Concrete is an artificial stone, reconstituted from three parts — an aggregate (sand, gravel, pebbles), a binder (cement, lime, tar), and the water that drives the reaction which sets them. A concrete boat presses the paradox to its limit: a hull of the very material that most wants to sink, held up by displacement rather than lightness. It is built as ferrocement — a thin, rich shell of cement mortar worked into a steel armature and mesh, strong and flexible enough to take the curve of a hull. The technique is old: Joseph Lambot built ferrocement rowboats in the 1840s and exhibited them at the 1855 Paris World’s Fair; during the First World War, with steel scarce, Woodrow Wilson approved the building of twenty-four concrete ships.
Over two weeks in April 2009 I built a thirteen-foot concrete boat on a beach near Los Angeles; the technique was later taught as a sculpture seminar, Concrete Boatbuilding: Its Technique and Its Future.
Ferrocement hull — the technique
The armature
Bend steel rod to the lines of the hull and tie it into a rigid frame; wrap and tie several layers of steel mesh over it, close enough to hold mortar.
The mortar
A rich mix — cement and fine sand, no coarse aggregate — wetted to a plastic, workable consistency. Rich mixtures are used for ferrocement forms precisely because a hull must be both strong and flexible.
To build
Force the mortar through the mesh from both sides and trowel it smooth, working in one continuous pass so the shell cures as a single body — nothing bolted, nothing welded shut. Keep it damp as it cures.
Proportions and fairing depend on the hull; the full build is documented in the Concrete Boat project and the sculpture seminar.