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Wednesday, September 23, 2009

The Four Roots of Boatbuilding

Old though it is (1976), Basil Greenhill's Archaeology of the Boat is the most useful book I've read in a long time toward understanding boat types and boat evolution. Probably the most important element is a typology for characterizing plank-built boats not by their planking method (i.e., lapstrake versus carvel), but into the categories of skeleton-built vs. shell-built. I'll get into these later, probably in a future post, but here I'll provide some of the background necessary for understanding this duology.

Greenhill identifies four "roots" of boatbuilding, from which, he argues, all boat types derived:

1. Rafts and raft-boats: a craft which is not watertight -- i.e., it does not enclose air. Its buoyancy comes from the buoyancy of the boatbuilding material itself. A "raft-boat" is basically a raft with a more or less boat-like shape, i.e., pointy at the bow, like the Brazilian jangada. Raft building materials include logs lashed together, bundles of reeds, and air bladders like inflated animal skins. Greenhill states that Chinese sampans may have evolved from rafts. The paired illustrations shown below are an intriguing bit of evidence to this effect.

Above: a sophisticated Chinese sailing raft, with movable daggerboards and capable of upwind sailing.

Below: the bottom of this sampan shows similarities to the raft above and lends credence to the theory that the sampan is one of the few extant boat types that derived from rafts, rather than dugouts.

(All images in this post are from Greenhill's book. All may be clicked to magnify.)


2. Skin boats: in which a skeleton built of sticks is covered by a skin, either animal skins or a waterproofed fabric. Obvious indigenous examples include kayaks, umiaks, coracles, and curraghs. These evolved into a very limited number of modern types such as folding kayaks and canoes. (Greenhill states that the basket-boat is a variation of the skin boat. The book, a survey, doesn't give evidence for this conclusion, and I'm skeptical that they share a common ancestry. I suspect that the basket boat may be a fifth, very small, root, which wasn't further developed.)

3. Bark boats: the American birch bark canoe being the obvious example, and the only well-developed one.

4. Dugouts. A hollowed log in its earliest iterations, the dugout was elaborated and expanded upon in many directions, and evolved into most of the boat types we know today. The evolutionary process took place in many phases, at many times and in many places, and it was far from a smooth, linear movement -- which is why we have such a diversity of plank-built boats today.

A log that has simply been hollowed out has limited seaworthiness and, as they get larger, they become quite unhandy. Simply but, they're too narrow and too shallow. The first stage of evolution, then, was the "expanded" dugout, in which the hollowed log is heated from the inside (top image in the graphic below) , usually with boiling water, and its sides are then forced apart with shores (left bottom). Not only does this increase the beam, but it forces the ends upward, creating a true "boat" shape that is both more aesthetically pleasing and more seaworthy.

The process does, however, make the boat even shallower (i.e., reduces freeboard), so the next step is to "extend" the hull by the addition of a single plank on each side (right bottom). Different cultures invented different methods of fastening the plank: pegs, stitches, mortise/tenon joints, dovetail inserts, and so on.


Top: dugout heated to soften. Left bottom: expanded. Right bottom: extended by the addition of a strake.

This was probably how plank-built boatbuilding got its start in Western culture. But since evolution doesn't progress linearly, it didn't work that way in all cultures and at all times. The Polynesian outrigger canoe, for example, is a dugout that is often extended but not expanded. The large cedar dugouts of the Pacific Northwest were expanded but often not extended. Of course, the Indians of the Pacific Northwest had enormous trees with which to work.

The balam (below), from the Bay of Bengal, is a sewn boat that Greenhill calls the "ulimate development" of the expanded-and-extended dugout. Based on a dugout hull that essentially acts like a backbone keel, it has been extended by the addition of five strakes.

A balam from the Bay of Bengal

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