Foamboat Construction





Introduction: 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.

Step 1: Trace Drawing Onto Components

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.

Step 2: Model Assembly

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.

Step 3: Magic!

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.

Step 4: P01 Grows Up

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.

Step 5: Vertical Web Attached

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.

Step 6: Butt Strap Cutout

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.

Step 7: Chine Plates Added

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.

Step 8: P01 Assembly Complete

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.

Step 9: Fullsize P01 Revealed

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.

Step 10: Real Deal, P05

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.

Step 11: P05 Revealed

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.
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."

Step 12: P05, One Mold and One Finished

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.



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    29 Discussions

    Was this cheaper and stronger than a stitch n glue hull.

    This hull form is easy to develop in ply and would take less time and pribably weigh the same or less if the hull was not sheathed in glass.

    Only major advantage of your hull would be ability to exclude water if holed

    how about sewing a skin of heat shrink dacron then shrink it and put a good paint..might be an interesting concept,,, cheap too,,,,cheap is good

    1 reply

    a long time ago, we had a canvas over wood frame canoe up in Elkhart -it was painted Red. Dacron is used for aircraft bodies like the Maule -I think you might have fun building something with it.

    im curious to know how the boat hull has held up over time. Is there any cracking or denting?

    1 reply

    michaelhoover, the hulls look the same today as they did the day I finished painting them, no dents, no cracking, no flaking of the paint. They have been stored outdoors uncovered year round, bottom side up, exposed to the weather and direct sunlight, Mid-Atlantic U.S. coast. To see more photos in my related Instructables, enter unclesam in the Instructables home page search box, page and scroll to see them all. Uncle Sam

    I would like to use this foamboat construction method to make me some outriggers for my canoe.

    2 replies

    Scott39, should work great for you canoe outriggers. See my related instructables by entering unclesam in the home page search box, then page and scroll to see them all. U.S.

    Thanks a lot unclesam, I will check it out. I have never played with fiberglass before, but I figure it can't be to hard.

    I have a fiberglass boat that was stored with the hull facing up.  The gel coat now looks horrible and I want to paint it with something.  I have sanded the bottom so whatever I used would have a good surface to adhere to.  Does the latex paint hold up ok? This seems like a very economical way to make my boat look better. And I don’t mind if it has to be reapplied now and then.
    3 replies

    if your gona go the cheap way instead of the better way that unclesam said, i wouldnt go with latex, it can get peeled off by the tape if your painting borders n stuff, also not all latex paints are water proof if any are. you can use latex but your gona need to water proof it.

    Please do not use latex, if you can not get your hands on some gel coat then an automotive enamel will work well.  If you're just going to paint the bottom of your boat, a quart would probably do.

    sdavisrt, sanding will really help any coating to adhere. See my other instructables about boat construction and fiberglassing by entering unclesam into the home page search box, page and scroll to see them all. I would use a primer under the latex paint. For the latex paint to be durable in this application, it needs to be premium, it needs to be white with titanium dioxide as its pigment, in order to block sunlight, such as Sears Weatherbeather exterior. If the paint you choose calls for a primer on its label, I would use that primer. A primer provides a stronger bond between the surface and the first coat of paint than does paint alone. On the two long hulls featured in my fiberglassing instructable, I used a sanding primer. It brushes on thick, like a coat of plaster, and fills pockmarks, cracks and the weave of fiberglass cloth. It sands smooth very easily, helps the coats of paint to adhere. The latex paint on those long hulls still looks perfect after over 10 years of direct exposure to the weather, all year long in Mid-Atlantic U.S.

    Do you think that this type of construction will allow me to make a sunfish hull?

    1 reply

    652800, I am not familiar with exact details of the sunfish, but it would seem to be a good candidate. FOAMBOAT will certainly allow you to quickly and easily build models to try it out. If the model works, the fullsize hull should also work. See my other related articles by entering unclesam into the instructables home page search box, page and scroll to see them all. You would need to imbed blocks of wood or plywood into the hull to allow you to attach mast, blocks, tiedown points, etc. These blocks would need to be attached to the internal plywood pieces that define the shape of the hull from within. You do not want your attachment points to pull out under the strain of sailing.

    ThanxLots Ive been shaping and wrecking foam for a while.
    In my quest for 2 identicle hulls for single man powered fishing catamaran.
    ILL send some deep water ocean fish
    Thank You.JC

    Mr. O'stache, my hulls are solid foam with fiberglass wrapped over them, some thin wood plates imbedded within to define the shape and add stiffness, as shown in the text of the Instructable. For more detail, see the expanded linked article at Flickr. Also see my Instructable "Fiberglassing tools and tricks," and its linked article on Flickr, for layup I used, which turned out to be way overkill. Also see my Instructable "Drawing Long Curves" and Flickr "Cutting Rigid Plastic Foam." To easily locate my Instructables, enter in Instructables search box the term unclesam, 2 pages of them. U.S.

    I relly love your desigin it allow me to build a kayak in the easy way, but do you have a plan for kayak?i want to build a kayak but I have no plan for building a kayak from foam

    1 reply

    mac969, see my Instructable "drawing long curves" for model of foamboat kayak. Also see my Instructable "fiberglassing tools and techniques." It would probably be cheaper and easier to buy rotomolded kayak than to make one foamboat way. I wanted a kayak that I could paddle with the long end moving forward, but could also row with the fatter end moving forward, using drop-in rowing unit. I have never seen a boat that could do both. My design had rectangular flat-bottom box as cockpit (represented by solid wood block in the model), with pointy bows added as snouts at each end. You can see the edges of the horizontal flat deck plate and the smaller vertical wood pieces that define the curves and add strength. I used the templates used for cutting the wood pieces to pre-cut foam pieces that were applied to the boat, rather than do the cutting on the boat itself. You can find more detail in articles I have posted at First click on "READ FIRST" to get instructions. Use search box to find articles and photos under tags "foamcutter," "curvetable". Meanwhile I will look for more kayak photos if I have them, will post them or perhaps create Instructables on that project. I will let you know when I do.


    Excellent! This gives me all kinds of ideas: a Hobie-type hull, power cat, foam outrigger canoe... Thankyou for posting.