3>Part Five

Balsawood-fiberglass construction

Note: The following intructions are for building a one-off, hand-molded boat with balsa core. I have built several boats this way (still have one today) and like them a lot. But balsacore has now been made obsolete by the new, polyethelene honeycomb core materials, sush as NidaCore and several others. I'm leaving these instructions here, in place, largely for historical reasons now.
End grain balsawood is sold in two foot by four foot sheets that consist of many 1"x2"x.5" blocks that are held together by a thin veil of fabric that is glued to one side of the sheet. Although it weighs very little, balsawood has tremendous compression strength against the .5" end grain. Covering both sides of a sheet of end-grain balsawood has the effect of creating a stressed skin panel. In other words, in order to bend a sheet of fiberglassed balsawood, it would be necessary to make one of the two layers of fiberglass stretch at a greater rate than the other layer. This is not easy to do, and that is why balsawood-fiberglass boat hulls are so stiff and so light at the same time. Balsawood boats are strong, fantastically light, and they row and handle better than any other boat on the river. And the only way to get one is to build it yourself. To build a balsawood boat, you have to cover a strongback with plywood to serve as a holding form on which you carefully drape the sheets of end-grain balsawood blocks. The exposed balsawood is then covered with a skin of epoxy- fiberglass. After the outer fiberglass skin has been completed, the boat is taken off the strongback and carefully turned over. Then the inside of the boat is skinned with fiberglass. After skinning the balsawood core, inside and out, two inch strips of balsawood blocks, four layers thick, are built up on the top-outside edge of the hull to form a balsawood gunwale. The balsawood gunwale is rounded off and covered with three or four layers of graphite fibers and fiberglass fabric, and the boat hull is then complete. Finishing a balsawood hull is much the same as it is for a plywood hull. Balsawood construction is more expensive and more time consuming than plywood construction. It is possible to use standard AC construction plywood to cover the sides of the strongback, but it isn't easy. Cheap construction plywood doesn't bend well, and it has a tendency to fracture at the sides of the boat near the front seat position. I have used construction plywood with success in the past, but you have to bend it with care. Wetting the outer lamination of the side panels does help a little, but you have to be careful, too much moisture in the plywood makes the problem even worse. I prefer to make my strongback side panels from AB exterior panels (that I get from Harbor Sales or any other marine-plywood distributor). AB plywood bends well, and it is available in full sized, 4'x16' and 5'x16' sheets.

Material List--Balsawood-FiberglassConstruction


10 gallons #105 resin
2 gallons #205 hardener
1 pair mini pumps
1 gallon #850 WEST SYSTEM solvent
1 bag #403 microfibers 1 bag #406 colodial silica
1 bag #407 micro balloons
1 bag #423 graphite powder
1 roll 1" graphite tape
1 roll 6" fiberglass tape
12 YD. 50" ten ounce fabric
23 YD. 60" ten ounce fabric

Other Materials

1 4'x16'x1/4" AB exterior plywood
1 5'x14'x1/4" AB exterior plywood
1 200sq' box 1/2" end grain balsa wood*
8 1"x6"x16' #2 pine boards for the strongback
2 sheets 3/8" CDX plywood for corner gussets
2 1"x4x"10' Fir or mahogany boards for seat rails. (both a rower's seat rail and a front seat rail can be cut from one ten foot board)
2 1"x3"x14' fir or mahogany boards for seat frames
6oz. Special T (a thick, cyanoacrylate "super-glue") 2oz.
Kick It cyanoacrylate accelerator
Special T and Kick It are available from Sheldon's Hobbies in San Jose California.
Sheldon's telephone order number is 1-800-822-1688
*One source for 1/2" end-grain balsa wood is King Fiberglass in Seattle Washington. King Fiberglass takes orders over the phone, and their telephone ordering number is 206-283-8818.

Prepare the strongback stations

The strongback stations are the same as they are for plywood construction, except that the side legs of each strongback station have to be approximately two inches longer. The overall size and shape of each station remains the same, but the side legs of each station should run past the gunwale edge of the side panel a few inches.

Prepare the side panels

Cutting out and laying out the two strongback side panels is the same as it is for plywood construction. The difference is that in plywood-fiberglass construction, the side panel became a permanent part of the boat. In balsawood construction the plywood is used only as a temporary holding form for the balsawood sheet material. The inside surface of the side panel for the plywood boat was pre-fiberglassed before installing it on the strongback stations. For balsa construction, the plywood is not fiberglassed, inside or outside, but it does get painted and then waxed. The outside surfaces of the two side panels (and the bottom panel in a later step) must be sanded, painted with two coats of glossy water based paint, and waxed with mold release wax prior to their installation on the strongback stations. Furniture wax can be substituted for the mold release wax. Installing the side panels and the bottom panel on the strongback stations proceeds in the same manner as the plywood boat.

