Since it is a non-graphic system, all the sets of offsets that define the change in hull curvature have to literally be marked on the templates themselves. This dependency on one set of templates to both mark the positions of the curvature control points on the timbers and define the curves between these points makes it difficult to add additional longitudinal controlling curves to the system. This increasingly becomes a problem in the ends of the vessel, where the change in curvature becomes more pronounced and such additional control is desirable. Therefore, the predetermination of frame shapes with Mediterranean moulding is limited to the portion of the hull between the tail frames.
The added ability to adjust the upper narrowing and rising enables the tail frames to be placed nearer the ends, since it allows for greater flexibility in narrowing and rising the bilge curve. Nonetheless, one of the big shortcomings of Mediterranean moulding is the fact that only one set of narrowing and rising increments controls the definition of the longitudinal curvature along the bilge of the hull. As can be seen in the drawing of La Belle's frames, as in the French drafts presented in Part I of the essay, moving away from amidships the curvature between the two lower diagonals progressively flattens and ultimately changes from convex to concave (Figures 5, 6, 7, 18, 19, 47). The use of two sets of bilge surmarks as points of control does not appear in any documented methods of Mediterranean moulding, and thus control of this area of transition is severely limited.
It is important to note that in French drafts using the "geometric fairing with diagonals" method, the curves between the lowest two diagonals are not single arcs. The procedures by which the control points on the diagonals were joined with transverse curves in these drafts are unknown, and this aspect of the design process is not clarified in later descriptions of this design method. In the reconstruction of La Belle's design, single arcs with radii changing from one frame to the next were used to draw the curves between the diagonal guide points (Figure 47), but this remains the most tentative part of the reconstructed design sequence. This ambiguity arises from the fact that the introduction of additional longitudinal control curves reduces the importance of the restrictions or rules imposed on the transverse geometry. The rules or restrictions for drawing the transverse shapes is a necessity with only a limited number of guide points, but this is only a secondary aspect to the evolution of geometric methods in Western ship design. The fundamental concept in these design methods is the quantification of longitudinal curvature.
French shipwrights were able to expand on this basic concept of Mediterranean moulding by merging it with the principles of orthographic projection. In the process of developing a graphic design method, the French shipwrights limited the curvature of longitudinal runs to flat design planes both before and abaft amidships. NEXT
© 2014 TARAS PEVNY