Foamboat Construction

Go from sketch to model to fullsize solid boat hull in easy steps using my FOAMBOAT method. Wrapping fiberglass over a pre-carved foam male mold is not new, it is regularly used to make custom surfboards. What I offer is a way to shape a mold that will yield smooth, symmetrical and repeatable results for those of us who do not have a sculptor's eye and who do not want to get covered in foam shavings and dust. I developed the technique for making long, narrow solid boat hulls, but it could be adapted to make all kinds of shapes and molds for other purposes. The photo is 1-inch to the foot drawing of my first pontoon, P1, used to work out the techniqes, and it is all that is needed to create all the components for a model and, scaled up, for the 15-foot hull itself. Vary the sizes, shapes and spacings of the lines on the sketch to create hulls, or other objects, that suit the user. To see my related Instructables, click on "unclesam" just below the title above or in the INFO box to the right. On the new page that appears, repeatedly click "NEXT" to see all of them.
Unclesam

These are all the parts for a model of P01, the very thin plywood, from hobby shop, cut directly from the drawing, bats of foam cut large enough to enclose the wood. Though their edges are curved, all wood components are flat, all assembly joinery is at right angles and easily made that way by the square cross-secion of the foam batts, which act as gluing cleats.

The model is assembled using wood construction glue, the component placing determined by the original drawing. Rubber bands keep everything in place until the glue dries. I use scraps of any rigid foam for the models, but when building fullsize hulls I buy white expanded polystyrene, one pound per cubic foot density. There is a factory that makes foam not far from me, for use as insulation in large industrial buildings by the truckload, but they will slice off sheets in thicknesses that I need, then I slice the batts myself. Foam is not something that is usually shipped long distances, so you can probably find a similar factory near you.

The curvaceous hull shape is revealed by cutting away the excess foam using a hotwire cutter, sliding the wire lengthwise along adjacent pairs of wood edges in turn. This is the beauty of my FOAMBOAT method, and the revelation never ceases to thrill! The foam cutter pictured is sold at hobby and other shops that sell foam shapes as bases for making wreaths and arrangements from plastic flowers. It uses two "D" cells, but I soldered wires onto mine so I could connect it to a power supply.
This is the first model I ever made using the method, and it worked perfectly. The fullsize hull uses the exact same process with the components scaled up from the original small drawing. For plans for a big-boy size foam cutter, link to my album "Cutting Rigid Plastic Foam," click page "READ FIRST" for instructions for reading and printing fine text. http://www.flickr.com/photos/sm_vermn/tags/foamcutter

Hull is assembled upside down. The plywood deck board is on the table and the first batt of foam glued to it along marked centerline. Foam batt thickness matches the drawing dimension.
I first scaled up the drawing onto pieces of MDF (medium density fiberboard), cut out the pieces and sanded their edges smooth. I used those as templates to mark the plywood parts, which I rough-cut out using saber saw, then used the MDF templates and pattern-following router and bit to trim the parts. Since P01 was symmetrical end-to-end and side-to-side, a template of only one quarter or one half of each wood piece was needed. The templates also ensure that molds for multiple hulls of the same design will be identical. I sanded the wood edges and painted them with epoxy to keep the hotwire cutter from catching a snag during the eventual carving.

Vertical web, or keel, is glued in place, most of the glue area is provided by the foam batts. I joined pieces of plywood, very thin marine plywood in this case, using butt joints backed up by strips of the same wood. All gluing, wood-to-wood, foam-to-foam and wood to foam, was done using Titebond II, which is water-resistant, (III now available), from home centers, and which has long "open" time. The assembly process needs to be done one step at a time, with adequate time between steps to allow the glue to set fully.

Foam was hollowed from the batts as needed to make room for the wood straps covering butt joints. Electric soldering gun holds loop of #26 solid copper bus wire that can be shaped for different small cutting tasks.

Two narrow pieces of plywood are glued along each side of the vertical web and to the two foam batts. Bricks will be placed on top of them, and bricks were used throughout the assembly process to provide clamping pressure.

Final foam batts glued in place. Since it was obvious that most of these two batts would be cut away if they had rectangular cross-sections, I pre-sliced one rectangular batt diagonally lengthwise to create the two pictured, saving some foam.

Excess foam carved away, yielding smooth, curvaceous, symmetrical, repeatable 15-foot solid mold for hull. In actuallity, the foam carving process is done in two steps. First, the hot wire is run lengthwise so the wire runs along only one edge of each piece of wood, in turn, to make all the edges visible. That leaves not so much foam left to be cut away in the final step, and it reveals all the edges, which makes keeping the wire running along two pairs of adjacent edges, in the final cut, easier to do. About the only mistake that can be made in the final cutting is that the wire is lifted off one edge, and that will merely leave excess foam, which can removed with another pass.

P01 served its purpose to proof the FOAMBOAT concept, develop tools and working techniques, and to show that a mold built in this way is extremely strong and stiff because of the very thin pieces of plywood encapsulated within it at 90 degree angles to one another. A crust of fiberglass and epoxy would add even more strength. P01 turned out much bigger and heavier that I wanted for my original purpose, so I did not even fiberglass it. I realized I did not need the 1/2" thick plywood deck board for strength, which made up half the weight of the mold. The next hull I constructed was the streamlined P05, and I used the same thin marine plywood for the deck plate that I used for the other wood pieces. I glued four blocks of 3/4" plywood to the inside side of the deck plate, at two locations where I could screw through the deck of the finished fiberglassed hull and into the blocks to make attachments. I marked the locations of the blocks on the outside side of the deck, which could be seen through the deck's layer of fiberglass.

P05 was made using the same easy steps of drawing to model to fullsize hull. Changing the lines on the drawing yielded the sleeker, slimmer hull I wanted.
More detail is available than can be included in the Instructables step-by-step format. Click link to photo album that includes article I had published in 01 Nov 1997 issue of MESSING ABOUT IN BOATS mag. Click link to the article, then click "READ FIRST" for instructions for reading and printing fine text. http://www.flickr.com/photos/sm_vermn/tags/foamboat
In addition, there is information about some related things that appear in these photos, see Instructables about them, enter "unclesam" in Instructables search box, look at "Table," "Drawing long curves," "Fiberglassing tools and tricks."

One finished mold and a twin after it has been fiberglassed and painted with white exterior latex house paint.
I will be glad to reply to comments and questions posted by interested folks who have first thoroughly studied this Instructable.
U. S.