Introduction: Foamboat Construction

Picture of 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

Picture of 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

Picture of 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!

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


indianabob (author)2011-11-20

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

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.

michaelhoover (author)2010-08-03

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

unclesam (author)michaelhoover2010-08-05

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

Scott39 (author)2010-07-08

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

unclesam (author)Scott392010-07-08

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.

Scott39 (author)unclesam2010-07-08

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.

sdavisrt (author)2010-04-22
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.
minipancho94 (author)sdavisrt2010-04-24

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.

whowild (author)minipancho942010-05-16

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.

unclesam (author)sdavisrt2010-04-24

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.

652800 (author)2010-04-22

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

unclesam (author)6528002010-04-24

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.

JoshWC (author)2009-10-20

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

lowpro (author)2008-11-16

Is the hull hollow? what was your layup schedule?

unclesam (author)lowpro2008-11-17

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.

mac969 (author)2008-05-04

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

unclesam (author)mac9692008-05-05

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.

Shinchan (author)2008-02-14

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

broham (author)2007-06-20

I want to use my old 10' jon boat for a mold (outside of boat ). I want to lay glass or kevlar over boat that has been treated with a release and copy about half of it (bow section). Then make a second copy of about half (bow section again) and join the two together, I now have a punt for shallow water work. How would I join the two halves together for the strongest bond?

unclesam (author)broham2007-06-20

Best would be to incorporate overlap at the joint, but I cannot figure out an easy way to do that without creating a bulge on the outside of the bottom of the boat. Next best would be to make butt joint and put glass and epoxy patch over it that overlapped several inches onto each half of the boat. It would make a small bump, but it could be feathered. Once crack had been covered on inside of the boat with several layers of glass, I would putty any crack on the outside between the two halves with epoxy filled with very short strands snipped from scrap fiberglass cloth, add at least one layer of glass over the underside of the crack in the boat. If you are worried about strength, you could epoxy strips of thin plywood over the butt joint on inside of the boat, then glass over it. U.S.

Jerobajas (author)unclesam2008-01-23

For a smooth, yet stronger butt joint, insert rigid discs called "biscuits" , wooden dowels or even carbon fiber spars buried 50% into each of the sections being joined. The discs, or other planar form such as battens should be placed in plane within the pieces' joint. A ribbed structure, sort of like a wing layed over equal lengths of each section would be a very strong way to reinforce a two-piece boat, if enough ribs are used, and enough epoxy and glass roving goes on top of them. I've seen a nice design which made use of the interior volume of the "wing" in the bottom of the boat by placing waterproof hatchways through the fiberglass into the cells between the ribs, for handy storage of gear. This design was made to stow nested, and be joined only during use; a sort of "collapsible boat", and though I dislike that juxtaposition of words, the virtues of the concept are undeniable for cruising yacht tenders or cartop use.

unclesam (author)broham2007-06-21

Second thought, make one half as you propose, make the second half several inches longer and the added inches slightly larger, nest the two open ends and epoxy the halves together. Before laying up the second half, tape band of cardboard across boat bottom and up sides, several inches wide and as thick as your expected layup. Fiberglass over the boat bottom and the cardboard band. Open end of first half should nest into those enlarged few inches of second half. Overlap should provide plenty of area for adhesive and provide extra strength there. U.S.

broham (author)unclesam2007-06-21

Hey thanks for your quick follow-up. I'll do as you suggested. I purchased plans for a 16' lapstrake row/power/sail called the "whisp" several years ago and never built it. I'm planning to make it out of foam and laminating it. Just started the learning curve building and working with foam. Thanks again for your help...bill

unclesam (author)2007-06-18

See model of FOAMBOAT kayak K01 at Instructables "Drawing long curves." Cockpit of model is block of wood, but fullsize cockpit is hollow rectangular flat-bottomed box. I wanted to put in seat and knee pads for using double-paddle, but also alternatively drop in Oarmaster II sliding-seat rowing unit. Boat's bottom and fore and aft snouts are FOAMBOAT construction. Fullsize MDF templates for cutting the plywood pieces were fastened together at right angles and used to pre-carve the foam pieces for the snouts, to prevent having to dig the foam and glue out of the actual craft if carving the very thin pieces of foam did not work out. Probably not necessary, but demonstrated another way to do things, which also worked. U.S.

flywoodkb (author)2007-06-17

Great work! I'm definitely going to try this on a small outrigger for a kite-powered boat. Thanks for posting it.

psychsurf (author)2007-06-17

Wow, that's great! I do a lot of work with EPS in surfboards, and hotwire my own blanks from block foam, but have never considered using this kind of templating to get the kind of curves you are doing. That's some great outside-of-the-box thinking there! Makes me want to make a kayak or something. :)

joejoerowley (author)2007-06-16

Wow!!!!!!!!!! That is really cool. I like that a lot. If I ever get in the boat making spirit I totally going to make this. Thanks

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