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Part Three Building the Plywood-Fiberglass Boat

Overall sequence of events, plywood-fiberglass construction

1) build the strongback (the boat hull is built upside-down)
2) cut and layout the plywood side panels
3) pre-fiberglass the side panels (pre fiberglass the inside surfaces only)
4) mount the sides on the strongback
5) fasten the two ends to the sides
6) make, pre-fiberglass, and install the bottom (pre-fiberglass the inside surface only)
7) fiberglass the outside of the boat
8) rollover the boat
9) install the gunwales and the temporary, cross gunwale braces
10) pull out the strongback stations and fillet the "inside- chine-corners" of the boat
11) apply a second layer of ten ounce fabric to the inside bottom
12) make and install the interior parts
13) paint the boat

Building the Strongback


The strongback is a temporary form on which the boat is built. The plywood side and bottom pieces of the boat are fastened directly onto the strongback. The strongback defines the shape of the boat and remains in place until a later stage in the construction process. The strongback consists of three trapezoids made from pine 1"x6" boards that are held together at the corners with scrap plywood gussets. These strongback trapezoids are nothing more than temporary boat ribs. Each Trapezoid is assigned a number: trapezoid number one being closest to the bow, and station number three being closest to the stern. Stations number one and three get legs screwed on to hold the whole assembly up off the floor of the shop. Keep in mind that the boat hull is built upside down. The side panels of the boat bend and take the shape of the boat as they are fastened to the strongback stations. The dimensions for the three strongback stations are given in the plans. Draw the stations full size on a sheet of plywood or on the floor of your shop, and assemble the stations over their corresponding drawings to keep the edges parallel and square. Use white glue and short drywall screws to fasten the plywood gussets at the corners of the 1"x6" boards.

When building the strongback stations I used to meticulously cut accurate side and bottom bevels on all strongback stations. I used to build old fashioned ribs-and-plywood riverboats. Eventually I began fiberglassing the bottoms of those early boats. Then, one day, while working on a half-finished hull that had already been fiberglassed on the outside, I got an idea and beat the ribs out with a sledge hammer. Then I fiberglassed the inside and the first MRB ribless boat was born. The strongback "stations," then, are nothing more than temporary ribs. Anyway, I used to put accurate bevels on the edges of my old boat ribs. So I did the same thing when I first started making "temporary ribs." Now I don't bother. I just use a round-over bit on a router to smooth off the outside edges of the stations. It's easier and it works just as well.

Once all three trapezoids are built add two vertically oriented 1"x4" or 1"x6" legs to station #1 and two legs to station #3. Make these legs approximately eighteen inches apart and forty-eight inches long. Once the stations have been built, it is time to layout the side panels.

Fastening the side panels with screws

Each station gets two holes drilled into its side edge. These holes are drilled into the center of the pine board that forms the side of the station. The locations of these holes are given on the plan sheet. Use a drill bit that corresponds to the size of the screws you will use (to hold the side panels onto the strongback).A 3/32" bit works well for 1.25" drywall screws. When drilling the holes into the sides of the stations, drill the holes at right angles to the beveled side face of the station. This means that your drill bit will break out through the side of the station in some cases, particularly at stations number one and two where the side bevels are the steepest. This is OK.

Fastening the side panels with finish nails

An alternative to fastening the side panels with screws (and corresponding drilled holes) is to use finish nails. If you use finish nails and drive them down flush, they will be difficult to remove. If you don't drive them down flush they don't hold well, and often allow the panels to shift. To get around this problem you can drive the finish nails (or air nails) through 1" x 1" x 1/4" plywood scraps. Then you can drive the nails down tight, and still remove them easily when the time comes.

Vertical Centerlines


Mark vertical centerlines on both sides of each station that bisect the distance between the chine edges and the distance between the gunwales at each trapezoid station location. These centerlines will be used to straighten the strongback in a later step. The final step in the construction of the strongback stations is to wrap the chine corners of each trapezoid with small pieces of visqueen, tacked on with masking tape. This will prevent the stations from adhering to the bottom of the boat in a later gluing step. Once the three stations have been completed, it is time cut out and layout the two side panels from the 4'x12'x1/4" side stock.

