A slab of pykrete

Pykrete is a composite material made of approximately 14 percent sawdust or some other form of wood pulp (such as paper) and 86 percent ice by weight. Its use was proposed during World War II by Geoffrey Pyke to the British Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier. Pykrete has some interesting properties, notably its relatively slow melting rate (due to low thermal conductivity), and its vastly improved strength and toughness over unmodified (crystalline) ice, actually closer to concrete.

Pykrete is slightly more difficult to form than concrete, as it expands during the freezing process, but can be repaired and maintained from the sea's most abundant raw material: water. The mixture can be moulded into any shape and frozen, and it will be extremely tough and durable, as long as it is kept at or below freezing.

[edit] History

This section contains too many quotations for an encyclopedic entry. Please help improve the article by removing excessive quotations or transferring them to Wikiquote. Help is available. (July 2009)

Pyke managed to convince Lord Mountbatten of the worth of his project (actually prior to the invention of pykrete) some time around 1942, and trials were made in two locations in Alberta in Canada. The idea for a ship made of ice impressed the United States and Canada enough that a 60-foot (18 m)-long, 1,000-ton ship was built in one month on Lake Patricia in the Canadian Rockies. It was, however, constructed using plain ice (from the lake), before pykrete was considered. It took slightly more than an entire summer to melt.

Plain ice proved to be insufficiently strong. Pyke learned from a report by Herman Mark and his assistant that ice made from water mixed with wood fibres formed a strong solid mass—very much stronger than pure water ice. Max Perutz later recalled:

Then, one day, Pyke handed me a report that he said he found hard to understand. It was by Herman Mark, my former professor of physical chemistry in Vienna, who had lost his post there when the Nazis overran Austria, and found a haven at the Polytechnic Institute of Brooklyn. As an expert on plastics, he knew that many of them were brittle when pure, but could be toughened by embedding fibres such as cellulose in them, just as concrete can be reinforced with steel wires. Mark and his assistant, Walter P. Hohenstein, stirred a little cotton wool or wood pulp—the raw material of newsprint—into water before they froze it, and found that these additions strengthened the ice dramatically.

When I had read their report, I advised my superiors to scrap our experiments with pure ice and set up a laboratory for the manufacture and testing of reinforced ice. Combined Operations requisitioned a large meat store five floors underground beneath Smithfield Market, which lies within sight of St. Paul's Cathedral, and ordered some electrically heated suits, of the type issued to airmen, to keep us warm at 0 °F (−18 °C). They detailed some young commandos to work as my technicians, and I invited Kenneth Pascoe, who was then a physics student and later became a lecturer in engineering at Cambridge, to come and help me. We built a big wind tunnel to freeze the mush of wet wood pulp, and sawed the reinforced ice into blocks. Our tests soon confirmed Mark and Hohenstein's results. Blocks of ice containing as little as four percent wood pulp were weight for weight as strong as concrete; in honor of the originator of the project, we called this reinforced ice "pykrete". When we fired a rifle bullet into an upright block of pure ice two feet square and one foot thick, the block shattered; in pykrete the bullet made a little crater and was embedded without doing any damage. My stock rose, but no one would tell me what pykrete was needed for, except that it was for Project Habakkuk.

—I Wish I'd Made You Angry Earlier, ^[1] Perutz, Max

Perutz would later learn that Project Habakkuk was the plan to build an enormous aircraft carrier, actually more of a floating island than a ship in the traditional sense. The experiments of Perutz and his collaborators in Smithfield Meat Market in the City of London took place in great secrecy behind a screen of animal carcasses.^[2][3][4] The tests confirmed that pykrete is much stronger than pure ice and does not shatter, but also that it sags under its own weight at temperatures higher than -15 C.^[5]

Mountbatten’s reaction to the breakthrough is recorded by Pyke's biographer David Lampe:

What happened next was explained several years after the war by Lord Mountbatten in a widely-quoted after-dinner speech. "I was sent to Chequers to see the Prime Minister and was told he was in his bath. I said, 'Good, that's exactly where I want him to be.' I nipped up the stairs and called out to him, 'I have a block of a new material which I would like to put in your bath.' After that he suggested that I should take it to the Quebec Conference."

The demonstration in Churchill's steaming bath had been most dramatic. After the outer film of ice on the small pykrete cube had melted, the freshly exposed wood pulp kept the remainder of the block from thawing.

—Pyke, the Unknown Genius, ^[6] Lampe, David

Another tale is that at the Quebec Conference of 1943 Mountbatten brought a block of pykrete along to demonstrate its potential to the entourage 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 ending up in the wall. The Admiral may or may not have been impressed by Mountbatten's unorthodox demonstration. According to Perutz's own account,^[7] however, the incident of a ricochetting bullet hitting an Admiral actually happened much earlier in London and the gun was fired by someone on the project—not Mountbatten.

