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.
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 (1942) before Pykrete was thought of. There is an 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 was probably more to do with the refrigeration equipment installed! 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.
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 liqefied 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, 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.)