A sheet can be glued to a tensile material and the composite bent so that the tensile material is on the outside. This will form part of a cylinder, if the sheet has extra thickness in the middle, the cylinder will have a bottom with less curvature. If the sheet has a cut out in front, it can curve up to form the bow of the boat, the rear can be just the overlapping tensile material. To keep the cylinder bent, ropes can attach the edges. These ropes will be under great tension, so the tensile material should have grommets at its edges to take the tension of the ropes across the width of the cylinder.

To get a feel for this take a thin, wide, long piece of styrofoam and apply tape lengthwise on one side of that styrofoam. Should the styrofoam be about one centimeter thick, it should be at least one meter long. Should it be about two centimeters thick, it should be at least two meters long, in general the styrofoam should be one hundred times longer and five times as wide as it is thick. This is for extruded styrofoam, expanded foam is more compressible, so for that kind, it need be only about sixty times as long as it is thick. Since the tape is stronger than the foam, once you have a taped piece of styrofoam you can bend it with the tape facing the outside of the curve to an enormous degree, even making its ends touch, without breaking the foam.

The tarp has to be stronger than the styrofoam it is bonded to for this to work. For ships up to about five meters wide, 12 mil thick UV resistant woven polyethylene with grommets every 0.6 meters is recommended. For larger ships, 25 mil thick polyethylene should be used. When ordering a tarp for a skin boat, be sure that they understand that you are specifying the final cut size.

To glue the stryrofoam to the tarp, two adhesives are placed on the styrofoam, one which is thick and takes a long time to bond, like Great Stuff foam sealant and a surface contact adhesive which bonds quickly and has good tack / initial stickyness like 3M Super 77 spray adhesive. When the styrofoam and tarp or another styrofoam sheet are brought together, the thick slow bonding adhesive keeps the surface contact adhesive from touching and there is plenty of time to align the surfaces. When the surfaces are pressed together, the thick slow adhesive is squished flat and the surface contact adhesive touches the other surface and binds. Even after the pressure is released, the contact adhesive holds the surfaces together and the slower foam sealant has time to bond and make a strong permanent connection. For a 7.2 meter boat, about 1.5 kg of foam sealant and 1.5 kg of spray adhesive are needed. The surfaces should have adhesive and sealant over the entire surface, especially at the edges, a line of foam sealant should be applied around the perimeter. Once you're used to this procedure, it takes about five minutes to apply the adhesives, position and bind each board.

To keep the structure in the boat shape, four millimeter diameter plastic rope can be used. For boats wider than five meters, multiply the mass per unit length in proportion. One side of the rope is tied to the grommets with a knot, the other is tied to a hook made of three millimeter diameter aluminum, the type used to ground lightning rods, bent into a hook. The aluminum is easy to work with, available at Radio Shack and other stores, rustproof and cheap. About ten centimeters of wire is needed for each hook and the aluminum wire usually comes in a twelve meter length, enough for about ten small sailboats worth of hooks.

The front of the foam panel has a right angle triangle cut out from its center to form the beveled bow of the boat. The front of the tarp is to be folded over flat repeatedly to keep it together. Then the flat folding itself is to be placed on one side of the bow, then folded back across itself and tied to the grommets at the front of the boat. This won't look too snazzy; but, all that matters is that it stays on.

Canoes are generally about four meters long, they are transportable and they can be stored at a marina for about a hundred dollars a year on a rack. Unfortunately, they can't carry as much as a sailboat or go as fast; but, they're still a good choice for recreation for people on a budget. The script page: boatcost.html?adhesive=2.0&compressibleThickness=0.04&floorCompressible=4.0&floorDensity=30.0&floorPrice=70.0&heightToBaseRatio=0.0&lengthToWidth=5.0&personal=50&power=0.0&powerPrice=30.0&shipLength=5.0&tarpPrice=20.0&wrapover=0.05 gives values for a canoe that is at five meters, slightly larger than average, slightly wider than usual for a canoe of that size and with a blunt stern. It has an extra thick floor for a prestressed boat of that size, 5 centimeters, so that it won't sag much when people step on it. This is for the shell of an open canoe, a small sailwing could be added to it later. This is about as small as a prestressed skin boat can be with half inch thick extruded styrofoam, which is the thinnest commonly available in hardware stores. This boat requires about seventy hours and ninety dollars to build. To make a smaller boat, specialty foam would have to be used or the thickness of the foam could be carefully reduced with a hot wire cutter.

To build the canoe out of commonly available styrofoam sheets which are 2.4 meter long requires cutting and throwing away about a quarter of their length. A version of this construction using exactly the length of styrofoam available from hardware stores is the 7.2 meter boat. The skin of this boat requires 12 sheets of 0.6 meter by 2.4 meter half inch thick extruded polystyrene panels. Twelve sheets of polystyrene are bonded to the tarp to form the bottom composite surface. This script page gives values for this boat which requires about a hundred and fifty hours and two hundred dollars to build.

A larger version of this construction using integral panels is the 14.4 meter boat. It has a floor with the same absolute thickness as that of the 7.2 meter boat, therefore having half the relative thickness. This boat is big enough for one person to live on. This script page gives values for this boat which requires about five hundred hours and nine hundred dollars to build.

An even larger version of this construction using integral panels is the 28.8 meter boat which is big enough for four people to live on. This script page gives values for this boat which requires about two thousand hours and four thousand dollars to build.

A 57.6 meter boat is fit to party in and thirteen people can live on it. This script page gives values for this boat which requires about ten thousand hours and nineteen thousand dollars to build.

A 115.2 meter boat can fit a tiny packed village of fifty people. This script page gives values for this boat which requires about fifty thousand hours and a hundred thousand dollars to build.

The script page can be used to calculate values for rigid boats that are really huge by simply increasing the ship length; but, after a while it becomes worth it to in effect thin the skin of the boat and make it more flexible by turning the continuous side sheet into a ribbed structure. How well this would actually work is unknown, reed ships might be necessary for cost effective larger sizes.
 
 

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