Install the stem and Transom

Make a stem and transom out of whatever you have on hand. paint and wax the outside of the transom. Install them according to the instructions for the plywood boat.

Straighten up and brace off the strongback

Refer the instructions for the plywood boat.

Make and install the bottom panel

Use a full-size sheet of 5'x14'x.25" AB plywood (purchased from Harbor Sales or any other plywood distributor) to make the bottom panel. The bottom panel installation is much the same as it is for plywood-fiberglass construction, however, there is no need to glue the bottom panel to the side panels. Draw a centerline down through the long axis of the 5'x14' panel. Place the bottom panel onto the form, square corners and all, line up the centerline on the panel with the station centerlines on the form, and then screw the panel temporarily in place. Draw around the edges of the panel. Remove the panel for cutting. Cut tight to the line with a saw set at twenty-seven degrees
Paint the outside surface of the bottom panel with two coats of glossy paint. Wax the panel with mold release wax or furniture wax. When painting and waxing, pay particular attention to the end-grain edges of the 1/4" bottom panel. The edges of the panel will need extra amounts of both paint and wax in order to seal them up, and to make these edges smooth enough to release from the completed hull in a later step.

Drape the balsawood on the outside of the form

The sheet balsa wood material is placed on the sides of the form first. The "fabric-side" of the balsawood sheet material is placed against the strongback. The bare, uncovered balsawood surface faces outwards. The mating edges of the 2'x4' sheet material are held together with spots of any thick, gap-filling cyanoacrylate super-glue. Make a glue spot every three inches or so. The super-glue I use is Special T. The hardening time for Special T is about thirty to forty seconds. Any two sheets of end-grain balsawood can be joined together by placing several spots of Special T onto the edge of one sheet, and by pushing the two sheets together and holding them tightly in place for forty seconds. This process can be speeded up dramatically with the use of a cyanoacrylate hardening accelerator, like Kick It. To join two sheets of end-grain balsawood with Special T and Kick It, start by spraying the edge of one balsawood sheet with Kick It accelerator. Then squeeze out several spots of Special T onto the edge of the second sheet. Then push the two sheets together and hold them for about six seconds.
In order to keep the balsa wood sheets from sliding off the sloped sides of the form, it is necessary to build a ledge at the "gunwale" edges of the strongback. This is why the side legs of the strongback stations were made longer than the side legs of the plywood boat. Rip two 1"x1"x16' lengths of pine and screw them to the side legs of the strongback stations, flush to the gunwale-edge of the boat. These two 1"x1" rippings form a convenient ledge that keeps the balsawood sheets from sliding off the sides of the form. The outside edges of the balsa wood at the stem, transom, and the chine edge of the boat are trimmed off to size with a razor blade knife and course sandpaper. Any areas of the balsa wood side panels that don't lie flat to the underlying plywood form can be held in place with 5/8" drywall screws driven through the plywood from the inside of the form. Have a helper push down on the bulging area of balsa wood on the outside of the boat while you drive the screw from the inside of the form. Once the sides have been covered with balsa wood. The bottom is ready to be covered in the same manner. Then cover the transom with balsa wood. Use Special T and Kick-It accelerator to glue the balsa wood on the sides of the boat to the balsa wood on the bottom of the boat. Use a spot Special T every two or three inches. Once all of the balsa wood is in place, the chine is rounded off by hand with a sheet of course, fifty grit sand paper.

Fiberglassing the outside of the boat

Fiberglassing balsawood isn't very different from glassing plywood, but there are a few things to watch out for. First, the end grain of the balsa wood is capable of soaking up large quantities of resin, and must be sealed off prior to the actual fiberglassing. To do this, spread a wet-out coat of resin over the entire outside surface of the boat with a foam roller, and then wait two or three hours for the resin to kick before proceeding with the lay-up. The outside lay-up itself is the same as it is for the plywood boat, except that a third layer of ten ounce fabric is added to the lay-up in the middle of the boat. This third layer of fabric is only nine feet long, and does not cover the length of the bottom, and it is sandwiched between the other two, full-length layers of fabric. The front edge of this nine foot, intermediate layer of ten ounce fabric starts at a point approximately twelve inches from the bottom of the stem, and continues backwards to a point approximately thirty six inches past the middle of the boat. The first layer of bottom fabric should lap the chine one inch, the second layer 1-1/2", and the third layer should lap the chine a little more than two inches. Remember to use just enough resin to get the job done, and try to leave the lay-up showing the rough, pebbly texture of the weave. Put at least 8 layers of fabric over the chine, with any combination of bottom layers overlapping side layers, plus additional layers of 6" and/or 3-1/2" fiberglass tape. Repeated layers of fiberglass tape over the chine can cause a slightly abrupt edge, at the bound edges of the tape, to occur. You can hide this edge during final finishing by trowelling on a mixture of resin and microballoons....just as if you were finishing off a drywall tape joint.

