But WHY is operation Sealion doomed for failure, as there weren't any beach defenses in Britain in 1940? And even the Civilian Patrolling squads/ guards whatever they were called had just been formed.
True, beach defenses were fairly light. True, the BEF hadn't proved that effective in the face of the Blitzkriege... But we're not talking about half a dozen Panzer divisions teleporting to southern England and driving towards London. Until the Germans secure a major port and get it functional all German armour, munitions, supplies and artillary will be coming over the beachhead (remember, we're talking crudely modified river barges as transports, not the purpose built LSTs, LSI, etc. used by the allies in 1944)... which means the opening battles (and quite probably the entire campaign) will be mainly infantry actions, due to numerical superiority (~300,000 troops evaced from Dunkirk plus several Dominion and Indian divisions plus other British divisions training plus home guard vs ~100,000 in the German's first wave) and being on the defensive the advantage goes to the poms.
I DO think about logistics and if the air would be ruled by the Luftwaffe, the canal be guarded by U-boats and sea mines as is read in the Wiki on operation Sealine WHY is it doomed to fail?
First, the Luftwaffe is unlikely to rule the air as: (1) both sides started with roughly equal numbers of fighters; (2) the british were producing new aircraft at a higher rate than the Germans and (3) if things were going poorly for Fighter Command the remains of 11 Group would have been pulled back north out of range of the Luftwaffe but ready to be recommited in the event of a landing, not left to be destroyed.
Second, the Luftwaffe is of only moderate anti-shipping utility as: (1) the Germans didn't have many torpedo bombers; (2) Level bombers could not reliably hit manouvering warships; (3) Ju-87 Stukas did not (in late 1940) carry sufficiently heavy bombs to be effective against capital ships and (4) As shown at Dunkirk, even against light units in confined waters the Ju-87 was far from deverstating.
Finally, U-Boat are great for slaughtering lightly escorted civilian shipping moving at 10-15 knots... they're rather less use against well escorted heavy naval units moving at 20 knots.
Putting those togeather with the major differance in size between the Kriegsmarine and RN and the chances of the poms breaking through and raising havoc amoungst the German supply ships and transports are very large indeed.