Foam Master Hull Construction

The use of high density rigid foam is becoming popular as a material for constructing ship hulls. The material is very light and inexpensive when compared with wood. It is also amazingly very sandable. Similar types of foam have been used in the taxidermy business for years and have become popular for making stage props. The material is dimensionally stable so warping is of little concern. Its high density provides good resistance to compression. It does possess some drawbacks and these are important to know. The standard high density rigid foam used in sheets to insulate new construction is extruded polystyrene foam. And being polystyrene, it is unable to handle many solvents and glues. If you want to turn you hard work into a pile of goo be sure to try any of the following products; acetone, gasoline, MEK, polyester resin, Bondo car putty, lacquer paint and any Cyanocrylate based super glue. What is left? Almost anything water based and epoxy resins such as those used for epoxy fiberglass work and epoxy adhesives. But to be safe, it is always a good idea to test the compatibility on a test piece first.

One can also use urethane foam, which is very similar, and is compatible with products such as polyester resin, but I found the cost to be prohibitive. A single 4' x 8' x 1" sheet of urethane was close to $100 at a local foam supplier while at the same time I can run to the local home supply superstore and pick up a sheet of polystyrene for under $10. Home Supply stores such as Home Depot, Lowes and many lumber yards carry polystyrene foam in a variety of thicknesses that include 3/4", 1" and 2". Sometimes a vapor barrier, a thin sheet of plastic, is laminated to one side and I avoid these types if possible. You will want to remove this barrier for cutting and sanding if it is included with the foam.

The first task to trace out the hull sections onto the foam. Now is the time to make the decision on how you plan on separating your hull. In my case, the hull is split along the centerline creating a top and bottom half. Depending on the shape of the hull you may decide to split the hull along the centerline creating a left and right half. The former method is more common but some odd shaped hulls and certain pre-nuclear age boats work better using left and right sides. Once you decide the split, you can create a set of templates from the hull sections of the plans. I used a heavy construction paper similar to thin cardstock for my templates as they would not bend as the felt tip pin was used to trace around them. Using a bright light directed up from under a clear surface, I was able to see the hull sections through the construction paper and trace the lines. A window and a sunny day can also be used. Since my set of hull sections were symmetrical I only created one side of the hull shapes. I would trace half of the hull section and then flip the plans and line them up to finish the other side of the section.

Foam is easy to work. You can cut it, sand it or saw it. But to cut out sections, nothing beats a hot wire cutter. The heated wire effortlessly slices through the foam leaving a satin smooth edge. There is no saw dust and you can cut in any direction or the smallest inside corners you could need. Several vendors make hot wire cutters and some make some table versions that resemble scroll saws. But they are sometime expensive on the order of $100. When I came across a small hand held version from Woodland Scenics for under $30 that I decided to rig up a little table for it. As you can see in the picture, it is nothing more than a piece of wood with a hole for the wire to pass through. One arm that holds the hot wire was mounted to the bottom side of the board and while the other arm was left hanging above the board. This arrangement provided a table mounted hot wire cutter with a 6" throat that could easily handle 2" material. The cutter is basically a wire connected across the terminals of a 9VAC wall mount power adapter and if you wanted to be fancy, you can purchase just the wire from Woodland Scenics and construct you own set of 'arms'. It is important to keep the wire at a 90 degree angle to the table. Any offset from perpendicular will cause the edge of the foam to be beveled.

After you cut out several sections and place them side by side, you will quickly begin to see the hull take shape. It is not unlike building a new home. The framing goes quickly and the shape of the home quickly comes into perspective. But the work has really only just begun.

For the most part I used 2" thick foam for the hull as can be seen in the pictures. But then again, this sub is 8 feet long. Only on the bow and sail did I use 1" thick foam to better follow the curvature of these shapes. If you don't like blue, polystyrene foam comes in a pink version that I have been told has the same properties as the blue version. My 'blue board' was manufactured by Dow and I suspect the pink version is a different brand such as Owens Corning. If someone has worked with both, please share your experience. In either case, they are both polystyrene foam so you have the same adhesive compatibility issues to face.

I used 40 grit sandpaper to quickly flatten the section's bottom. I also put together a simple fixture to assure the bottom would remain perpendicular to the sides. Using some boards to create a 90 degree block, I used some double-sided tape to stick the sandpaper down to the block. With the sections resting on its side against the cutting table the block was slid back and forth to sand the bottom flat. I first sanded the bottom on each section and stood them side by side to check that things were looking correct. I then replaced the 40 grit paper with some 80 grit paper to sand the remaining edges of the sections. The 80 grit sandpaper provided a smoother finish for the exposed section edges.

Starting with light pressure, the foam sands cleanly and very quickly. The 80 grit sandpaper doesn't try clog up and will remain sharp for a long time. A stiff bristle brush is useful for sweeping away the foam dust that collects on the sandpaper. The dust will tend to be electrostatically charged and cling to nearby surfaces. The dust does seem to settle quickly and it won't fill the air such as wood sawdust does, but it is a good idea to protect yourself from it with a simple dust mask as the particles are not easily ridded by the body's respiratory system.

With all the coatings and adhesives you can not use on polystyrene foam, most epoxy resins and adhesives will not attack the foam. To glue the hull sections together, 5 minute epoxy is just the ticket. It only takes three small dabs of glue to hold the pieces together. If you need to disassemble a part for any reason, you'll be disappointed if you went overboard with the adhesive. Pay special attention that you keep the adhesive away from the edge. With epoxy being hard to sand, you will not want to have any forced out of the joint to the outside. I usually glued together 8 to 10 sections at a time. A clamp was used to press the pieces firmly together while the epoxy cured. Keep the number of pieces to a manageable size and leave off any small end pieces until all other pieces have been assembled. The clamps only need to be lightly tightened. Always keep an eye on the pieces during glueup to make sure they don't shift.

Don't sand all they way down to the mating surface at first. Leave this last inch or so until both halves have been sanded. Push a few short finishing nails, head first, into the mating surface of one half leaving half of an inch of the sharp end protruding. Then push the other half down onto the nails and secure the two halves together with some straps. With the two halves held firmly together, sand around the mating seam. This assures proper alignment between halves.

Currently this is where my Typhoon sits. And unfortunately, it will be a little while before more progress is made. But when I get back to it, this will be my game plan.

When I am finally happy with the sanding, I'll glue the sail to the top and then use drywall compound to fill any dents and gouges in the surface. I will also skim over any rough areas with the drywall compound to produce an smooth even surface over the entire hull. Some final sanding with 150 grit and dust removal and the hull is ready for the hard coating. I will coat the halves, one at a time, with a mixture of epoxy resin and thixotropic filler. The filler will thicken the resin so it clings to the sides and provide a sandable surface. Pure epoxy resin is very tough and hard to sand. It may take a couple coats of the mixture to build up the desired 1/16" thickness. I may also lay a thin layer of fiberglass cloth after the first coat for extra strength. The mating surfaces will also receive a coat or two of the resin mixture.

After the resin has cured, I'll then sand out the hull starting with 100 or 150 grit sandpaper and work all the way up to 1200 grit wet sanding. I'll then proceed to mark out all the scribing detail on the hull. First mark, then scribe is the rule here. Scribing will take a long time as there are over 12,000 tile to scribe on the surface, but the result will be impressive. Once the scribing is done, the master hull plug is ready to be prepped for mold making.

Again, I invite anyone with questions, and especially those with suggestions, to drop me an email. The purpose of this site is to both teach and learn, for you the reader and myself.