Make the side panels


Dimensions and layout locations are given in the plans. Study the plans, and then read the following layout instructions. To make a Buffalo Boat you need to make one 4x12' piece of plywood by scarfing together two smaller pieces. It would be great if you could buy 4x6' plywood, as you could make one 4x12' piece without having any leftover, scrap. But there is no such thing. So you have to make a 4x16' panel (by scarfing two 4x8's) and the cut a 4x4' square off the end. You can use this leftover plywood to make seats and/or dry storage lockers later on.So make sure the plywood you are working with is 12 feet long. Notice the way the two side panels are taken from the one 4'x12' sheet. A long diagonal cut is made from end to end that describes the gunwale edge of both panels. The chine edges of both side panels are described by the original factory edges of the plywood. Measure along the forty-eight inch axis of one end of the panel. Make a pencil mark at twenty-six inches from one corner. Go to the other end of the sheet and make a second pencil mark that is twenty-two inches from the opposite corner of the first mark. Snap a chalk line connecting these two marks, from one end of the sheet to the other. Actually, you can cut the side panels down some. It depends a lot on how and where you want to use the boat. If, for instance, you plan to float mostly on slow moving, steady current tail water fisheries, that have no white water and are often windy, then you might want make side panels that taper from 24" at the front down to 18" or so at the rear. I like to make the side panels full width, so I can still negotiate big waves when I have to.The diagonal line you cut down through the middle of the 4x12 side panel sheet defines the gunwale edge of boat panels. Hook your tape measure on one end of the panel near a corner that corresponds to the bow end of a side panel. You will be measuring along a chine edge. Measure back toward the stern 12" and make a mark. Connect a straight line from this mark upwards at a slant to the top front corner of the side piece you are working on. This point corresponds to the tip of the stem. The slanted line connecting these two points describes the front edge of the side panel, where it meets the front transom.

Then run your tape all the way out to twelve feet and leave it there. Measuring from the bow end of the panel along the chine edge you will have marks at 12", 48", 72" and 96" and 134". The marks at 48 72 and 96 correspond to temporary rib stations #1, #2 and #3. Mark an 'x' on the rear side of each mark (you started measuring at the front end). This means the ribs (stations) will be mounted flush to the layout lines, on the rear-facing edge of each line.Square all three layout lines up to the gunwale edge of the panel (make the layout lines at right angles to the chine edge, so they extend up to the gunwale edge). The last mark is at 134" from the bow, or 10" from the rear end of the panel. This last mark is connected with a slanted line up to top rear corner of the side panel to describe the transom cut. Repeat on the other side of the chalk line to mark and layout the other side panel. Use a sharp saw and a steady, patient hand to cut out the two side pieces.

At this point the two side panels have been laid out, and cut out. But notice that the layout lines on one panel correspond to the outside of sheet, and the layout lines on the other panel correspond to the inside of the sheet. This happened because of the way the two side panels were oriented on the original 4'x12' panel. Turn both panels over, and repeat the layout procedure on the two opposite sides. The layout lines will now appear on both the inside and the outside of both side panels.

Pre-fiberglass inside side panel surfaces
Pre-fiberglass the "inside" surfaces of the two side panels
Now that the side panels have been cut and laid out, it is time to fiberglass the inside face of each sheet. Understand that you are to fiberglass the inside face of each side panel only. If you were to fiberglass both faces of a sheet, you would be unable to bend it onto the form.

Place four sixteen foot 2"x4" boards over a pair of saw horses, and then place both side panels on top of the 2"x4"'s with the inside faces up. Precut two pieces of ten ounce fiberglass cloth so they are about one inch bigger than each side panel on all sides. You can cut both side pieces from one 12' 6" length of 50" wide fabric in much the same manner as you did the plywood sides. Roll the fabric up and set it aside. Refer to the fiberglassing review at the beginning of these instructions. Don't use too much resin.

Make the transoms


Dimensions for the transoms are given in the plans. To make the panel that will eventually become the bottom of the boat, you will have to scarf two 4x8 pieces of 3/8" plywood, in much the same way you made the ¼" side panel pieces. But you'll only about 10' of it to make the bottom panel. So, from one end of that 4x12x3/8" sheet you cut the two transom ends. Any plywood left over after that can (also) be used to make seats in a final step.The transoms have a bottom bevel and two side bevels that are given in the plans. Draw vertical centerlines down through the middle of the transom on both sides before cutting out the transom. Fiberglass the inside surface of the transom and set it aside. In about two or three hours, or whenever the resin on the side panels reaches a half-cured, rubbery state, use a sharp knife to trim off the excess fabric from the edges of the two side panels. Once the resin is tack free, but not necessarily fully cured, you can apply another coat of resin with a foam roller, and then flatten it out with a drywall trowel as necessary to fill the weave of the fabric and to achieve a smooth finish.

