16th International Symposium on Boat & Ship Archaeology

58 16 th International Symposium on Boat & Ship Archaeology Nathan Helfman Department of Maritime Civilizations, University of Haifa, Haifa, Israel The keelson: An engineering-archaeological analysis of a milestone in medieval maritime architecture J.R. Steffy remarked, when analysing the shell-first construction, that ‘the keelson would only serve its purpose hundreds of years later’. The thousand years of the first millennium CE bear witness to a slow but deliberate revolution-evolution in marine architecture and propulsion. Wooden ship hull architecture morphed from robust edge-joined planks to skeletal framed-based ships whose frames were joined to a single back-bone keel. Their skins were composed of longitudi- nal strakes nailed to the frames. During this same period square sails were being replaced by settee and triangular lateen sails. However, these hull and sail inno- vations engendered their own unique challenges: frame-based ships required ex- tensive longitudinal reinforcement while triangular sails, heading up-wind, result- ed in slide-slippage and heeling. Two representative shipwrecks standout in their structural uniqueness: the Tantura B (9 th century CE) and the Serçe Limanı (11 th century CE). Both ships, less than 20 meters in length, possessed frame struc- tures nailed and bolted between a keel and a massive keelson. This structural archetype appeared anomalous given that marine architectural principles clearly demonstrate that in wooden ships of this length a keelson is redundant. Upon employing structural Finite Element Analysis (FEA) Computerized Fluid Dynamics (CFD) accompanied by laboratory validation, it was demonstrated that the Serçe Limanı maintained a high degree of structural integrity which was not influenced by the addition of the keelson: the keelson was not a longitudinal reinforcer. This finding contradicted the assumptions previously published in archaeological lit- erature. The keelson did, however, enable both the lengthening and thickening of the keel, which when applying CFD simulations, it was demonstrated that the modified keel contributed to the significant reduction of side-slippage caused by upwind sailing characteristic lateen sails. This research was inspired by P. Pomey’s commentary that ‘…the nailing or bolting of frames to the keel, and eventually to the keelson, needs further study [and] cannot be interpreted simply…’.