Molds and Setup, Part 1

Making Molds Few operations offer as much of a guarantee for getting your boat’s shape right (and symmetrical) as creating a properly set up construction jig made with accurately built and properly positioned station molds. A good construction jig will prevent the formation of unsightly sags or bulges, which detract from the desired curvature of the hull. A construction jig can be enhanced and adjusted to reflect the complexity of the hull. A simple skiff might require just a few molds, as its shape is relatively simple and the planking is usually thick enough to fair itself between supports. On the other hand, a boat built with thin and floppy plywood planking, or with narrow strip planks, might need twice as many station molds to keep things fair. A sophisticated hull shape like the Haven 121⁄2 might require a mold for every single steam-bent frame. In this article, we’ll take a close look at molds: how to ascertain their shape by using information from the lofting board and how to build them. In part two, (in our next issue), we’ll get into the finer points of setting up molds on the building jig. Visualize and Create the Shapes The stations are transverse planes that are analogous to the slice a knife makes in cutting through a loaf of bread. Station molds, because they have thickness, are like the individual bread slices. Only one face of the piece of bread, or mold, can be considered a station. Before making molds, determine whether the lines on the lofting board (or the full-sized patterns, if your plans include them) have been drawn to the inside or outside of the hull planking. Older plans, especially those drawn to lines taken from historic round-bottomed hulls, are likely to be drawn to the outside of the planking, whereas hard-chined craft such as dories or sharpies are likely to be drawn to the inside. In most cases, there will be a notation on the table of offsets indicating whether dimensions are to the inside or outside. If the stations are drawn to the outside of the planking, you will need to subtract plank thickness in order to make an accurate mold. If the planking is thin (less than 9⁄16" or so, as it is on most small boats), it is a simple matter to reduce the station outlines to achieve the correct shape of the molds (see Figure 1). Take a small piece of wood that is the same thickness as your planking, and, on the body plan, run it along the inside of each station’s curve (from sheer to rabbet) while making a series of pencil marks to graphically subtract the planking thickness. Alternatively, you can set a pencil compass to the plank thickness and run a series of arcs from the station line. Connect the points or crowns of the arcs with a flexible batten and draw in the line with a pencil. This gives you a trustworthy inside-of-plank station line from which to build the molds. The easiest molds to make (and to modify) are those with square cut edges rather than beveled ones. Most small boats have been built with squared