Pykrete

— Pete's Personal Perspective —

... or, The Myth that Wouldn't Die...

When I was a kid, I was greatly taken with the idea of an iceberg aircraft carrier — helped I'm sure by at least one adventure serial that featured the idea. I remember an argument or two with my Dad, who had a somewhat different perspective (as you can see from his article)! From the steady stream of hits on this site, I guess it still fascinates... (:-))

Years later, I suppose I have to accept the impracticality of the concept, even though there's still that sneaky thought that it's a pretty cool idea. [Pun probably intended, but why not...?] However, reading the various postings that fly around the net, I see a certain amount of mythtifcation [Eh — I'm doing it again] on the topic of Geoffrey Pyke, Pykrete, and Habakkuk, so it's probably worth coming down to earth a bit and checking some history.

The Myths

The first thing to make quite clear is that Pyke himself did not invent Pykrete! The idea came fairly late on (Spring 1943). The original discovery was by a couple of researchers in New York, and it was worked on further by later-Nobel-prize-winner Max Perutz (see below) in London.

As I understand it, Pyke simply conceived of using plain ice to build a ship — probably thinking that blocks would be carved from the Arctic ice sheet. After all, icebergs do survive fairly well into temperate waters, and perhaps if they were refrigerated and fitted with propulsion...

Another misunderstanding that ought to be squelched is that Pykrete was used in the Lake Patricia pilot project in the Canadian Rockies. In fact that test 'ship' was built from plain ice cut from the lake — the experiment was under way (early 1943) before Pykrete was thought of. See the link above, and there is also another article on the project that has a good description of it and the actual time sequence. So the idea that it was Pykrete that made it "last through the summer" is pure nonsense. That might be more to do with the refrigeration equipment installed! (Though that was turned off before long.) Anyway, a large mass of ice is quite good at surviving if it is well insulated: we kept the icebox in my uncle's cabin, at much-lower-altitude Lake of the Woods, stocked from Ken Whitie's icehouse across the lake, which also was simply filled with ice from the lake during winter and provided for the community all summer.

I tend personally to seriously doubt the story of Mountbatten dumping a chunk of the stuff into Churchill's bath. For a start, as the project had been initiated — with Churchill's blessing — before Pykrete was invented, he wouldn't have needed to convince him that way. As the true properties of Pykrete aren't that magical, it wouldn't have proved much. The tale could be true, but the only reference I've heard of is David Lampe's book on Pyke. I never heard it from my father, or from any other source.

(I also notice that Lampe is quoted in the Wikipedia article on the subject as claiming that the name 'Habakkuk' was from Voltaire's "Candide", not the biblical character. As the word does not actually appear anywhere in that text, this seems just a tiny bit unlikely! I begin to suspect that Lampe's book is a nice bit of creative writing, with occasional correspondence to fact...)

'Habbakuk' or Habakkuk'?

This word is one of those where I, at least, find it a bit hard to notice misspelling, so confusion isn't surprising, and if you web search for the wrong ('two-bs-one-k') spelling, you'll find quite a few, quite unrelated to the ice ship!

The 'one-b-two-ks' (well, actually three, but I don't count the last...(:-)) is 'biblically' correct, and both Goodeve and Perutz conform to it. However, the spelling seems to have been corrupted at some point in the chaos of wartime — quite probably by Pyke hmself. In most of the actual wartime documents it is in fact spelt 'Habbakuk', exemplified by the draughtsman's labelling of the project drawing. [In earlier editions of this page I claimed, based on the sources I knew, that the project was titled the 'proper' way. A visit to the National Archives to read the originals proved I was wrong.]

Experimental Approach

Of course Pykrete is hardly an exotic material, so if you really want to know what the stuff is like, it's simple: make your own! Mix some wood pulp with water, and freeze it. Wood pulp might not seem too easy to get hold of, but it's really just a precursor to paper, so I suggest torn-up newspaper should do just about as well. (Some recent experimenters used sawdust, but I suspect that's less fibrous — see below. Might be good enough, though.) The exact proportions used in the original composite are vague, but 10% by weight is a good starting point. (Perutz says that 4% is sufficient.)

To date I've only done very quick and rough experimenting (taking the newsprint approach). My samples were mostly ice-cube sized [for probably obvious reasons]. With those, I could see no superiority of Pykrete as far as melt-resistance goes. Letting two 40g samples — one plain ice and the other Pykrete — sit side by side on absorbent paper, the Pykrete retained its shape and weight while the ice melted away, but in fact the Pykrete turned out to have liquefied all the way through when 1/4 of the ice was left — it just didn't 'slump'. (This is in line with expectation: the only thing the added pulp can really do to slow melting is provide insulation, and there wouldn't be much of that with the quantities I was using.)

