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.
The lofting board body plan (or full-sized patterns) will provide the information needed to build accurate station molds.
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).
Figure 1 Use a compass or a block the thickness of the planking to generate an inside-of-plank line on the station. This method is fine for hulls that do not have extreme shapes (such as a bluff bow).
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-edged molds, so, for simplicity’s sake, we will limit our discussion to boats built this way. We’ll also set up the hull for upside down construction to facilitate framing and planking, but refer to “top,” “bottom,” “upper,” and “lower” as though the boat were right-side up.
Create Contact Points
With the desired shape of the station molds depicted, the next step is to establish a construction baseline on the body plan that can be used as a reference to set the molds at their proper height relative to one another. This line will run parallel to the baseline that’s shown on the lines plan. The idea here is that the upper end of each U-shaped mold will extend to (and be cut off at) this construction baseline.
The port and starboard sides of each mold will then be connected with a narrow piece of wood (called a “cross spall”) whose upper edge will be aligned with the drawn construction base. Now that each mold has a common line or base, when the molds are erected like a stack of inverted Us, they will all be at the correct height relative to one another—sort of like standing up a set of dominoes.
So where does this construction baseline come from? It may already be drawn on the full-sized patterns. But usually, you have to make the command decision of where to locate the line. Generally speaking, it will have to be higher than the top of the stem. So, if the top of the stem is 33″ above the original baseline, you could set your construction baseline at 34″, 35″, or perhaps 36″.
With that done, all that remains is to extend each station line vertically from sheer to the construction baseline. Once these steps are completed, we can begin to build the molds.
Making Molds
Molds can be made of either plywood or boards of solid wood. At first blush, plywood seems tempting—it’s strong, and you can make each mold out of only one or two pieces of plywood. On the other hand, plywood does have its drawbacks. Inexpensive plywood (the most popular choice) warps easily so it will need to be reinforced, and it is hard to cut with edge tools as it is brittle and is riddled with voids. In most cases, building the molds from joined pieces of construction-grade softwood boards (like 1″×10″ pine, spruce, or cedar) is the better choice for mold building.
Now let’s take a closer look at the body plan (or full-sized station patterns, if that’s what you have). The first thing you’ll notice is that the plan likely shows only one half of the stations. That’s actually a good thing, as it’s much easier to get symmetry by joining two identical halves made at the same time to create a whole, symmetrical mold. We’ll build the station molds right on the body plan. Cut a straight piece of 2×4, at least as long as the boat’s beam width.
Screw it down to the far side of the construction baseline with the near edge aligned with it. This will be your stop against which the tops of the molds will be pushed. Like the ledge on a drafting table, this stop will aid in keeping everything square, aligned, and generally right where it ought to be. ’Midship station molds will be made up of two boards per side (four in total) with a gusset securing the two halves and an elbow gusset at each side near the turn of the bilge or at the chine. Often, the nearly straight, forward-most molds can be made from a single piece on each side without an elbow. You can easily determine the rough lengths of your mold boards by eye. Cut sets or pairs of rough stock for each station as needed.
Now let’s turn to making one side of a ’midship mold. Lay the two pieces on top of the body plan. Cut the far end of the outer piece so that it will butt up to the 2×4 stop and the other piece so that it will overshoot the centerline. The opposite ends where the two pieces meet will overlap, and be located at the turn of the bilge, about halfway around the station. Roughly bisect the angle formed by the overlapped pieces and draw that line onto the top piece.
Cut it to the drawn line on the bandsaw. Lay it back on the body plan with the top pushed against the 2×4 stop as before and the end you just cut overlapping the uncut piece. Trace this angled end onto the uncut piece; take the uncut piece to the bandsaw and cut on the drawn line. The two pieces should then come together in a tight butt joint and meet the construction line at the top. Next, transfer the vertical centerline onto the bottom piece and trim it on the bandsaw.
The next job is to accurately transfer the curved line from your body plan to the mold stock. While there are a number of ways to do this, (see WB No. 137), perhaps the easiest is the “bed of nails” technique (see Figure 2). Lift up the unfastened pieces and lay down a series of 3d box or ring nails with their heads laid right along the curved edge of the station line. (Some builders grind off one edge of the nail’s head so they don’t roll, while others tap them into place with a hammer.)
Gently lower one of the pieces of mold stock back in place on top of the station, then press it onto the nail heads. Cautiously stomping on it is one good approach; another is kneeling and gradually bringing weight to bear.
Figure 2 Use a straight piece of wood, fastened along the construction baseline, to establish mold ends. Once mold boards are roughed out, place nails along the station line and imprint points on the pieces. Connect the points to create the mold edge line.
Lift the piece, turn it over, and you’ll see a whole series of impres- sions and/or nails stuck in the wood. Use a straightedge and a flex- ible batten to connect the points and then mark the shape with a pencil. Lay it atop the second piece you cut for the opposite side at this station and drywall-screw them together. Take the fastened pair to the bandsaw and cut to the curved line. Return the mold pieces to the body plan and see how closely your cut fits the drawn line. Touch them up with a plane, if necessary, and then anchor each assembly to the body plan with drywall screws to keep them from shifting.
After fitting and anchoring, transfer waterlines onto the molds, squaring them up from the body plan and scribing them onto the edges and faces of the mold pieces. Cut some gussets (just squares of plywood or scraps of mold stock) that will be used to join the pieces of the mold together. Attach the first gusset to the top (exposed) side while the pieces are still tacked to the body plan. Drywall screws work well for this job. (Many builders like to add a shot of glue under their gussets as well.) After one side has been gusseted, unscrew the joined pieces (a five-piece assembly) from the body plan, leaving the mold assembly fastened together. Then, glue and screw the second gusset to the other side of the pair.
Figure 3 With the upper and lower pieces cut to length, attach the pair to two identical boards; cut and shape the mold pieces in tandem.
Now separate the two sides of the mold. One side is lined back up with the drawn station on the body plan and temporarily anchored into place. The other (mirror-image) side mates up to the first, butting up to the centerline at one end and against the construction line ledge on the other. Measure to ensure that the second piece is exactly equidistant from the centerline, matching the first. The drawn waterlines on the edge of the mold should line up with waterlines on the body plan. Gussets will be on the same side.
Figure 4
Top—Anchor the pairs (four pieces) to the lofting board. Transfer waterlines onto the mold pieces and then gusset the joint of the topmost pair as shown. Pull the whole assembly from the board and gusset the opposite side of the bottom mold pieces.
Bottom—Release the two sides; they will mirror each other. Secure the appropriate side on the station (gusset-side up), mate the other side to it, gusset the two halves at the centerline, and add a cross spall, placing its upper edge along the construction baseline.
After giving the whole business one more check for accuracy, nail down the second section and attach the joining gusset at the bottom (on the same side as the other two gussets) with screws and glue. In the same manner, join the two half-pieces together with a cross spall (a 2×4 works well) at the construction base ledge location. Mark the center-line onto the cross spall and center-line gusset (see Figure 4). Lift it off, cut the notch for the keelson, and repeat the process for the next mold (and the next…).
Greg Rössel is a contributing editor for WoodenBoat.