Floating a big idea: MIT demos ancient use of rafts to transport goods

Dewan also analyzed the materials used for the construction, including the lightweight balsa wood used for the hull. Besides having to study the aerodynamics and hydrodynamics of the craft and the properties of the wood, cloth and rope used for the rafts and their rigging, she also ended up delving into some biology. It turns out that one crucial question in determining the longevity of such rafts had to do with shipworms-how quickly and under what conditions would they devour the rafts? And were shipworms always present along that Pacific coast, or were they introduced by the European explorers?

Shipworms are molluscs that can be the width of a quarter and a yard long. “Because balsa wood is so soft, and doesn't have silicates in it like most wood, they are able to just devour it very quickly,” Dewan said. “It turns into something like cottage cheese in a short time.”

That may be why earlier attempts to replicate the ancient rafts had failed, Dewan said. After construction, those replicas were allowed to sit near shore for weeks before the test voyages. “That's where the shipworms live,” Dewan said. “One way to avoid that is to minimize the amount of time spent in harbor.”

Dewan and Hosler did a simulation of the amount of time it would take for shipworms to eat one of the rafts and concluded that with proper precautions, it would be possible to make two round-trip voyages from Peru to western Mexico before the raft would need replacing.

The voyages likely took six to eight weeks, and the trade winds only permit the voyages during certain seasons of the year, so the travelers probably stayed at their destination for six months to a year each trip, Dewan and Hosler concluded. That would have been enough time to transfer the detailed knowledge of specific metalworking techniques that Hosler had found in her earlier research.

While Hosler's earlier work had shown a strong likelihood that there had been contact between the Andean and Mexican civilizations, it took the details of this new engineering analysis to establish that maritime trade between the two regions could indeed have taken place using the balsa rafts. “We showed from an engineering standpoint that this trip was feasible,” Dewan said. Her analysis showed that the ancient rafts likely had a cargo capacity of 10 to 30 tons-about the same capacity as the barges on the Erie canal that were once a mainstay of trade in the northeastern United States.

Hosler said the analysis is “the first paper of its kind” to use modern engineering analysis to determine design parameters and constraints of an ancient watercraft and thus prove the feasibility of a particular kind of ancient trade in the New World. And for Dewan, it was an exciting departure from her primary academic work. “I just loved working on this project,” she said, “being able to apply the mechanical engineering principles I've learned to a project like this, that seems pretty far outside the scope” of her work in nuclear engineering.

Written by David Chandler, MIT News Office