Churchill's Aircraft Carrier Icebergs

In 1942, the Allied forces were losing a significant amount of merchant shipping in the Atlantic Ocean, due to German submarine forces and the lack of adequate air cover in the mid-Atlantic. The range of operating aircraft was not sufficient to cover this area and aircraft carriers badly needed in other theatres of operation. If only some kind of large floating platforms in the mid Atlantic could be created to carry aircraft which could assist in convoy protection. Lord Louis Mountbatten was Chief of Combined Operations and part of the work of this department was to develop technology and equipment for offensive operations. He encouraged scientists to produce their ideas, however outlandish they might seem. Many ideas did not get past the drawing stage, but others were taken up and experimented with before being abandoned. One such idea was that of an iceberg aircraft carrier, and this project was enthusiastically endorsed by both Mountbatten and Churchill.

Project Habakkuk, as it became known, was a plan by the British Navy in World War II to construct an aircraft carrier out of pykrete (a mixture of wood pulp and ice), for use against German U-boats in the mid-Atlantic, which were beyond the flight range of land-based planes at that time. The idea came from Geoffrey Pyke an English journalist, educationalist, and later an inventor.
Pyke conceived the idea of Habbakuk while in the US organising the production of M29 Weasels for Project Plough, a scheme to assemble an elite unit for winter operations in Norway, Romania, and the Italian Alps. He had been considering the problem of how to protect seaborne landings and Atlantic convoys out of reach of aircraft cover. The problem was that steel and aluminium were in short supply and required for other purposes. Pyke realized that the answer was ice, which could be manufactured for only 1% of the energy needed to make an equivalent mass of steel. He proposed that an iceberg, natural or artificial, be levelled to provide a runway and hollowed out to shelter aircraft. From New York, Pyke sent the proposal he had composed on Habbakuk via diplomatic bag to COHQ (Combined Operations Headquarters) with a label forbidding anyone apart from Mountbatten from opening the package. Mountbatten in turn told about Pyke's proposal to Churchill, who was enthusiastic about it.

Pyke was not the first to suggest a floating mid-ocean stopping point for aircraft, nor even the first to suggest that such a floating island could be made of ice: German scientist Dr. Gerke of Waldenberg proposed the idea and carried out some preliminary experiments in Lake Zurich in 1930. The idea was a recurring one: in 1940 an idea for an ice island was circulated round The Admiralty but was treated as a joke by officers.

The decision was made to build a large scale model at Jasper National Park in Canada to examine insulation and refrigeration techniques, and to see how it would stand up to artillery and explosives. At Lake Louise, Alberta, large ice blocks were constructed, and a small prototype was constructed at Patricia Lake, Alberta, measuring only 60 by 30 feet (18 by 9 m), weighing in at 1,000 tons and kept frozen by a one-horsepower motor. The work was done by conscientious objectors who did alternative service of various kinds instead of military service. They were never told what they were building. Bernal informed COHQ that the Canadians were building a 1,000 ton model, and that it was expected to take 8 men 14 days to build it. The Chief of Combined Operations (CCO) responded that Churchill had invited the Chiefs of Staff Committee to arrange for an order to be placed for one complete ship at once with the highest priority, with further ships to be ordered immediately if it appeared the scheme was certain of success.

The Canadians were confident about constructing a vessel for 1944. The necessary materials were available to them in the form of 300,000 tons of wood pulp, 25,000 tons of fibreboard insulation, 35,000 tons of timber and 10,000 tons of steel. The cost was estimated at £700,000.
Meanwhile, Perutz had determined via his experiments at Smithfield Market that the optimum structural properties were given by a mixture of 14% wood pulp and 86% water. He wrote to Pyke in early April 1943 and pointed out that if certain tests were not completed in May, there would be no chance of delivering a completed ship in 1944. By May the problem of plastic flow had become serious and it was obvious that more steel reinforcement would be needed as well as a more effective insulating skin around the vessel's hull. This caused the cost estimate to increase to £2.5 million.
In the early summer of 1943, naval architects and engineers continued to work on Habbakuk with Bernal and Perutz. The requirements for the vessel became more demanding: it had to have a range of 7,000 miles (11,000 km) and be able to withstand the largest waves recorded, while the Admiralty wanted it to be torpedo-proof, which meant that the hull had to be at least 40 ft (12 m) thick. The Fleet Air Arm decided that heavy bombers should be able to take off from it, which meant that the deck had to be 2,000 ft (610 m) long. Steering also raised problems; it was initially projected that the ship be steered by varying the speed of the motors on either side, but the Royal Navy decided that a rudder was essential. However, the problem of mounting and controlling a rudder over 100 ft (30 m) high was never solved.