Smooth off the outside lay-up

Smoothing the outside of the boat proceeds in the same manner as the plywood boat.

Marking the center of the boat

Before taking the boat off the form and turning it over, the center of station #5, at the 98" layout line, must be marked on the balsa wood sides of the boat. Drill four small holes in the side of the boat, two holes on each side, right into the center of station number five. Make one hole near the gunwale, and one near the chine on each side of the boat. Immediately patch these holes with resin and micro balloons. When the boat is taken off the form and turned over, these holes will appear as small red circles. A pencil line can then be extended down through these holes to the floor of the boat, and then connected across the floor of the boat. This line marks the middle of the boat for a future leveling step.

Take the boat off the form and turn the boat over

Build a cradle for the boat and set is aside on the shop floor. Jiggle the boat until it breaks free from the form. You may have to swear a few times make this happen. Carefully lift the hull straight up and off the form. Turn the hull over and set in on the cradle. Be careful. The boat is still delicate and easily broken at this point. Dismantle the form. A balsa wood boat does not relax the way a plywood boat does, and no special precautions have to be taken to hold the shape of the boat.

Fiberglass the inside of the boat

Fillet the inside edges of the boat. Then apply one layer of ten ounce fabric on the sides of the boat, and two layers of ten ounce fabric on the inside bottom. Remember to let the first wet-out coat of resin kick for a few hours before applying the fiberglass.

Build the gunwale

Gunwales--stage one

Building a balsa wood gunwale is possible for one worker to accomplish on his own, as is the whole boat building process for that matter, but it is a lot easier to accomplish with a helper.
Cut 116' of two inch strips of balsa wood blocks. Remember that the balsa wood comes in two foot by four foot sheets of two inch blocks. These blocks are aligned in parallel rows, so it is easy to bend a 2"x4' row of blocks over at ninety degrees to the horizontal, and to run a sharp knife blade through the fabric backing of the sheet to separate a 2"x4' strip of blocks. You'll need twenty nine such 2"x4' strips to lay-up the gunwale. You'll also need many more "C" clamps than you did for the oak gunwale on the plywood boat. Sixteen clamps are a minimum, which allows you to do one side of the boat at a time. Thirty two or more clamps is better. Most rental shops rent C- clamps. I have approximately sixty C-clamps in my shop, and I use them all when making a balsa wood gunwale.
Spread a sheet of visqueen over a work bench to make an area where you can work with balsa wood strips and a sloppy glue mess at the same time. To wet out the strips, you'll need a two inch foam roller. You will probably have to buy a two inch roller with a knap cover at the paint store (and throw away the knap roller cover) to get a two inch roller frame. If the paint store doesn't sell two inch foam rollers, you can cut down a larger one. Cut four 8'x2"x1/8" strips of masonite or .25" plywood, and set them aside.
Wet out the outside, top two inches of one side of the boat. Wet out the fabric side of about eight to twelve four foot strips of two inch balsa blocks.
Start at the transom end of the boat by placing a strip of balsa wood onto the top outside edge of the boat. Place the fabric side of the balsawood sheet material flush against the outside of the boat. Hold this first strip temporarily in place with spring clamps, or if you don't have spring clamps, use "C" clamps. Then add a new strip of balsa wood onto the top edge of the boat, immediately toward the bow from the first strip. Then wet out the exposed, outer surface of the balsa wood strips with the roller. Then cut a third strip in half to make a 2"x24" starter- strip. Start a second row of balsawood blocks with the 2"x24" starter strip right on top of the first row. Readjust the clamps to hold both rows. The two foot starter strip for the second row staggers the joints between the first two rows, which helps somewhat in the assembly process. Run these two rows up to about amidships, and then go back again to the transom and start a third row. But this time, when you readjust the clamps to hold all three rows together on the gunwale, cover the balsa wood with a six inch strip of visqueen, and then place an 8' strip of masonite on the outside of the whole works. The masonite helps to spread out the clamping pressure and hold things in alignment. The visqueen keeps the masonite from adhering to the balsa wood. Keep adding strips of balsa wood like building blocks until the length of the gunwale is covered with three layers of blocks and clamped (with masonite strips) the whole way. You do not need very much clamping pressure. If you have enough clamps, immediately repeat on the other side. At this point you can quit for the day and let the resin harden completely, or you can take a break for about two hours to let the resin kick to a thick, very viscous and sticky state, like days old grape jelly on a picnic table. And then proceed with the next step.

