The idea was to put a powerful naval force including three 28,000-ton battleships into the Baltic, attacking Germany from behind. To evade German defences in the Baltic approaches - the Kattegat, between Denmark and Sweden - Churchill proposed to send the massive warships through an unguarded but shallow channel. Normally, the channel would have been too shallow for the giant vessels, so Churchill came up with a brilliant idea: float them through buoyed up on airbags.
At the outbreak of the Second World War on 3 September 1939 Churchill was re-appointed First Lord of the Admiralty - a position with no modern equivalent but which gave him enormous power as political and military supremo of the Royal Navy. Sir David's research, which he described as "a kind of hobby", began several years ago when he was browsing through Churchill's Admiralty papers.
Like Churchill's ill-fated brainchild of the First World War, the Gallipoli campaign, the potential strategic advantages were enormous. Churchill's aim was to divert huge German naval and air forces into the Baltic, cut off Germany from the supplies of iron ore it received from Sweden, threaten Hitler from the rear and possibly bring Sweden and Norway into the war on the British side.
None of these materialised: the cost and complexity of the operation, and the risk to the force weighed against it. The first battleship to be earmarked, Royal Oak, was torpedoed the next month. But in the end , the decisive factor may not have been the cost, but the risk that a British fleet in the Baltic might start war with the Soviet Union - then a formal ally of Nazi Germany.
According to papers discovered by Sir David, Churchill's plan relied on reducing the 30ft draught of the 28,000-ton Royal Sovereign class battleships by 9ft so they could pass through "a certain channel where the depth is only 26 feet". Later correspondence reveals the channel was the "Vengeance Shoal", between Fyn Island and Zeeland.
"There are at present no guns commanding this channel", wrote Churchill, "and the states on either side (Sweden, Denmark) are neutral. Therefore there would be no harm in hoisting the armour belt [on the battleships, normally below the water-line] temporarily up to the water level."
Churchill proposed to do this with "caissons" - air-filled floats , designed to lift the giant battleships nine feet. Codenamed "galoshes", they would be attached in two layers either side of the battleships, increasing their beam from 102 to 141 feet.
Churchill recognised that a naval force operating in the Baltic would be subject to heavy air attack. He also proposed reinforcing the armour on the battleships' decks, codenamed "umbrella". The force would also need a dozen smaller vessels with strengthened bows to withstand bumping into mines, and ships designed to take on enemy aircraft, including ships carrying barrage balloons.
During December, Operation Catherine was in effect cancelled. The First Sea Lord, Admiral Sir Dudley Pound, wrote that "sending a fleet of surface ships into the Baltic is courting disaster". But On 9 April 1940, the shock news came that Hitler had invaded Norway. Thatmorning Churchill received a telegram from Admiral Lord Cork, who had been in charge of Operation Catherine: "If only Catherine had gone ahead. What an ideal force we would have together to go right in and break up the German fleet. It would have been ready, according to your last date, nine days ago."