have been in trouble".
Now in the past I've been part of an overly long thread (which went to nearly 15000 posts) on the subject. It came about because one of the forum users was a die hard German fanboy and wouldn't take no for an answer, so in due course the rest of us had to dig up a lot of information to prove the point. And this led us to have a pretty good understanding of the subject. The YouTuber's comment got my inner-self muttering and so, here's the abridged version of Operation Sealion.

After Dunkirk and the fall of France both sides got ready for the next battles. It was Germany with the initiative, and looking at the time it took to prepare the invasion fleet and tides and moon conditions the best sort of time for the Germans to launch the invasion is about late September 1940, with about the 21st being the best combination of factors.

Now the first, and possibly the biggest myth of the entire scenario is that the German fleet, famously assembled from river barges could be sunk by a destroyer moving at high speeds. The Germans spent about 1/3rd of their fleets total carrying capacity on weight used to make modifications to the barges to improve their sea handling abilities. Unsurprisingly they also tested them, and found they did pretty well.
German Barge modifications in progress
No; Germany’s problems lay elsewhere. First Germany lacked the manpower to crew the barges. After trawling through the entire armed forces for anyone who had any experience of ship handling, even just sailing a dinghy on a lake at weekends, they were still several thousand sailors short. To complicate matters the plan for landing required the flotilla to approach the English coast then turn 90 degrees, sail parallel to the coast then to turn again to run into shore. All of that in darkness.
Next you'd have the issue of how ready to fight would the German troops be? The flotilla would have taken 24 hours to sail across the channel.

However all these issues pale in comparison to the biggest of Germany’s problems, being outnumbered. The Germans could muster ten destroyers for the protection of the invasion flotilla. Against this, in just the waters around the UK the Allies had 104 destroyers. In the area covered by the invasion alone the British had 40 destroyers. In smaller craft, such as MTB's and E-boats the situation was if anything even worse. The Germans could muster about 200 small craft. In the invasion area the British had about 2000. Some of the German plans to address this imbalance were laughable, such as the idea of taking car ferries and deploying 88mm AA guns on the decks.
This'll stop a Destroyer!
The other thing to remember was that the British had ships all over the world. The Admiralty had a codeword, Blackbird, which when received the ship was to immediately make best possible speed for the Channel. So the British forces would rapidly swell with reinforcements, while the Germans wouldn't get anything.

What about submarines though? Well here's where it gets even more interesting. At that period in the war the British actually had more submarines than the Germans! These were on patrol watching the channel ports for the departure of the invasion flotilla. When they saw it, they would radio the news back to the UK and then commence attacks on the flotilla.
From the German side things looked bad. Due to the removal of all the river barges to form the invasion flotilla, the German economy was in a dire way. Production was dropping, and in the case of torpedoes it had dropped so badly that the German stock would have been utterly exhausted by early September.

The Germans did however also plan to mine the channel, laying huge mine barriers on either side of their flotilla giving a safe corridor. Two issues here are even adding all the mines from captured and allied nations together they only had at best, half the required mines. Secondly British efforts towards minesweeping were clearing the mines faster than the Germans could get them into the sea.
Of course there's also Germany's air force, surely they could stop the Royal Navy. Well no, at the Dunkirk evacuation against destroyers moving slowly or stationary in coastal waters the might of the Luftwaffe managed to sink four destroyers. Instead they'd be up against destroyers moving at full speed in the open sea.
Equally it'd be at night. The timings were such that the forces from the north of the UK would arrive amongst the flotilla just as darkness fell, then have twelve hours to smash the flotilla moving at a speed of four knots, and get beyond the range of the Luftwaffe.
A final issue for the Luftwaffe was at the time they lacked any armour piercing bombs capable of hurting the deck armour of British warships.
Lets however ignore the above and assume the Germans made it ashore, and that they could even supply their forces (Consider the life expectancy of a German merchant ship in the Channel, or parked on the coast trying to unload without a port, with MTB wolf packs sailing about and bombers from coastal command overhead). First you have to consider the terrain. The area selected as the logical point for the invasion was as close to the French side of the Channel as possible. The British had known this was the biggest danger for over a century. During the Napoleonic Wars a large waterway was constructed as an obstacle to bottle any would be invader up in that area of the country and also be easily defensible. This is called the Royal Military Canal, during the period it had forces dedicated to manning it.
The British had dug in in depth, but in the Midlands there was a fully equipped Armoured Division waiting to counter attack. Its interesting to note that after Dunkirk British tank production increased, while at the same time decreasing the amount of light tanks they built. Against this wall of armour the Germans had a few Tauchpanzers and PAK-36 anti-tank guns. The British were so confident that during August 1940 they were shipping divisions out to go and fight in North Africa.

So where did the popular thinking on the period come from? The one about the British stalwartly defending their homes with knives lashed to broom handles? Its all a brilliant piece of propaganda designed to make the country pull together and fight. Although there was simply no threat to the UK, the appearance of a threat as displayed by the British Ministry of Information got the entire population to move from its peacetime ways of thinking onto a Total War footing, something Germany didn't manage until much later in the war. Equally the story of the few, the RAF's pilots defending the UK from certain defeat came about as the British morale needed a victory.
But of course if you don't believe me, there is one other thing to consider. In 1974 the Royal Military Academy Sandhurst held a wargame to simulate the German invasion as best as is possible. Before any accusation of bias gets levelled at the exercise there was a team of umpires. The British umpires were:
Air Chief Marshal Christopher Foxley-Norris
Rear Admiral Teddy Gueritz
Major General Glyn Gilbert

From Germany:
General Adolf Galland
Admiral Friedrich Ruge
General Heinrich Trettner

So people who had been on opposing sides, and all in position during the war. All the umpires agreed that the German force was wiped out.

Image credits:, and