Britain once planned an aircraft carrier made of frozen sawdust
In the Second World War, the inventor Geoffrey Pyke proposed building a gigantic aircraft carrier, closer to a floating island, out of ice. Plain ice was too weak. Mixed with wood pulp, though, it became pykrete: slow to melt, hard to shatter, and, according to one story, tough enough to bounce a bullet at an Allied conference.
The recipe is roughly 14% sawdust or other wood pulp to 86% ice by weight. The fibres make it far stronger and tougher than ordinary ice, comparable to concrete as long as it stays frozen, and its low thermal conductivity slows melting. It expands as it freezes, which makes it fiddlier to cast, but it can be patched with seawater. The weakness is creep: above −15 °C it slowly sags under its own weight.
Pyke won over Lord Mountbatten around 1942, before pykrete existed, and a 60-foot, 1,000-ton test vessel of plain lake ice was built on Patricia Lake in the Canadian Rockies. It took more than a summer to melt, but the ice was too feeble. The fix came from Herman Mark and Walter Hohenstein in Brooklyn, who found that stirring cotton wool or wood pulp into water before freezing strengthened it dramatically. The scientist Perutz then tested the material in secret in a meat store five floors under Smithfield Market, hidden behind animal carcasses.
Mountbatten said he once dropped a block into Winston Churchill's bath at Chequers; after the surface film thawed, the exposed pulp shielded the rest. Another account has him firing a pistol at pykrete at the 1943 Quebec Conference, where the bullet ricocheted and grazed Admiral Ernest King's trouser leg. Project Habakkuk still never went ahead, short of money and overtaken by a war turning the Allies' way.
Since then it has stayed a curiosity. A 1985 plan for a quay in Oslo was dropped, a Vienna team's 10-metre dome in Obergurgl lasted three months in 2011, and a Finnish dome collapsed within days in 2014 with no visitors inside. MythBusters built a pykrete boat in 2009 that reached 23 miles per hour before leaking after 20 minutes.
Source: Pykrete
