From Derek Robinson "Invasion 1940"

For Germans:

This was the enormous gamble of Sealion. Its planners assumed that all would go well, or at least fairly well.They could tolerate losses of 10 or 20 per cent in the crossing, but they must have enough shipping to get back to France, re-load, return, and hustle reinforcements to the men on the beaches who, after 48 hours, of fighting, must be running out of strength and hope and ammunition. The RN existed to shatter that plan. Even if it sank only half the enemy ships each night, that was enough to kill off Sealion.

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What evidence is there that the RN was competent to find and to smash Sealion at night? Here is what it did in September 1940.

On 11 SEptember , every port from Holland to Cherbourg got entered and shelled. Next night, British destroyers, MTB`s and fast gunboats from the Nore, Portsmouth and Plymouth carried out what was almost a tour of inspection, entering the mouth of the Maas, Flushing and the Scheldt, Ostend, Dunkirk, Calais, Boulougne, Le Touquet and even heavily fortified Cherbourg surveying them for signs of invasion preparations and shelling...any vessels..they encountered. All the attackers got back to England unharmed.And so it continued. The biggest operation was in the early hours of 11 October. The Battleship Revenge, with an escort of seven destroyers, bombarded Cherbourg harbour and left it in flames. A week later Calais got similar treatment. A total of 45 salvoes hit the harbour. No British ships were lost.

If the RN, night after night, in fair weather or foul, could penetrate the invasion ports and sink enemy ships, there can surely be no doubt that the same British warships could find the Sealion fleets in mid-Channel, and reduce them to wreckage and corpses. The German Army and Navy expected to land 60,000 men on S-day. Few would have lived to see the dawn.

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