Pykrete and Habakkuk

Wednesday, November 1st, 2017

When commenter Sam J. mentioned building ships out of concrete, I went searching for an old post on Pykrete, only to find that I hadn’t ever posted anything on Project Habakkuk:

Project Habakkuk or Habbakuk (spelling varies; see below) was a plan by the British during the Second World War 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, who worked for Combined Operations Headquarters. After promising scale tests and the creation of a prototype on a lake in Alberta, Canada, the project was shelved due to rising costs, added requirements, and the availability of longer-range aircraft and escort carriers which closed the Mid-Atlantic gap the project was intended to address.


Pyke conceived the idea of Habbakuk while he was in the United States 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 were required for other purposes. Pyke realized that the answer was ice, which could be manufactured for only 1 percent 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.


The project’s code name seems to have been consistently (mis)spelled Habbakuk in official documents at the time. This may in fact have been Pyke’s own error, as at least one early document apparently written by him (though unsigned) spells it that way. (However, post-war publications by people concerned with the project, such as Perutz and Goodeve, all restore the proper spelling, with one “b” and three “k”s.) The name is a reference to the project’s ambitious goal: “… be utterly amazed, for I am going to do something in your days that you would not believe, even if you were told.” (Habakkuk 1:5, NIV)


In early 1942 Pyke and Bernal called in Max Perutz to determine whether an icefloe large enough to withstand Atlantic conditions could be built up fast enough. Perutz pointed out that natural icebergs have too small a surface above water for an airstrip, and are prone to suddenly rolling over. The project would have been abandoned if it had not been for the invention of pykrete, a mixture of water and woodpulp that when frozen was stronger than plain ice, was slower-melting and would not sink. Developed by his government group and named after Pyke, It has been suggested that Pyke was inspired by Inuit sleds reinforced with moss. This is probably apocryphal, as the material was originally described in a paper by Mark and Hohenstein in Brooklyn.

Pykrete could be machined like wood and cast into shapes like metal, and when immersed in water formed an insulating shell of wet wood pulp on its surface that protected its interior from further melting. However, Perutz found a problem: ice flows slowly, in what is known as plastic flow, and his tests showed that a pykrete ship would slowly sag unless it was cooled to –16°C (3°F). To accomplish this the ship’s surface would have to be protected by insulation, and it would need a refrigeration plant and a complicated system of ducts.

Perutz proceeded to conduct experiments on the viability of pykrete and its optimum composition in a secret location underneath Smithfield Meat Market in the City of London. The research took place in a refrigerated meat locker behind a protective screen of frozen animal carcasses.

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 pykrete would stand up to artillery and explosives. Large ice blocks were constructed at Lake Louise, Alberta, and a small prototype was constructed at Patricia Lake, Alberta, measuring only 60 by 30 feet (18 metres by 9 metres), weighing 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 eight men fourteen 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, and that further ships were to be ordered immediately if it appeared that 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 per cent wood pulp and 86 per cent 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 cold 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 addition, the Canadians had decided that it was impractical to attempt the project “this coming season”. Bernal and Pyke were forced to conclude that no Habbakuk vessel would be ready in 1944.

Pyke was excluded from the planning for Habbakuk in an effort to secure American participation, a decision that Bernal supported. Pyke’s earlier disagreements with American personnel on Project Plough, which had caused his removal from that project, were the main factor in this decision.

Naval architects and engineers continued to work on Habbakuk with Bernal and Perutz during the summer of 1943. 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, and 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 would 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.


  1. Anomaly UK says:

    The oddest part for me is the research location under Smithfield market. The use of refrigeration for the market is the justification, but it just seems so small, awkward and overly central (and vulnerable) a location. There’s a car park under the market now, accessed via a spiral road under West Smithfield Rotunda

  2. Sam J. says:

    I think if they would have taken Pykrete and used it as the material was and worked with it, it could have been a great game changer. Even if it were only good for the winter, a huge mass of them could have been mass produced and carried a stupendous amount of material. Compare Pykrete to a Liberty ship. One good hit on a Liberty ship and I’m sure it went straight to the bottom.

    I apologize about babbling so much on your blog about concrete aircraft carriers, but I posted mostly the same thing on Scipio Americanus’s site, and it didn’t show up. I wanted to get the idea out there as I had been thinking about it a while. While he may not run the Navy’s spending budget ,it’s amazing where an idea at the right time with the right person moves things sometimes seemingly inexplicably. I have a cousin who was an Air Force general, and at the top and at various choke points in the bureaucracy there’s often very few people making long-lasting decisions. Reading Jerry Pournelle for years also clued me into this, as he and others were doing very much the same sort of work as Scipio Americanus, and it gave us the Air Force we have today. My worry is that the US defense budget seems to be following the path of the book, The Rise and Fall of Great Powers, with almost complete play by play. It’s very disturbing to watch what I perceive as folly blindly unfolding.

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