How light is **REALLY** light? Look at some of the Platt Monfort Aerolite designs. Even his lightest design might have a few ounces shaved off from some areas, and they would still be functional, and safe, in calm water.

The things to look at are what stresses you are dealing with, and where they are.

For the most part, the biggest stress you have to deal with is the squeeze exerted by water pressure on the outside of the boat. In calm water this is fairly predictable, and you can use some math and physics to figure out the displacement, and how much pressure is exerted at which points along the hull. For rougher conditions you beef up the design (if you are the designer), or take the simpler solution of adding more layers of glass (if you are the worried builder).

Overall the amount of pressure exerted on a kayak hull is minusule. If you figure that a 16-foot-long boat with a beam of 24 inches represents a skin area of about 16 square feet exposed to the water, then a 144 pound paddler will exert only an ounce of pressure per square inch, or 1/16th of a psi. If you could avoid punctures, you could get by with a skin made of saran wrap -- and people do sometimes test the frames of their skin boats with such flimsy materials.

Perhaps someone who makes kayak models will paint one with two coats of liquid latex, remove that, and then stretch it like a balloon to cover a full-sized boat frame. (Hmm, sounds like I've dreamed up a Kondom Kayak!)

An inflatable boat has no wood or metal frame inside it. The air pressure in the boat's hull tubes is somewhere around 4 to 15 psi. That is more than enough to resist the squeeze of the displaced water. If you used extremely flexible inflatable sponsons or float bags inside an extremely lightweight kayak you could build one with an extremely light frame, or no frame at all. I believe Stearns has some inflatable kayaks on the market which are like this.

If you are not going to make your boat as an inflatable model, then you need a frame. Otherwise, about half of the displacement is going to be pushing at you from each side, trying to collapse that boat and grip you inside. Somehow you need some supports to keep the sides out where they belong. On a canoe you have thwarts, or rigid seat frames, which are simply props that keep the gunwales spread open. On a kayak you have the deck doing much the same thing. The deck beams on a skin boat are nothing more than thwarts which are bent up so they an lift the deck and coaming high enough to let the paddler's feet fit in.

The gunwales on a canoe are curved into arches, which gives them some structural strength to resist the pressure of the water outside the boat. Like an archery bow, the curved gunwales can flex, but it takes some energy, or strength to make them do that. On very short canoes you don't even need a thwart. The gunwales alone are strong enough. A thwart in the middle of a longer canoe lends its strength to that of the gunwales. It doesn't need to carry the full load.

On a skin kayak's frame there is a chine which falls in the same place as the gunwales of a canoe. In some designs this is a fairly thin piece of wood or aluminum supported by transverse frames, and in other designs it is more substantial and serves to support the ribs. On a stitch and glue designs there will be seam between two panels along this line. There might be a wood sheer chine, or the glass-filled fillet on this seam line acts like a fiberglass chine. On a strip-built kayak you could draw lines around the area where the deck and hull meet. The compound curvature in this area serves the same purpose as a very strong "rib" (or gunwale).

By fortunate coincidence, most builders connect the deck to the hull at this critical area, and in so doing they add a few more layers of glass along the "gunwale" line, adding even more strength and stiffness to their boats.

Once you get past the design for this area you can make the rest of the boat paper thin. Ribs (on the inside of the boat) transfer some of the stress from the compression of the water to the gunwales, and by their design, they resist some of this pressure, too. In some designs multiple ribs are replaced by a few bulkhead-type frames. You just need enough solid material in there to keep the flexible skin from being pushed in (and stretched) beyond its breaking point.

Before the advent of fiberglass strip-built canoes were constructed with the strips nailed to interior ribs. Now, we use the strength of the interior fiberglass for this task. With a composite "skin" of glass-wood-glass the very shape of the hull helps strip-built kayaks share stresses nicely over large areas of their surface, and the strength in any given inch is thousands of times greater than the minimum needed.

Just a few thoughts, and some shaky engineering, to inspire you as you shave more weight off of your strips.

Good luck with your project.

PGJ