On the other hand, in the matter of strength there was no comparison. I could hit a piece of Pykrete fairly hard with a hammer and it might or might not split, but it never shattered. A much lighter blow on a similar sized ice chunk was enough to result in very-satisfying complete demolition...

The assertion that "a one-inch column of Pykrete could support a car" I would like to have better evidence for [or even some! Where did that claim come from anyway? I guess from the statement that Pykrete has a "Compression Strength of 3000 lbs/sq. in."], but it does appear that the stuff is at least a much better construction material than ice! It might be really interesting to see what could be done with it in regions where ice is already a feasible building material. This all does not make Habakkuk itself any less of a fantasy, of course. Even with 'Super Ice', you still need all that refrigeration and insulation. And of course if wood pulp really does block heat transfer, it wouldn't make that refrigeration any easier! (The refrigerant pipes have to extract heat through the pulp, too.)

Crazy Ideas

So yes, Habakkuk was almost certainly a pretty crazy idea. Pykrete itself perhaps less so. But then, crazy ideas are not at all a bad thing sometimes. And the Admiralty department that Dad headed during WWII, the "Wheezers and Dodgers" (a.k.a. The Directorate (or Department) of Miscellaneous Weapon Development), thrived on wild ideas. Tales of the "Great Panjandrum" and "Hajile", for instance, definitely bear retelling. There is a great description of the Panjandrum adventures here.] [And in 2004 BBC2 ran a series, "Crafty Tricks of War", that had some fun building a mini-Panjandrum...] I couldn't find anything about 'Hajile' on the web, so I've put up my own short essay on it.

Enough of the department's projects were successful, though, (like the Hedgehog and Double-L Sweep) that they did contribute to the war effort in a major way. How do you tell which crazy ideas will work, and which won't? Only by pursuing them a ways, I guess. I think I'll accept Dad's argument. though, that Habakkuk was pursued just a little too far.


Max Perutz

I've finally found Max Perutz's own account of his involvement in the Habakkuk affair [the essay "Enemy Alien" in his book "I Wish I'd Made You Angry Earlier..." (1998 -- republished 2002)]. His verdict on the project — "Imaginative and absurd". (The essay covers much more than Habakkuk itself, and is fascinating if you can track it down.)

He clarifies a lot of details, in particular the actual development of Pykrete. He met Pyke first in the Spring of 1942, but learned nothing about ice-ships until more than six months later. The effect of adding wood pulp (or even cotton wool) to ice was originally noted by two researchers at the Polytechnic Institute of Brooklyn NY, Hermann Mark and Walter Hohenstein. Pyke found their report "hard to understand", so he passed it on to Perutz to investigate. Perutz had no idea at the time what the stuff was needed for. He does say, though, that he himself was the one who later tagged it with the name "Pykrete".

The essay includes the full text of Churchill's original memo on Habakkuk. From that it's clear that the original concept was simply to cut 'bergships' out of the Arctic ice, or else to freeze water in situ there. It was only when the flaws in that idea (like ice fracturing under its own weight if lifted by waves!) became apparent that Pykrete entered the picture.

Perutz also locates the incident of a ricocheting bullet hitting a senior officer to a briefing in the UK — months before the high-level Quebec meeting in August 1943 — and the gunman wasn't Mountbatten, just a naval associate of Perutz. The victim was the Chief of the Imperial General Staff (not injured, just displeased...).


• In 1946, Perutz gave a talk to the British Glaciology Society on Pykrete and its properties. This has some actual research-established facts so I have summarized it here.


How not to extrapolate...

I'm not sure where the figure of "3000 lbs/sq. in." came from, and in fact it is not correct, but even if true it would be unwise to deduce that a one-inch-square column would "support a car".

I remember an alleyway in University College London, that had a stern notice: "Load on these pavings must not exceed 150 lbs per sq. ft.". Well, let's see — the average student probably weighs at least 150 lbs, and with both feet on the ground covers not much more than half a square foot... Oops! As scores of students strode down that alley every day, I don't know why we didn't all disappear into the unknown depths!


Wood pulp vs. sawdust

I recently tracked down an article (from way back… 1953!) on the properties of wood pulp, and from reading it I imagine wood pulp is likely to be superior to sawdust as reinforcement. This is because the fibres in the pulp (pine or spruce) are typically 3 to 5 millimetres long, much larger that the usual diameter of a sawdust grain (and where the fibres might be firmly bound in lignin anyway).

Anybody up for some experimenting…?


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