Finite Element Analysis of Shell-First and Longitudinally Reinforced Frame-Based Wooden Ships

Abstract

During the period between the 5th century BC and the 6th century AD, two ship construction technologies were prevalent around the Mediterranean Sea: shell-first and framebased. The shell-first concept was of a strong rigid hull, comprising edge-joined strakes reinforced by transverse frames which were not joined to the keel. In the frame-based technique, transverse frames were connected to the keel to form the skeleton, and the strakes were then nailed to the frames. The hull was strengthened with longitudinal reinforcements. These concepts did not exist in isolation, and at times both techniques were combined. An initial global finite element study (FEA) which analyzed the two technologies, the shell-first Maʻagan Mikhael (400 BC), and the frame-based Dor 2001/1 (6th century AD), revealed that the ships were on a structural par. In that same study, a controlled FEA experiment on generic models was constructed to simulate shell-first and frame-based ships, showing, however, that the shell-first technique exhibited significantly higher structural integrity than the frame-based method. These results prompted a secondary study to explore the mechanical-structural contribution of longitudinal reinforcements to framebased constructs. Using the same frame-based generic model as in the previous study, longitudinal reinforcements were added in various configurations. The FEA results from this study demonstrated that three critical interdependent factors determined the relative strengths of shell-first and frame-based techniques: the number of transverse frames, the number of longitudinal reinforcements, and their relative locations.For that study, this hypothesis was proposed: The inherent mechanical advantages of the frame-based technique contributed to the transition from shell-first to frame-based ships. The 'inherent advantages' would be manifested in degrees of rigidity and reduction of internal stress on the hull. This experiment, which applied standard computerized analysis techniques, demonstrated overall global structural parity. However, using simplified generic models, the shell-first design exhibited a significant structural advantage, thus refuting the above hypothesis. The subject of this paper deals with the question, 'What structural and mechanical measures were necessary and employed by shipwrights to achieve structural parity to attain ultimate dominance?'. The authors, being motivated by archaeological evidence, engineering principals and the promptings of scholars, including Pomey, Reith, Kahanov (Pomey et al. 2012:236) and van Doorninck (van Doorninck 1976), developed a further hypothesis: 'Increasing the number of hull frames and longitudinal reinforcements establishes equal levels of increased rigidity and reduction of stress in shell-first and frame-based ship hulls'.This engineering-archaeological research approach to the transition from shell-first to frame-based ship construction cannot disregard the peripheral constraints, which include simplification of construction, less use of skilled wor...

Numerical Reconstruction in Maritime Archaeology

“…While the structural analysis lacks both precision and the data required for the potential repetition of the analysis, the approach to the problem was consistent, and the study conclusions indicate a consistent and significant advantage of the shell-first method over the frame-based method. Additional details about the aforementioned analysis are provided in the following paper by Helfman et al [45]. Here, three critical interdependent factors are studied: the number of transverse frames, the number of longitudinal reinforcements and their relative location.…”

“…Three main ship hull reinforcement schemes were examined and the corresponding rigidity was determined, as shown in Figure 16. It has been shown that the insertion Additional details about the aforementioned analysis are provided in the following paper by Helfman et al [45]. Here, three critical interdependent factors are studied: the number of transverse frames, the number of longitudinal reinforcements and their relative location.…”