To build their vessel for this year's National Concrete Canoe Competition, engineering students from California Polytechnic University in San Luis Obispo, Calif., placed a 1/8-inch-thick layer of concrete in a foam mold.
To build their vessel for this year's National Concrete Canoe Competition, engineering students from California Polytechnic University in San Luis Obispo, Calif., placed a 1/8-inch-thick layer of concrete in a foam mold.

If you don't know much about boatbuilding, start by welding rebar together to frame the gunwales and keel. Then form the hull with layers of chicken wire, and trowel a stiff 1/2-inch-thick mortar mix onto the mesh. This simple technique was used by engineering students at the University of Illinois back in 1971 to build the winner of the first concrete-canoe race. Though it weighed 360 pounds, the ungainly boat managed to defeat its closest (and only) competitor, a craft that was sleeker and lighter by 235 pounds but had trouble staying afloat.

However, if you have a few thousand spare man-hours of labor available, plus boatbuilding software and access to computerized routing equipment, you're ready to build a state-of-the-art concrete canoe. Settle on a design, then cut the female mold out of foam blocks. After assembling the blocks, line the mold with plaster of Paris, sand it smooth...

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