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Mast Step and Keelson: The Early Development of a Shipbuilding Technology

Mark Geanette
Thesis: December 1983
Chair: Bass
Nautical Archaeology Program


Mediterranean mast steps and keelsons spanning the fifteen hundred years from the sixth century B.C. through the first millennium A.D. were central structural elements in a seafaring tradition whose primary vessel was the efficient merchant ship. As such, these ship timbers exhibit characteristics distinct from the Gallo-Roman tradition of inland waterway towed transport or the Scandinavian tradition of longships, designed for speed but not for commerce.

Excavated evidence from some twenty wrecks indicates that the complex of support cavities found in the mast step is likely the footing for a "boxing" arrangement which is a direct descendant of the ancient Egyptian tabernacle system.

Early examples point tot he mast step being an isolated ship member, increasing in size in proportion to the enlargement in ship dimensions but remaining essentially static in design. However, by the first centuries A.D. there developed an independent concept in the minds of ancient shipwrights, the keelson. The keelson was an integral element of the trend from shell-first to frame-first construction and the concomitant increased reliance upon skeletal support. While the "hybrid" longitudinal timber was now fastened by bolts to the keel for enhanced structural support, the need for a massive mast step, stabilized by its own weight, diminished. There had thus evolved a delicate balance between mast step and keelson whereby the two had merged quite naturally into a single longitudinal member by virtue of their identity of location.

This development of the mast step-keelson relationship mirrored the broader evolution in ship construction techniques. Careful documentation and analysis of these ship timbers will, therefore, supplement and confirm archaeologists' knowledge of the history of seafaring.

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