Lofting is the arcane craft of determining the full-size shape of parts
for the fabrication of boats
. In the past, this was accomplished by making a full size drawing
of the vessel on the floor of the loft above the construction facility, hence "lofting". In the present, lofting of most craft is accomplished on computers
, and differs little (but significantly) from other product-design disciplines.
An important duty of the lofter is ensuring the lines of the full size drawing, or 3D model, are "fair". Fairness does not have a mathematical definition, but can include smoothness, continuity of curvature, tangency and hydro (or aero) dynamic suitability. Fairness must often be ensured in 3 dimensions.
Lofting takes into account the limitations of the construction materials. For instance, for a boat hull to be most easily built of steel, the shell plating must be developable, that is, curved in one direction only (a Gaussian curvature of 0). If the material is to be welded, the lofter must compensate for the predicted weld shrinkage by enlarging the patterns. If the material is a composite, such as fiber-reinforced plastic, the lofter must ensure the part can be removed from the mold.
The data the loft creates so each part can be fabricated takes different forms depending on the material. In the past, for metal ship construction, patterns were made of wood and given to the shop. Now, data is transmitted from the lofter's computer, to computer controlled burning machines, which cut out the nested parts from large sheets of stock. For composite construction, 3D computer models may be sent to fabricators who use the data to program 5-axis milling machines for mold making.
Nesting is the task of orienting parts on stock to achieve the best usage of material. It is measured by "scrap rate" (or inversely, "usage rate"). Nesting of parts on sheet stock is often done by computer, but this is a Hard Problem. Japanese shipbuilding experts claim women arranging paper cutouts of parts on scaled-down sheets have achieved the best scrap rates.
A large part of lofting in a typical shipyard is bookkeeping. Thousands of parts can be generated during the lofting of a large vessel, which are assigned numbers that can indicate location, sub-assembly and assembly.
The loft often generates material orders for the purchasing department, since it has the best data on part size and quantity. For the same reason, the loft tracks weight information on parts and assemblies.
The loft is often full of drama during construction. It is the first stop for builders encountering fitting problems on the shop floor. The lofter can be the greatest hero or most reviled character in the shipyard. The courage to resist management efforts to rush the lofting process rewards the thoughtful lofter by speeding the construction process, much to management's later delight.