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Sunday, January 11, 2009

Indonesian Canoe Outriggers -- Pretty Clever

The outriggers of Indonesian double-outrigger canoes (jukungs) are as sophisticated in their design as they are rudimentary in the technology of their construction.

Many Balinese canoes have distinctive upward-curving outrigger booms, giving a graceful gull-wing appearance. The booms are lashed to the hull, and separate connector pieces are attached to the ends of the booms in a scarf joint, usually with a single wooden peg and lashings. The bamboo floats are attached to the lower ends of these connectors by lashings around the floats. Sometimes, the connector penetrates a hole in the float.

This arrangement has several advantages, some of them surprising. According to Adrian Horridge in Outrigger Canoes of Bali and Madura, Indonesia, "To an engineer it seems obvious that the direct connection (i.e., eliminating the intermediate connector piece) is the one most likely to survive (for a short time) in a rough sea... The direct connection has fewest weak points and is the least likely to work loose."

But Horridge goes on to cite three advantages to the indirect connection:
1. It brings the float down to the surface of the water, thus allowing the use of the gull-wing boom form, or of other booms shapes (even straight ones) that ride higher above the surface and so don't catch waves.
2. It provides "a flexible structure that will absorb the strain energy as the boat moves in the waves."
3. It allows the height of the float to be adjusted, for example, to accommodate different boat loadings or sea conditions.

Item #2 is the most fascinating. To quote Horridge again:
That a flexible structure lashed together from short pieces of wood can be the preferred design may seem strange if one is used to structures made of steel tubing; in fact, when a structure must be light and resist shocks and shaking, some designs that are apparently not firm or well engineered, and that are also troublesome to make, turn out to be the most resilient. Cloth, straw hats, leather shoes, baskets, chain armor, even human skin resist breakage for two reasons: the loads are distributed among many fibers or components, and forces that change suddenly are dissipated as friction between the parts. Gymnastics performed upon a hard wooden chair will soon loosen its joints and break it, but on a basket chair the gymnast will be exhausted before the chair collapses. When a structure receives a hard blow it will break unless the energy of the blow can be absorbed as heat energy in the structure, which is why composite materials such as wood, horn, bone, and fiber glass do not shatter when hit. So, when an outrigger hits a reef or the beach on coming into land, it is stronger if designed to absorb the blow. Melanesian outriggers, in particular, were built like basket chairs. Also, at sea, the continual kinetic energy of the waves is dissipated if the outrigger is flexible...

The floats are made of bamboo, with the shiny outside layer scraped off to reduce weight and to remove nodes, to make the surface smoother and reduce resistance. This also allows the fibrous portion of the bamboo to dry out between uses, and so prevents rot. Often, a small hole is bored in each segment of the bamboo to prevent cracking, and modern users sometimes coat the floats with epoxy. Even so, floats are removed after every usage and stored under cover.

Interestingly, the floats extend beyond the front of the canoe's hull, and they are always toed-out a bit toward the front. According to Horridge, native users explain this as being a guard against pitch-poling, but Horridge states that its more important function is to maintain sailing balance:
I think it more significant as a control of the "weather helm." (A sailing boat of any kind should preferably tend to turn into the wind if the wind suddenly blows harder. This feature is called "weather helm.") If there is a gust, the float on the leeward side tends to dig into the water. If the float extends far forwards, it will then suddenly shift the center of lateral thrust far forwards, creating more weather helm. How far the floats protrude forwards may be more important than how much they are turned outwards.

The line art, from the Horridge book, shows a Balinese jukung model, accurate except for the smaller number of seats than in an actual canoe. Note especially the connections between the outrigger booms and the floats.

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