NEMAH: The Birth of a Skiff - Part III
   
 
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NEMAH: The Birth of a Skiff
Part III: Fabricating the Plywood "Shell"

by Tracy O'Brien
http://www.tracyobrien.com/

Part I - Part II - Part IV

In Part II of this series of articles on the design and construction of the sharpie skiff, Nemah, I detailed how various structural "sub-assemblies" can be combined to produce a boat that is both strong and lightweight. Here in Part III, we will consider plywood composite bonding techniques, and then go through the actual process of fabricating a plywood "shell" for Nemah.

Plywood Composite Variations

There area variety of plywood boat construction systems that utilize fiberglass tape-bonded joints* Some are actually plywood "plank on frame" structures that utilize taped seam joints to eliminate the tricky bevels of chine logs or seam battens, but are otherwise made up of frames and other structural members. At the opposite end of the spectrum are hulls that could be classified as "cold molded structures fabricated from pro-bonded sheet stock," the difference being the degree to which the designer has relied on the plywood shell itself to cany the structural loads of the finished boat. Neinah falls near this end of the design spectrum, while the smaller Constant Camber structures carry the concept to its logical conclusion.

Bonding Techniques

The goal of any bonding process is to create a joint that duplicates the structural characteristics of the materials to be joined. In the case of plywood composite structures, we want to design a joint that is as stiff and as strong as the plywood, but is not so stiff that it would cause the plywood to fail near the joint by concentrating all the bending loads at one point. To accomplish this, we will utilize two materials: epoxy saturated fiberglass tape which supplies the needed tensile strength; and a fillet of epoxy, wood flour, and colodial silica which supplies compressive strength as it forms a radius on the inside of the joint to which the fiberglass tape will be bonded.


Figure 1- Section Through Chine

In this type of construction, if the inside and outside layers of fiberglass tape are not held far enough apart, or if the inside or outside radiuses of the joint are too small, the fiberglass tape can flex and fail. A typical plywood composite joint is shown in Figure 1. Note how the layers of fiberglass tape are feathered out, reducing the concentration of bending forces.

Fabricating The Shell

As we mentioned in Part II of this series, Nemah's bottom panel has it's face grain running across the width of the hull. This is accomplished by scarphing several 40" x 48" panels together, edge-to-edge, to form a single sheet long enough to cut the bottom out of in one piece. The panel is laid out on both sides of it's centerline at stations measured from the transom end. This is easily accomplished using a tape measure and framing square. After the station points are laid out, the curves are drawn with the aid of a wood batten stapled directly to the panel face. Cut the panel out using a jig saw or a circular saw as shown in Figure 2. (Note: I prefer a wormdrive circular saw, such as the one shown, which is much more accurate to use, as they are gyro-stabilized by the blade and armature turning at right angles to each other.) Plane the edges of the panel as necessary to smooth the curve to the line.


Figure 2
Cutting the Panels to Shape

Both side panels can be gotten out of a single 4' x 16' sheet of I /4" marine plywood. If you are scarphing your side panels out of 4' x 8' sheets, rip them in half lengthwise and scarph each panel separately, as it is easier to align a narrower scarph joint for gluing.

The side panels are laid out along their top edges. (Be sure that the panel edges you are working from are straight, as they form the sheer of the boat.) Stations are laid out along the edge of the panel and lines are squared down to locate the chine line. Once the layout is complete, with the chine, stem, and transom end lines drawn in, the two panels are sandwiched and cut out together.

The transom is assembled using a solid lumber core, with fore and aft faces of plywood (or from solid stock with its grain running up and down).
For the prototype, I used 7/8" red cedar for the core, plywood for the forward face, and tapered strips of mahogany arranged in a sunburst pattern for the aft face. After the transom is assembled, it is cut to shape, the side and bottom cuts being given the specified bevels.

Prior to assembly, all components should be precoated with epoxy. The side panels and transom are given three rolled coats on each side. These can be applied in one operation, moving from panel to panel in a cycle until they each have received three coats.

Sheathing the inside face of the bottom panel is done easily. The fiberglass fabric is spread out while the panel is still dry, and then is wet out with epoxy, using a squeegee to work the resin into the fabric. A second coat can be squeegeed on after the first coat has had a chance to set up a bit A final coat should be applied by roller, which will finish leveling the weave of the fabric.

Because the outside of the bottom panel will be sheathed with fiberglass after the hull is assembled, you don't need to coat it with epoxy prior to assembling the boat.


Figure 3 Section Through Stem

After the precoated panels have had a chance to cure for a day (or more, depending on the weather), you can wash them down with warm water to remove any wax that may have formed on the resin as it cured, and then sand the panels. Sanding can be done with either a random-orbital (DA)
pneumatic sander or an electric orbital sander. If you use a DA, an 80 grit, c-weight disc works well; if you use an electric sander, you will want to start with 50 or 60 grit, followed by 80 or 100 grit. This twostep procedure speeds the operation, as the 80 or 100 grit paper will load up quickly if used initially.

Assembling the Shell

Before we "stitch" the shell together, the lower edge of the bottom panel must be relieved with a router or block plane as shown in Figure I, and the inside edges of the side panels need to be beveled at the stem as shown in Figure 3.


Figure 4

Once the beveling is complete, we can proceed with the actual "stitching" process, starting with drilling and wiring the side panels together, as shown in Figure 4. (Note that the side panels are held in alignment with clamps during this process.)


Figure 5

The side panels are now inverted and spread apart 45 degrees or so. The bottom panel is placed between the side panels and is supported on boxes or sawhorses, so that it is easily brought into alignment with the side panel assembly. The first two wire stitches are then put in place, as shown in Figure 5. Note how two pairs of pliers are used to pull the short lengths of waxed iron tie wire up snug, prior to twisting them tight. After the ends are given a couple of twists by hand, the stitch is completed with two or three additional twists using a pair of Vice-Grip pliers.