Mount the Plywood Sides on the Strongback


It is possible to proceed alone at this point. In fact the entire boat can be built while working alone, but it is a lot easier if you have a helper for the next step. Stand up station #3 and fasten one of the two side panels to it with one screw through the corresponding holes that are nearest the chine corner of the station (or with a finish nail, if you choose that method). Your helper will have to hold the station to keep it from falling over. Drive the screw (or nail) in half way only. Let the front end of the plywood droop down to the floor. Repeat on the other side. Then stand up station #1. Pick up the front end of either side panel and fasten it to station #1 with one screw (or nail) through the hole nearest the chine edge of the station. Drive this screw in half way. Repeat on the other side. Then drive in the bottom screws at stations #3 and #1, driving the screws only halfway. Install trapezoid two, driving these screws in all the way as you go. Drive these screw in halfway only. Then drive all remaining screws in tight.

Install the Transoms


Wet out both side edges of one transom (front or rear) and the edges of the plywood sides where that transom will go. Apply thickened resin to the transom and then fasten with 1 1/4" drywall screws (or pneumatic air staples if you have them). If you use nails or screws, don't bother to try to fasten at a steep angle, into the middle of the thin, 3/8" tansom panel. Fasten at right angles to the side panel, and let the nails or screws break out through the outside face of the transom. Once the resin has cured (thus gluing the transom to the side panel) you can then back out the screws and putty the screw hole. Repeat at the other end.Straighten the Form

Now that stations 1-3, the sides, and the two transom ends are in place, it is time to straighten up the form. If you are working over a wood floor, move your boat aside and snap a 12' line through the middle of your work space. Then screw a straight, sixteen foot 2"x4" board to the side of the line. Screw straight scraps of 1"x4" or 1"x6" boards vertically along the centerlines of the strongback stations that run from the floor of the shop up to the top of each strongback station. Use screws or toenails to fasten these vertical "centerline legs" to the straight edge that is now mounted on the floor of the shop. Pulling the vertical center legs over to the straightedge quickly straightens up the whole form, sides and all. Screw a 1"x4" to one side of the centerline that runs down through the middle of the transom so it reaches down to the straightedge on the floor of the shop. Toenail or screw the 1"x4" to the straightedge. Screw a 1"x4" to the flat, inside surface of the stem so it reaches down to the straightedge on the floor of the shop. Screw or toenail this board to the straightedge. Add braces here and there as necessary to firm up the whole assembly. Once things are all centered up and braced firmly in place, you are ready to make the bottom panel.

If you are working over a concrete floor, you cannot fasten a 2"x4" to the floor of the shop, so you will have to straighten up the form without fastening it rigidly to the floor. To do this, you will need a 1/4" x 12" x 12' length of smooth exterior masonite siding material. Slide the masonite board underneath the boat. Climb under the boat, lift up the masonite board, and use drywall screws to fasten it to the top cord of each station. Fasten the ends to the top edge (closest to the floor) of each transom end, and then cut off any waste ends (extending out past the front and rear ends of the boat. Because one edge of the siding board is now lined up with the center lines (that you made in an earlier step) on the trapezoid stations, and with the centerlines on the transom ends, the strongback should now be quite straight, although it is still "free-floating," and not fastened in any way to the concrete floor. Use any remaining scraps of wood to add extra legs and diagonal braces here and there until the strongback assembly is rigid and firm all around. Now you are ready to proceed with the next step making and installing the bottom.

Make and Install the Bottom


Working with a sharp block plane or a wood rasp, plane off the high "outside" edges of the 1/4" plywood side panels so they are flush with the bottom of the strongback stations (and parallel to the shop floor). Use a five foot straight edge to check your work as you go. Do not plane anything lower than the inside edge of the plywood sides.

The five foot by 16 foot bottom panel has already been cut down about 3' shorter when you made the two transom pieces. Snap a line down the middle of the bottom panel from end to end. Place the bottom panel over the strongback, so the centerline down the middle of the panel lines up with the centerlines on the stations. Push down on the front end of the bottom panel until it touches the bottom edges of the transom (which is closest to the ceiling, because the hull is still upside down). Drive a screw through the panel into the transom to hold the panel temporarily in place. Have your helper walk around to the other end of the boat and push down on the rear end of the panel until it touches the other transom. While he or she is doing this, drive a series of screws, two to each station, through the panel into the bottom cord of each station. Start at the bow and work back toward your helper at the stern. Look underneath the boat periodically to make sure the centerlines on the stations are lining up with the centerline on the panel. Finish up with two screws driven through the panel into the bottom edge of the transom. Reach under the edges of the bottom panel with a pencil and draw a line around the outside of the boat to describe the final shape of the bottom. Back out all the screws and take the panel off the form. Turn it over for cutting. Set a sharp power circular saw or a jig saw at twenty seven degrees and cut tight to the line. If you use a power circular saw, set the depth of the cut just deep enough to cut through the plywood.