Gunwales--stage two

If you wait until the resin has cured, you will have to sand out the glue bumps that form into the folds of the visqueen before you can proceed. If you don't wait for the resin to cure, you can remove the clamps after a few hours (actual kicking time varies tremendously with temperature and humidity conditions) and proceed without sanding. Remove the clamps, the masonite, and the visqueen. Any thick, sticky blobs of resin on the outside of the balsawood gunwale can be smoothed out with a little solvent and a putty knife. Wet out the outside surface of the existing balsa wood gunwale, and then wait for a few hours for the resin to kick. Roll out a seventeen foot length of graphite fibers and hold it at both ends. (have your helper grab the other end if your arms aren't that long) Place the graphite fibers onto the outside of the gunwale with a maximum of tension on the fibers. Wet out the fibers with a bristle brush. Cut a long slanted cut on one end of a strip of balsa wood blocks, similar to one half of a scarf joint, so the long end of the cut is on the fabric side of the strip. Wet out the fabric side of the balsa wood strip. Place this new strip of balsa wood right on top of the wet graphite fibers so the slanted end points back toward the transom from a point twenty four inches forward of the corner of the transom. Keep adding strips of balsa wood on top of the wet graphite until you reach a point about twenty four inches back from the stem. Finish off with a slanted cut at the front. Clamp off the fourth row of balsa wood with visqueen and masonite as before. Now you do have to wait a day for the resin to cure. Go ahead and start work on the interior parts while you are waiting.

Fiberglass the gunwale

Once the gunwale resin is hard, round off all three edges of the gunwale with course sandpaper. Turn the boat over just long enough to fillet the underside of the balsa wood gunwale, where the gunwale makes a square corner with the outside of the boat. Then turn the boat over again. Wet out the balsa wood gunwales and wait a few hours for the resin to get thick. Take off your shirt and crank up the heat if you want to hasten the curing time. Wrap the length of the gunwale with six inch fiberglass tape. Pull on the ends of the tape to put a maximum of tension on the weave of the tape. Repeat on the other side.
Don't rush the lay-up, and don't use too much resin. You won't be able to get the edges of the tape to lie down flat at first. Resist the temptation to keep brushing on more resin. Be patient and wait for the resin to get thick and sticky. Then brush down the tape. As soon as the first layer of tape is in place, cover the entire gunwale with a second layer of tape. Stagger the edges of the tape so both edges aren't directly on top of each other. Make sure the second layer of tape laps over the fillet on the underside of the gunwale. Let this resin cure for a day.

Make the oarlock holes

Locate the oarlock positions with the help of the reference marks you made on the inside of the boat. One oarlock hole goes 3" to the front of the reference line, one 3" to the rear. The third oarlock position is located 15" to the rear of the 98" layout line. To make the oarlock holes, drill out the gunwale all the way to the bottom with a 1.5" spade bit, but don't drill through the fiberglass on the bottom of the gunwale. This makes a 1.5" hole in the gunwale that has a thin fiberglass bottom to it. Use a 5/8" twist drill to bore through the thin layer of glass on the bottom of the hole.
Cut a 2.5" inch piece of 5/8" pine dowel. Drill a 1/4" pilot hole through the center of the pine dowel, parallel to the long axis of the dowel. Stick this dowel into the 1.5" hole so it just protrudes out through the 5/8" hole in the fiberglass at the bottom of the gunwale. Reach under the gunwale and use a hot glue gun to fasten this dowel temporarily in place. Mix up a batch of microfibers and resin, and fill the balance of the 1.5" hole with thickened resin. Repeat for all six holes.
When the resin is cured, sand off the top of the wooden dowel and any lumpy resin until the top of the gunwale is flat again. Use a 5/8" twist drill to bore down through the dowel to open up the oarlock hole. The bit should track straight because of the pilot hole pre-drilled in the dowel, but be careful. Then stretch one more length of graphite fibers on the outside of the gunwale, and cover the whole works with one last layer of six inch fiberglass tape. The graphite fibers and the six inch tape only need to cover the portion of the gunwale that is four layers (of balsa wood) thick. The boat hull is now complete.

Interior Parts

Finishing the interior parts of the hull is much the same as it is for a plywood hull. You can make the interior parts (the parts that correspond to full size patterns) out of plywood, but I prefer to use more balsa wood. Use the full size patterns to cut slabs of balsa wood sheet material to shape. Use Special T and Kick It accelerator to piece together scraps of sheet balsawood when making these parts. Once the parts that correspond to the full size patterns have been roughly assembled from balsawood scraps and super glue, and once these parts have been marked and trimmed to shape with the help of the patterns, any outside, exposed edges must be identified and "capped" with super glue and 1/2" x 1/4" strips of fir or mahogany. Then fiberglass both sides of the balsa wood with ten ounce fabric. Then proceed as if the balsa wood were slabs were plywood. Cutting, fitting, and gluing instructions for balsa wood interior parts are the same as for plywood parts

Paint the boat

Sand and paint the boat.