Figure 6

Because of the comparatively thin plywood used in Nemah, it is possible to drill the stitching holes through both the side and bottom panels in a single operation, as shown in Figure 6. Because this eliminates the need to work from under the shell while installing the wire ties, the operation proceeds quite quickly.

As stitching progresses aft (alternately working short portions of both sides), move the loose end of the bottom panel up or down to bring it into alignment. Position the last stitches about 2 inches ahead of the transom end of the hull.


Figure 7

After the stitching operation is complete, the hull can be righted and placed on low horses. At this point the temporary spreaders are installed in their specified positions, as shown in Figure 7. (Note the centerline, which is marked on the spreader during fabrication.) Once the spreaders are fitted, adjust the hull on its horses so that the centerlines of the spreaders are in visual alignment.

The transom can now be bonded in, being fastened in place with bronze ring nails through the bottom and side panels. (Note: If the side and bottom panels do not come out even in length, align the transom with whichever is shorter, being sure to keep it parallel with the edge of the longer panel.) After the transom is in place, a temporary stringer is fitted down one side or the other of the centerline of the spreaders and attached to a block screwed to the inside face of the transom. This stringer can be a long, straight 1x6 or a couple of 5-inch by 8-foot plywood rippings. If plywood, they can be fitted to either side of the centerline with one extending forward and one extending aft. Again sight down the spreader centerline. If it does not line up with the stem, adjust the positions of the sawhorses to correct the misalignment.

Applying The Structural Fillet

The "recipe" I currently use for structural filleting has proved to be both economical and easy to work with. It is made up of one part colodial silica (Cab-o-sil), two to three parts wood flour, and enough epoxy/hardener mixture to yield a creamy peanut butter consistency. I start with about 3 ounces of resin and add the dry ingredients, using a coffee scoop as a measure. (Larger quantities tend to be hard to mix evenly and can heat up on you if not used quickly.)


Figure 8

Using a 3/4" x 8" stir stick, daub the filleting blend into two or three feet of joint. Then, using a paddle of the proper radius (1/2" radius for the transom and stem; 1" radius for the chines), go back over the joint leaving a smooth fillet, as shown in Figure 8. Remove any excess immediately with a putty knife, as it is difficult to remove after it cures. Continue working your way around the perimeter of the boat, making up additional batches of filleting compound as needed. You may have to do a little "sculpting" to fill in at some of the corners.

After the fillet cures, scrape or sand away any irregularities, using a radiused block and a piece of coarse sandpaper.

Interior Taping

Because the finished joints arc to be feathered out, it is a good practice to use a different width of tape for each layer of the joint. For Nemah, I used 8.5-ounce tape in 4", 3", and 2" widths, applying the 4" width first, followed by the 3" and 2" tapes.

Precut your tapes for each joint; including the chines, the perimeter of the transom and the inside of the stem; allowing about 2" extra on each end for lapping in the comers. Just prior to applying the tape, apply epoxy to the area of the hull where the tape will contact, paying extra attention to the fillet itself, which is often porous.


Figure 9

Pre-wet the fiberglass tape using a roller, as shown in Figure 9. Note how the longer lengths are rolled up on a short length of broom stick after wetting out. Figure 10 shows how the tape is applied to the joint, rolling out a foot or so at a time and smoothing it out with a gloved hand. All three layers of tape should be applied in one continuous process, If you should have to stop, and what has been applied sets up, you will need to sand any irregularities and then re-wet the joints before continuing.


Figure 10

After the interior tape has set, brush on one or more additional coats of epoxy to fill the weave of the fabric.

Bonding the Exterior

After the interior bonding is complete, invert the hull and stabilize it on saw horses. Clip one side of each of the wires down close to the plywood, and then ease them out, using a claw hammer.

Using a block plane followed by a sanding block, radius the chines as shown in Figure 1. Round off the bottom and sides of the transom in a similar fashion. Take care when radiusing the stem that it remains straight when viewed from the side. The stem heel should be well rounded off, as it will receive quite a number of layers of tape, and could develop a slight bulge.

After lightly sanding the whole bottom to remove any splinters and so forth, position the sheathing fabric and smooth it out. Wet out the bottom sheathing with epoxy, using a squeegee, taking care not to allow excess resin to run down the sides of the hull. At this point you have the option of immediately trimming the fiberglass fabric around the perimeter of the bottom panel with a sharp utility knife or a single edge razor blade, and then going ahead with the taping of the outside pints; or letting the bottom sheathing cure and trimming it afterward. If you plan to let it cure, you can squeegee a second coat of resin into the weave as it sets. If you do trim it after it cures, be sure to sand it to a feather edge before
applying the outside tape.

Outside taping is essentially a repeat of the procedure for interior bonding, except that it is much easier to apply, owing to easier access. As with the interior, be sure to wet out the edges of the joint before applying the tape. Apply one or two additional coats of epoxy to fill the weave.

Allow the tape to cure for a day or so, and then feather sand as shown in Figure 1. Use care while sanding to prevent excess removal of material or sanding through the epoxy coating of the side panels. (You need not sand the inside tape in the areas where the fore or aft flotation chambers will be fitted) Take extra care when sanding in the area of the radius itself: You may want to hand sand this area to prevent sanding through the tape. After sanding, apply one last coat of resin to fill in any irregularities.

Next time, in the final installment of this series, we will bring Nemah to completion, trimming her out and fitting her rig.

Visit Tracy's website:
http://www.tracyobrien.com/