Lines and Lofting

Shipbuilding begins with a shape. A hull must float at the right waterline, carry its load, move through water efficiently and stay stable in waves. That shape is recorded in the lines plan.

The lines plan

Naval architects draw the hull as three sets of curves. Sections are slices across the ship; waterlines are horizontal slices; buttock lines are vertical slices parallel to the centerline. Each point on the hull appears in all three views, and the curves must agree. Adjusting them until they do, with smooth, continuous curvature, is called fairing. The offsets, a table of measurements, record the shape in numbers.

The mold loft

Small-scale drawings magnify errors. Traditionally, loftsmen redrew the lines full size on a mold-loft floor, using long flexible battens held by weights (ducks) to find fair curves. From the full-size lines they made wooden templates for frames, plates and other pieces. The half model, a wooden half-hull built up from layers, was an older way to design and take off shapes.

Mass production

During World War II, American yards built more than 2,700 Liberty ships using standardised designs, templates, prefabricated sections and welding in place of riveting. Build times fell from months to weeks.

Digital lofting

Computers took over fairing and lofting from the 1960s and 1970s. Today hull shapes are modeled in 3-D; the software nests plate parts to reduce waste and sends cutting files to CNC plasma or laser tables. Plates are marked with lines for fitting.

Programs

Shipbuilding / Marine Fabrication, Drafting / CAD, Fiberglass / Composites, CNC Machining.