Despite these tests, the main Project Habakkuk was never put into action due to limitations in funds and the belief that the tides of the war were beginning to turn in favour of the Allies using more conventional methods.^[8]

According to the memoirs of British General Ismay:

A good deal of consideration, much of it highly technical, was also given to the feasibility of building floating platforms which could either be used by fighters to support opposed landings until such time as airfields ashore were available, or act as staging points for ferrying aircraft over long distances. The idea as originally conceived by a member of Combined Operations staff, and vehemently supported by Mountbatten, was that these floating platforms should be constructed out of icebergs. They would be provided with engines which would enable them to steam at slow speed, and with refrigeration plants to prevent them melting. They would be unsinkable. The whole thing seemed completely fantastic, but the idea was not abandoned without a great deal of investigation. Various alternative methods of construction were then considered by the United States naval authorities, but in the end there was general agreement that carriers and auxiliary carriers would serve the same purpose more effectively."

—The Memoirs of Lord Ismay, ^[9] Ismay, General Lord

Since WWII pykrete has remained a scientific curiosity, unexploited by research or construction of any significance. In 1985, pykrete was considered for a quay in Oslo harbour.^[10]

New concepts for pykrete however crop up occasionally among architects, engineers and futurists, usually regarding its potential for mammoth offshore construction or its improvement by applying super-strong materials such as synthetic composites or Kevlar.

[edit] Durability

The durability of pykrete is up for debate. It is often asserted that pykrete has a compressive strength of greater than 3,000 psi (21 MPa) so a very short 1 in (25 mm) column could support the weight of a typical car. However, no published figures support this. Perutz himself gives a value of around 1,100 psi (7.6 MPa)^[8]

A September 1943 proposal for making smaller pykrete vessels included the following table of characteristics. Strengths are in units of pounds per square inch:^[11]

[edit] MythBusters analysis

[edit] Television

In an April 2009 episode of the popular television show MythBusters entitled "Alaska Special 2", pykrete's properties were put through a series of tests:

• The first test involved subjecting a wastebasket-sized chunk of ice and another of pykrete to a close range impact (approximately 10 ft/3.0 m) by a .45 caliber round. The ice shattered upon impact and the bullet easily penetrated it. However, the pykrete chunk held together and successfully stopped and deflected the bullet while sustaining only a small (approximately 1 in/2.5 cm deep) impact crater.
• The second test involved comparing the strengths of ice and pykrete by placing increasing amounts of weight on a cantilevered slab (approximately 2 in/5.1 cm) thick until they reached the failure point. While the ice slab failed at approximately 40 lb (18 kg), the pykrete slab successfully supported 300 lb (140 kg) (and only failed after it was repeatedly struck with a hammer.)
• Further experimentation led to them testing a pseudo-pykrete (or super-pykrete) made with wet, layered newspaper instead of sawdust. Like the original pykrete, it held more than 300 lb (140 kg) and resisted breaking with the hammer even longer. Newspaper was used due to the longer length of individual cellulose fibers within the ice block, which allowed for greater overlapping and hence greater strength.
• The third test involved determining how long small-scale boats made of each material (ice, pykrete, pseudo-pykrete) would maintain its integrity in warm water before total failure due to melting. The ice boat failed in approximately 1 minute; the pykrete failed after 5 minutes; while the pseudo-pykrete maintained its integrity for almost 60 minutes.
• The final test involved the use of pseudo-pykrete to build a small boat. Once completed, the boat was fitted with a 500 lb (230 kg) outboard motor and tested in Alaskan waters (which were ideal as the temperature of the water is only 34 °F (1 °C). Despite carrying both the motor and the two hosts, the boat remained buoyant and functional for approximately 20 minutes and reached speeds of up to 25 mph (40 km/h). However, the boat then developed serious leaks which ended the test. After returning to shore, the boat continued to deteriorate quickly.

In the conclusion of the segment the original idea for pykrete, to build an aircraft carrier, was deemed "plausible but ludicrous" by the show's hosts. The hosts concluded that while pykrete is extremely strong and resistant, the speed at which it melts would make any ship constructed from such a material viable for only a very limited period of time, even under ideal conditions.^[12] The originally proposed refrigeration plants^[13] were not incorporated.

[edit] MythBusters web site

In additional footage shown on the MythBusters website, two additional tests were shown.^[14]

• Blocks of ice and regular pykrete were placed on nails against a board at room temperature. The ice melted significantly faster than the pykrete.
• A "shock test" was performed on the three materials by dropping a block of each approximately six feet onto the ground. The block of ice shattered into many pieces. The block of regular pykrete broke into two pieces, but did not shatter. The block of pseudo-pykrete would not break, even when slammed down from the same height.

[edit] See also

• Cement-bonded wood fiber

[edit] Footnotes

[edit] References

• Ismay, General Lord (1960). The Memoirs of Lord Ismay. Heinemann. 
• Lampe, David (1959). Pyke, the Unknown Genius. London: Evans Brothers. 
• Perutz, Max. "A Description of The Iceberg Aircraft Carrier [1948]". in Gerald Seligman. The Journal of Glaciology. Volume I (January 1947 - October 1951). The British Glaciological Society. 
• Perutz, Max (2002 - paperback). I Wish I'd Made You Angry Earlier. Oxford University Press. ISBN 0-19-859027-X. 

[edit] External links

• Pykrete - Ice Ships in the Rockies
• WWII Aircraft Carrier
• Pykrete ... or, The Myth that Wouldn't Die...