Pre-fiberglass the "inside surface" of the bottom panel


Place the panel flat on a pair of saw horses with the inside surface facing up. Fiberglass this face as you did the side panels. Trim off the excess fabric when the resin is at a half-cured, rubbery stage. You can turn the bottom panel over and fasten it permanently in place as soon as the resin is tack free, or you can take the rest of the day off and resume work again when the resin is totally cured.

Install the Bottom


Wet out the mating surfaces with unthickened resin. Trowel a heavy bead of thickened resin onto the .25" chine edge of the plywood sides. Use microfibers for the thickening. You'll need a helper when lowering the bottom panel down onto the strongback in order to avoid a glue mess. (The chine corners of your strongback stations should have been covered with masking tape in an earlier step) Drive the first screw back into one transom end, and work back toward the other transom as you did before. The bottom panel should now be held in proper alignment, but the outside edges will not be held down tight enough for a good glue bond (to the edges of the side panel). Walk around the perimeter of the boat, drilling adjacent pairs of small holes through both the bottom of the boat and the side, an inch or so back from the chine. Then use 8" lengths of bailing wire to sew the joint together tightly. Sew the wire through both holes and twist it up tight, just as if you were tying off re-rod in a concrete form. If you have an air stapler (of almost anykind) you can staple the bottom of the boat down into and onto the ¼ " side edges. Don't worry if a large number of these staples miss their mark and/or split out through the sides of the plywood near the chine. It doesn't matter. When the glue has dried, pull out any bailing wire or staples, and then round off the chine with a wood rasp or a grinder. Any stuck wires can be removed by heating them up first, with a torch or a soldering iron. Round off the chine as much as you dare without grinding right through the joint. If the joint between the bottom and the side panel fractures while you are rounding off the chine, ignore the crack for the time being, and finish rounding off the chine. Then work some fresh resin into the fractured area of the chine, and nail or clamp the chine-joint one more time. Once the grinding and rounding of the chine is complete, back out all of the screws in the bottom panel of the boat. Drive a new set of screws through the side panel into the side of each strongback station at the gunwale edge of the boat. You now have three screws (or nails) in the side cord of each station, but not for long, as you should now back out all side-cord screws except the ones you just placed near gunwale edge of the boat. Before proceeding to the next fiberglassing step, carefully rub some soap or candle wax into the phillips head notches of the sixt screws that remain along the gunwale edge of the boat. You will be covering these screws with fiberglass in the next step, but you will eventually need to dig these screws out. Working with a sharp block plane or a wood rasp, plane off the high "outside" edges of the 1/4" plywood side panels so they are flush with the bottom of the strongback stations. Use a five foot straight edge to check your work as you go. Do not plane anything lower than the inside edge of the plywood sides. Use a block plane and sandpaper, or a wood rasp and sandpaper to round off the chine edge of the boat. Round off the chine as much as you dare without grinding right through the joint. If the joint between the bottom and the side panel fractures while you are rounding off the chine, ignore the crack for the time being, and finish rounding off the chine. Then work some fresh resin into the fractured area of the chine and nail or clamp the chine-joint one more time. Once the grinding and rounding of the chine is complete, back out all of the screws in the bottom panel of the boat. Drive a new set of screws through the side panel into the side of each strongback station at the gunwale edge of the boat. You now have three screws in the side cord of each station, but not for long, as you should now back out all side-cord screws except the ones you just placed near the gunwale edge of the side panel. Before proceeding to the next fiberglassing step, carefully rub some soap or candle wax into the phillips head notches of the sixteen screws that remain along the gunwale edge of the boat. You will be covering these screws with fiberglass in the next step, but you will eventually need to dig these screws out.

Fiberglassing the outside of the boat


The sides of the boat will be fiberglassed first, and then the bottom. The outside lay-up can be broken into a series of steps over a period of several days, but you will achieve the best results if you complete the lay-up all at once. Sweep off the floor of your shop to make a clean cutting area and roll out a 12' 6" length of 50" wide fiberglass cloth. Cut out two pieces of fabric similar to the ones you glassed the inside of the side panels with in an earlier step.

