The Tao Philippines guys have spent over 2 years and a small fortune on building an “authentic” dual-outrigger boat, pictured below. They’ve gone to great lengths to document and employ original materials and construction methods, and so have done anthropology a great service. The boat features intricate carvings done by master swordsmiths (I suspect that isn’t a word, but it ought to be) that make the entire vessel a work of art. National Geographic has done small stories on the construction, and may be planning a larger feature in the future. Kon Tiki it ain’t, but still very impressive.
(If you aren’t interested in my musings on naval engineering, best stop here.)
There is great national pride among the seafaring community here that modern boats here still employ a similar design, but with modern methods and materials. True enough. They’re described as the perfect vessel for navigating the shallow and reef- and wreck-strewn waters of the Philippines. Less true. And they were described to us by our guide as follows; “These boats never sink, except when they turnover.” Not quite true, but certainly very revealing.
The dual outrigger design is well-suited to these waters–they have exceptional roll stability and shallow draft. But there’s a fatal design flaw that limits, or should limit, them to only calm waters. That is to say they work well battling surf, but storms become deadly. In fact, the very boat that we were on was capsized in a tropical storm while anchored, killing a sleeping crewman–this was it’s first voyage since renovation. While it’s true that these boats rarely sink except when they turnover, they turnover rather frequently. The problem is that the outriggers exert maximum anti-roll force when in the neutral position. As soon as the boat begins to roll, it’s passive ability to counter the roll begins to fade. (Specifically, as the “down” outrigger is forced into the water the buoyancy forcing it up remains constant* with depth. Similarly, as the “up” outrigger is forced out of the water, the downward pull of gravity, i.e. the weight of the outrigger, remains constant. But both of those forces operate perpendicular to the Earth’s surface**. The boat is no longer perpendicular to the Earth’s surface, so the portion of those forces that effectively act perpendicular to the boat, in the direction to counter the roll, are a fraction of what they were when the boat was flat on the surface.) At some point (less than 90 degrees given a center of gravity above the waterline), the same outriggers that previously countered the roll now guarantee the roll. This is a boat that’s happiest upside down.
This is a perfect example of fail-danger design, as opposed to fail-safe. In other words, once things start to fail, the ability to recover is diminished. That isn’t to say the outrigger is a worthless design. It had it’s purpose, and lots of boats are happiest upside down. The real problem is that the outrigger is dominated by the catamaran, and the dual outrigger by the trimaran. For a given tonnage, the -maran draws less water, has even more exceptional anti-roll stability, and is more efficient through the water.
For the local shipbuilding industry of yore not to have discovered improved designs is unimpressive. To still not have employed improved designs is unthinking. To be proud of both is stupid.
*Not exactly true. Buoyancy is the difference in pressure at the top and bottom of an object. Water isn’t exactly incompressible, but is effectively so. In reality, buoyancy increases with depth, but by a negligible amount.
**Alternatively, the effective torque arm acting on the boat is reduced in length. The outrigger remains the same distance from the boat, but the distance between the boat and a line drawn from the outrigger to the center of the earth, along which the anti-roll forces act, is now shorter. The math is equivalent either way.