Historical Materials: Vessel Types

Ship models as aids in the design of a ship played an important role in America from the late eighteenth century to the early twentieth. Most widespread was the builder's half-model, a half hull to scale, such as 1/2" = 1' (1:24 or one twenty-fourth actual size). These half-models were made in three ways: block half-hulls, "hawk's nest" models, and "lift" half models.

The block half-hull was the simplest—the shape of one side of the hull was carved from a solid piece of wood. "Stations," locations of frames at convenient intervals, were marked on the hull and "templates" wooden shims carefully shaped to fit closely at each station were made from thin sheets of wood. The templates could then be traced on paper (or wood, if paper was too expensive) marked with a grid for measuring dimensions at regular intervals. The paper grid was redrawn on a mold loft floor to the full size of the vessel, and measurements were made where the grid lines intersected the traced curves. Wooden patterns called "molds" were made from the measurements and used as patterns for full-size frames.

The "hawk's nest" model consisted of solid "frames" (actual molds), mounted on a "back board," and fitted with a keel, stem, stern post, and thin strips of wood outlining the run of the hull planking. More importantly, the strips, or "stringers," aided in the "fairing" of the frames so planks would fit to the frames without showing bumps or hollows.

The "lift" half model was the method of choice from the 1840s onward. Carefully planed boards of uniform thickness were screwed together and carved to the shape of the envisioned half-hull. After the desired hull form was established, the surface was carefully smoothed and "faired", aided by flexible battens and "sighting" the curved surfaces. The model was then disassembled and each lift traced, taking care that the lifts were accurately superimposed on the tracing sheet. The hull profile was also traced, usually in alignment with the lift tracings. The locations of frames (or sometimes only every second or fourth frame) were carefully drawn on the tracings and their "half breadths" measured and tabulated. When the half-breadths were converted to full size dimensions, they became "offsets" used to lay down the molds for the frames. Sometimes, the offsets were written on station lines scribed on the top surface of each lift, making the model its own table of offsets.

Builders' half models are priceless documents of  nineteenth-century ship design and have been the essential starting point for many important reconstructions of the vessels of that period, whether in model or full-size reconstruction. They rank among the most important objects in marine museum collections.

– Erik Ronnberg

Reference:

Howard I. Chapelle, The National Watercraft Collection (Washington, DC: Smithsonian Institution, 1962).