Wet out one side of the boat with a foam roller. Pick up a piece of side fabric with you at one end and your helper at the other end, and lay the fabric onto the wet side of the boat. Place the fabric onto the boat with the ragged, scissor-edge lapping over the gunwale. Place the clean factory edge of the fabric up so it laps one inch over the chine. Pull on the ends and the edges of the fabric to put tension on the weave of the fabric. Use a drywall trowel or a rubber squeegee to work out any remaining wrinkles. Stroke the trowel from the middle of the fabric out toward the edges. Cut the front edge of the fabric down through the middle of the stem. Wrap the rear end of the fabric over the side edge of the transom. Trim back the rear end of the fabric so it laps the transom by an inch or so. Repeat on the other side of the boat. Wait until both side pieces are in place before applying more resin to the fabric. Try to leave the fabric with its rough, pebbly texture unobscured by slick puddles of resin. On the other hand, cloudy- white areas that haven't become fully transparent don't yet have enough resin.

Once the sides have been glassed, you are ready to stop for the day, or immediately go on to glassing the bottom of the boat. I recommend going ahead with it. Wet out the plywood bottom panel with a foam roller. Roll out a 14' 6" piece of 60" wide fabric and cut it off. Carefully lower the fabric onto the bottom while pulling tight on the ends of the fabric. Pull on the edges of the fabric, and stroke out any wrinkles with a rubber squeegee or a 6" drywall trowel. Trim the edges of the fabric to follow the chine of the boat. This first layer of bottom fabric should lap over the chine edge by about one inch. Once the first layer of bottom fabric has been lowered into place and trimmed to size, then finish wetting out the fabric. Once again, use just enough resin to make the fabric transparent.

You have to let the resin in the first layer of fabric kick for a while before you can install the second layer of fabric. The edges of the fabric (where they lap over the chine) rarely stick down at first. Resist the temptation to brush on more resin. Ignore these edges until the resin has become thick and sticky. Once the resin is thick enough, the edges can be pressed down and they will stay down. While you are waiting for the resin in the first bottom layer to get thick and tacky, it is a good time to fiberglass the transom. Cut a piece of ten ounce transom fabric that is approximately an inch large all around. Wet out the transom and then put the fabric in place. Trowel out any wrinkles, and then continue wetting out the fabric until it becomes transparent. Then cut the fabric flush with the edges of the transom. The side fabric already overlaps the transom, so there is no need to lap this corner once again with the transom fabric. Once the resin on the bottom of the boat has become thick and tacky to the touch, like raspberry jelly left out in the sun on a picnic table, it is time to install the second layer of bottom fabric. Spread a fresh wet-out coat of resin with a foam roller. Roll out a second 14' 6" length of 60" wide fabric, and lay it down on top of the first layer. Pull on the edges of the fabric to work out any wrinkles. Trim back the side edges of this layer so they lap the chine approximately two inches. Then add more resin until all cloudy-white areas have disappeared. Take a break now and give the resin in this second layer time to kick. This second layer will kick a lot faster than the first layer because of heat generated by the chemical reactions taking place in the first layer. Once the second bottom layer has become thick and tacky, you are ready to apply two layers of fiberglass tape over the chines.

Wet out the chine were the first layer of 6" tape will go. Then cut a ten foot length of six inch fiberglass tape and lower it down onto the boat so it straddles the center of the chine, starting from a point twelve inches back from the stem. This will place the rear end of the tape approximately 36" from the transom. Pull tightly on each end of the tape to tension the weave and to pull out any wrinkles. Place your hands over the midsection of the tape (you're still wearing rubber gloves) and stroke your hands out toward the ends to squeegee out any air bubbles in the tape. Repeat on the other side. Finish wetting out both tape pieces. As always, use as little resin as is necessary to get the job done. Use six inch tape to cover the stem and both transom edges. Take another break. Once this resin has become tacky, you are ready to tape the chine from end to end with a second and final layer of fiberglass tape. Clean up your tools with solvent and take a bath.

Once the resin in this outside lay-up has completely cured, you should wash it with a tack rag and acetone. Then put on a good dust mask and sand off any bumpy resin drips, and smooth off the lumpy edges of the chine tape. Fifty grit silicon carbide floor surfacing paper in an orbital sander will grind off a lot of resin in a hurry. A fifty grit disk in a right angle grinder is even faster, but be careful and use a light touch. Change grits periodically and finish off with 180 grit paper. You may need to do some intermediate resin patching during this smoothing process. If so, use a mixture of resin and #407 micro balloons. It may take two or three additional coats of resin (with intermediate sanding) to really slick up the bottom of the boat. The smoother the bottom of the boat is, the better it will eventually row. Once you have faired or smoothed the hull to your satisfaction, mix up a fresh batch of resin and thicken it with #423 graphite powder. Mix in the powder at about twenty percent by volume. Roll this mixture on with a foam roller, Use a bristle brush to work out any lines left by the edges of the roller. Carry this thickened mix up the side of the boat to the edges of the six inch chine tape.