: Ok I am thinking of how to build a new yak. Here is one of my probblems, a
: flat piece of 1/8 inch ply is to flexible. I am hoping that by making a
: tri-planed hull it will help make the hull a bit stiffer. Now how would
: you go about making this hull even stiffer? Add a second layer of ply or
: lay in carbon fiber, or is there a method that I am not seeing right away?

: Sage

The easiest way to stiffen a flat sheet of thin plywood is to change it from being flat. Think along the lines of "I" beams or box beams when you create your internal bracing. And, consider the lowly egg shell when designing any curved shapes into your hull concept. While you are at it, think of egg crates.

High tech airplane wings are stiffened with a honeycomb-shaped material whih ads thickness and stiffness without adding a lot of weight. You can make a low tech analog from your 3mm or 1/8th inch plywood by cutting it into strips and then notching those strips so they interlock into a grid work (egg crate. It takes very little glue or epoxy to hold these things together, and you can make them stiffer by making them thicker -- use wider strips. You can reinforce curved shapes with these by scribing the shape of the curve in any given place along each strip in your egg crate, and then cutting the strip to fit the space. Do it strip by strip. Strips which go from side to side will be fitted into place like bulkheads, or ribs. Space them one to two inches apart. Then fit those which go the length of the boat -- or the length of the area you want to reinforce, and the whole thing will lock together.

You can fill the squares with foam if you want some flotation, too. Cut pieces of styrofoam home insulation a bit larger than the squares and force them into place, or use the more expensive expanding foam which comes in a can and squirt some in each space.

If egg crate reinforcement won't work in the areas you are thinking of, consider bending the plywood. A rounded bottom can be created over forms and maintained by a frame, rib or bulkhead on the interior.

If you need to have a flat bottom or side, build a long. hollow "box" from your plywood and glue it onto the part you need to stiffen. You could make just three sides of the box and have your flat boat section be the 4th side. A hollow 2x4 made from strips of 1/8th inch plywood is not going to weigh anywhere near what a real, solid, 2x4 will weigh, but it should be nearly as stiff. Of course you won't need to make something that large, either.

Hollow boxes made from 1/8th inch ply are not going to be very study, so reinforce them by having an occasional "bulkhead" or two along the length of the box, or, fill the box with foam. Again, this will provide some flotation and help support the box.

Glass over the box beams-- it holds the seams together and makes them sturdier. Just use epoxy on the egg crate to seal the wood.

If you are unable to use a wooden reinforcement, or if you are committed to using a cloth (glass, carbon, etc.) for space reasons, then consider using a polyester resin with a thick (9 ounce or more) glass cloth as a cheap but efficient method. The polyester resin is stiffer than epoxy and a thick glass layer used with that stuff makes a thick, strong, stiffener.

Have you given any thought to using 1/4 inch ply in those areas most likely to suffer from flexing? If you want that advantage of thicker plywood you couls laminate two layers of 1/8th inch. Or, laminate 1 inch wide "battens" of 1/8th inch ply abotu an inch apart, giving you a part whih alternates between being 1/8th inch thick and 1/4 inch thick. The orientation of these battens can make them serve as stiffeners or ribs.

Just a few thoughts

Good luck with your design

PGJ