Naval architects had produced three alternative versions of Pyke's original concept, which were discussed at a meeting with the Chiefs of Staff in August 1943:

Habbakuk I (soon discarded) would have been made of wood.

Habbakuk II was closest to the COHQ model and would be a very large, slow, self-propelled vessel made of pykrete with steel reinforcement.

Habbakuk III was a smaller, faster version of Habbakuk II.

Air Chief Marshal Portal asked about potential bomb damage to Habbakuk III, and Bernal suggested that a certain amount of deck covering might be ripped off but could be repaired by some kind of flexible matting. It would be more difficult to deal with bomb holes in the centre portion, though the roof over the aircraft hangars would be made proof against 1,000 kg bombs. Bernal considered that no one could say whether the larger Habbakuk II was a practical proposition until a large scale model could be completed and tested in Canada in the spring of 1944. He had no doubts about the suitability of pykrete as a material, but said that constructional and navigational difficulties remained to be overcome.

The final design of Habbakuk II gave the bergship (as it was referred to) a displacement of 2.2 million tons. Steam turbogenerators were to supply 33,000 hp (25,000 kW) for 26 electric motors mounted in separate external nacelles (normal, internal ship engines would have generated too much heat for an ice craft). Its armament would have included 40 dual-barrelled 4.5" DP (dual-purpose) turrets and numerous light anti-aircraft guns, and it would have housed an airstrip and up to 150 twin-engined bombers or fighters.

According to some accounts, at the Quebec Conference of 1943 Lord Mountbatten brought a block of pykrete along to demonstrate its potential to the bevy of admirals and generals who had come along with Winston Churchill and Franklin D. Roosevelt. Mountbatten entered the project meeting with two blocks and placed them on the ground. One was a normal ice block and the other was pykrete. He then drew his service pistol and shot at the first block. It shattered and splintered. Next, he fired at the pykrete to give an idea of the resistance of that kind of ice to projectiles. The bullet ricocheted off the block, grazing the trouser leg of Admiral Ernest King and ended up in the wall.
Max Perutz gives an account of a similar incident in his book, I Wish I Made You Angry Earlier: a demonstration of pykrete was given at Combined Operations Headquarters (COHQ) by a naval officer, Lieutenant Commander Douglas Grant, who was provided by Perutz with rods of ice and pykrete packed with dry ice in thermos flasks and large blocks of ice and pykrete. Grant demonstrated the comparative strength of ice and pykrete by firing bullets into both blocks; the ice shattered, but the bullet rebounded from the pykrete and hit the Chief of the Imperial Staff (Sir Alan Brooke) in the shoulder. Brooke was unhurt.

Later that year Habbakuk began to lose priority. Mountbatten listed several reasons: the great demand for steel, permission had been received from Portugal to use airfields in the Azores which facilitated the hunting of U-boats in the Atlantic, the introduction of long-range fuel tanks that allowed British-based aircraft extra patrol time over the Atlantic and increased numbers of escort carriers
In addition, Mountbatten himself had withdrawn from the project. The final Habbakuk Board meeting took place in December 1943 and it was announced that "The large Habbakuk II made of pykrete has been found to be impractical because of the enormous production resources required and technical difficulties involved". The use of ice had actually been falling out of favour before that, with other ideas for "floating islands" being considered, such as welding Liberty Ships or landing craft together (Project TENTACLE).