Okay, let's grant for a moment that
1) the Luftwaffe can win the battle in the air, knocking out enough of Fighter Command to make them withdraw from Southern England's airfields, without the LW itself being crippled.
So we have a damaged LW which has to maintain the following tasks:
a) maintain superiority over the RAF.
b) degrade the Royal Navy's ability to interfere with the landings and supply convoys.
c) disrupt the British Army's ability to fight, to move, and to co-ordinate against landing beaches.
You've got the priority list backwards. Picture the defences to Sealion as three legs of a stool - the RAF, the RN and the British army. Now, remove the RAF completely. Pretend not one plane existed in Britain. That's bad, but the RN and British army could still defeat the invasion, right? Now, remove the RN - no ships. Dicier, but the RAF and British army can still defeat the invasion, right? Now, remove the British army. That's checkmate. Without a British army there is no concievable way to defeat Sealion. Britain will fall.
The British army is the key leg of the stool. That is the point that obsession with the RN always misses. If the British army fails, Sealion
will win the war. If the British army succeeds, Sealion is thrown back.
So the operational objectives of the Luftwaffe during are backwards to what you wrote; c,b,a
As far as I can tell, that is an impossibly tall order - remember that this is a force which OTL failed to achieve just point a) - and I know Iain argues it was an impossibly close run thing; perhaps - so the Luftwaffe was narrowly insufficient to deal with the RAF on its own.
Think of Sealion as an invitation to the Royal Navy to prove that it can sink as many ships and do as much damage as you think it can. Sort of a
put up or shut up call to some boasting claim. Don't forget that the ditches of history are paved with navies that failed to perform at crunch time.
As the Atlantic campaign showed, it is harder to defend a convoy against attack than to attack one; there are more RN warships than KM, and the RN warships have more heavies.
As many WW2 convoy battles showed, it could prove very difficult for warships superior in numbers and firepower, even in perfect visibility conditions, to translate their advantage into heavy enemy losses when attacking convoys. And that was true even later in the war when ships fighting at night could actually
see in their environment with radar. Here, you're asking the RN to fight blindly at night with nothing but luck and some starshells against smoke screens and other visibility impediments.
The effect of the RN would probably be more to break up the organisation of the invasion fleet than do heavy attritional damage.
In this instance, where the targets are barges, and most of the defending ships S-bootes or smaller, a "heavy" is probably a destroyer and up. Now imagine the carnage when a destroyer with 4 4.7" QF guns, and 10 torpedo tubes gets within striking distance of unarmoured barges?
At night without radar pretty much 99% of all ammunition fired is going to hit nothing. If the RN destroyers were to close to point blank (where their guns could hit maybe with 10% of their shells) then they are also within lethal radius of the embarked guns of the invasion fleet.
I've seen Slapton Sands mentioned - there really is no comparison between an S-boot (2 torpedo tubes and a 37mm gun if it's lucky) attacking LSTs (Landing Ship Tank), and an I-class or up destroyer attacking Rhine barges.
During WW2 the Allies and Axis fought dozens of small sea battles using small ships and destroyers, kind of like a potential Sealion battle. In practically NONE of them, despite fire control radar and better intel, did Allied destroyers approach ANYTHING LIKE the kill ratio being glibly assumed here.
Do you have any idea how hard it is to see something like a barge at night at sea? And that doesn't assume Murphy - fog, smoke screens, or good old fashioned smoke from burning ships and gun cordite.
The LST is about ten times the displacement of a barge (4.8kT against ~.5kT, as far as I can tell), and the destroyer easily 5-times as well armed as the S-boote.
Think of S-boats something like the Ajax and Archilles at River Plate. The Exeter, (ie, the barge fleet) is wounded and helpless. The Graf Spee (ie, the RN destroyer) is trying to concentrate on sinking the Exeter. The light forces are hounding it, distracting it, preventing it from acting, causing it to be inefficient.
Or in other words, where 9 S-boote sank 2 LSTs and damaged 2 more, I would expect 9 destroyers to sink more like 100 barges and damage perhaps 100 more.
Before calling kill rates of 20:1, you might actually want to take a look at the dozens of night destroyer sea battles in WW1 and WW2 to see what sort of average kills rates were actually generated. For example, 3 Austrian cruisers in 1917 managed to sink 14 out of about 50 small lighters on the Oranto Barrage. Their "poor" performance was a result of the fact that they actually had to go out in the middle of the night and find these tiny boats, then manage to fire enough shells to sink them. I imagine that if the Austrians were fighting the Oranto Barrage on the internet, their cruisers would have posted a result of 50 kills.
And Glenn, you keep talking about how it will be a major win for the Battle of the Atlantic even if Sealion is tried and failed; it will, for the Allies. Do you expect the Kriegsmarine to be able to avoid committing U-boats to support the landings?
The RN might lose 60 destroyers in a Sealion campaign -sunk or badly damaged. The German navy lost 6 U-boats in the Norwegian campaign. That looks about right for Sealion.