Advice seriously needed.

Build a kayak and sail it 10 miles ALONG the beach.

Buy some books on the web for practically nothing on S&G building which also includes plans.

I had to laugh, Jay - but of course, that's excellent, likely life-saving advice.  Personally, I never paddle further than I'm willing to swim.  I don't have a 'bullet proof' roll, and if conditions are such that I find myself in the drink, it's unlikely I'm going to get back in the saddle.  Of course, YMMV.

[q]Okoume 3 mm furniture grade beautiful looking plywood at $12/ sheet.  No name, no grade epoxy pumped out of a huge drum + hardener.   Choice of  parawood (rubberwood, potentially obtainable 'green') or knotty pine kiln dried import.   Plenty of copper wire.  [/q]

That is the plywood I used for a few of mine. As long as the epoxy sets OK there should be no problems. Make sure to stick to the recommended mix ratio.

My double, basically one of my first 2 kayaks with half a sheet length in the middle. This means the equivalent of cutting it in half, the first cockpit moved forward a little, the aft cockpit a little further aft. I'd checked out other doubles around the world for their dimensions and about 6 came out about the same proportions as I was going to build.

Seemed to be OK, circumnavigated Vanua Levu, Fiji, 38 day trip, a few months after it was finished.

Those first 2 singles, still got one of them, don't know what happened to the other. Then ran a building class (4 were built) doing slightly bigger singles before the double.

I f size and weight is a concern have you considered something like this?

http://gentrycustomboats.com/Annabelle.html

"Did you use this 3 mm for all the hull or only for the deck?  Some people on the internet recommend a combi:  4 mm for hull + 3 mm for deck."

As far as I remember, for the double, I had some reasonably light 5 mm for the bottom planks and 4 mm for everything else. In your case, with 4 mm available (only?) I'd use that.

I didn't make any comment before on the epoxy filler though it did cross my mind. The possible problem with sawdust is any water could tend to make it swell. I'd suggest mixing some up and abusing it to see what happens. Make up a block, maybe sand it so it might have open pores, measure it accurately, soak it for a week, measure it. This will be equal to the very worst case situation as hopefully joins wouldn't be so open.

The usual filler is a fibrous material. From a quick web search, this -

"Some builders only add a thixotropic material like cabosil to glue in joints while some add wood flour. Where more strength in the epoxy is needed, glass microfibers are often used. For fillets with glass tape, many use wood flour or sawdust to bulk and extend the epoxy."

It appears part of the filler's job is to add a little bit of flexibility and stop the epoxy cracking. Again, a long strip with and without filler, bend them and see how they respond.

Cleaning - acetone will clean off epoxy BUT would you drink it? No? Use white vinegar, it does the job well and can, if necessary, be drunk (without epoxy in it). This means, don't use acetone.

Eddie

Wood flour is simply sawdust that is as finely ground as the wheat flour used for baking bread. It can even be as fine as flour for baking cakes. Typically you get sawdust this fine as a byproduct of sanding wood with a fine grit sandpaper. However, you can make it in other creative ways. Get a blender in the kitchen? Process a handfull of coarser sawdust originally produced by actually sawing some wood. Play with different speeds on the blender until you have very fine powder. Save it and process another handfull the same way. Repeat until you have about a liter. Then wash out the blender. 

3mm plywood is lovely stuff to use for smaller kayaks. As the boats get longer the materials used tend to get thicker. Otherwise the boats would flex too much. If all you have is 3mm then simply laminate two sheets to produce a piece 6mm thick, and use that. This actually makes one operation much simpler: stretching the plywood to length. Usually you will need to turn 8 foot long sheets of plywood, or their metric equivalents, into sheets that are  a bot longer than the finished length of the boat. For your double you would probably want materials 18 to 20 feet long for a 17 foot boat. Rather than cutting wide tapering ends for scarf joints, you can just butt two panels together, slather on some glue or epoxy, and drop another panel on top. Stagger the seam lines by at least a foot and built panels as long as you like.  You’ll have a crack at each joint, but fill that with a mix of your wood flour and epoxy and sand smooth before applying the glass cloth. 

Another option is to build with the 3mm plywood and add more layers of glass fabric. This makes a heavier boat, and usually a more expensive one. However, the 3mm plywood is very easy to bend into shape so original construction time is reduced.  If you are thinking of using fancier reinforcing fabrics such as carbon fiber, or kevlar, then you have a great base  to put them on. 

Or, you can use chine strips running the length of the boat to stiffen the hull. You could make these from very flimsy materials such as rubber foam. Cover them with two or three layers of glass cloth in epoxy and you’ll form solid reinforced plastic tubes which will be quite rigid and strong despite the foundation they are applied over. Your finger-jointed lumber would be great here. 

Finally, consider some of the older designs for fabric-covered kayaks. Then cover the frames with sheets of your thin plywood instead of canvas. Interior glass cloth is not needed, and if you put any on the outside it can bee as thin as you like. It will simply give a thicker layer of epoxy to keep things watertight. You don’t need it for structural support—that is provided by the frame.  There are plenty of free or cheap plans for these older designs on line or in books.  Look at www.Clarkcraft.com to start.

I am also wondering if you have epoxy or perhaps a different type of resin. Epoxy resin and hardener mix in fairly close proportions.  Typically 1 part resin to 1 part hardener, or  sometimes a smaller amount of hardener.  It might be 3 or 4 parts of resin to 1 part of hardener.  The hardener is as thick and viscous ad the resin.  However, most fiberglass work is done with a polyester resin which uses a very concentrated and watery hardener.  With this resin you might use a single drop of hardener for 3 cc of resin, or maybe 30 cc hardener to a liter or resin.  A quick look will tell you: is the bottle of hardener a lot smaller than the drum of resin? 

Just some ideas for you. 

PGJ

Since you're a sailor not a paddler why start now?  And you don't really want to paddle.

So instead of a kayak design which won't sail well,  go for a sail boat.

And what better than a wonderful sailboat design from your part of the world, an outrigger sailing canoe. More like your trimaran.

If you seal all wood surfaces with three coats of paint, the boat will likely last longer than you.

Buy - Building Outrigger Sailing Canoes, by Gary Dierking.

Great book full of plans and how to. Wish I had the time to build one, very exciting! Too many boats, too little time.

Please let me know how I may be of further assistance. 

Live Long and Paddle/Row/Sail,

Rob Macks

Laughing Loon Custom Canoes & Kayaks

http://www.laughingloon.com/

207-549-3531

“People who say it cannot be done should not interrupt those who are doing it.” - Chinese Proverb

I'm 100% with Rob on this one - kayaks don't sail very well at all...... unless you lead a charmed life and everywhere you go, you have a following wind.

If you can't handle your existing sailboat, a sailing kayak will be more of a handful than you'd like, I think.

A proa or sailing canoe will be much more fun and more balanced under sail than a kayak.