Somewhat Catboat

I must be missing something.

The Stevenson Project's POCKET CRUISER looks like a great platform to adjust to your specific needs. See, jump ahead to the point where most of the engineering and histological computations are done. Well done avoiding the tedious portion of the design process.

 

Francois Vivier is a true NA, it's not the case of a lot of designers of small boats in the States . His boats sail well upwind and downwind, row well, and are easy to trailer. His designs can be built in classic. strip plank and plywood. It's the safe way. There are also other designers.
I'm not so positive about Stevenson. His designs are let's say gently controversial. Not about the method of building, a classic plywood stitch and glue, but about the ability of sailing. A lot of people had to modify deeply and costly his designs to get a true sailing boat .
Flat bottoms are notoriously bad for sailing monohull boats, when heeling, all sailing monohulls heel, the waterlines are frankly disastrous, and the sailing qualities go to the bottom.
I have some building experience. For small simple boats, the winner is plywood stitch and glue. The method is fast and simple, strong enough, no need of big tools.
With some conditions or you get a kleenex boat, or you have a garbage boat.
First of all a correct marine plywood, that looks expensive but that pays. There are many reasons for not using exterior plywood, except for expendable prams.
Second marine epoxy, Raka for example in the USA. Polyester and wood are not compatible, except for an expendable pram. And polyester stinks...
Third use a female jig or your life will be miserable.
The composite marine plywood/glass fiber/epoxy is very strong, easy to maintain and durable. Con: it's not on the dirt cheap side. Good boats are never dirt cheap, except expendable prams.
In short a 2 or 3 chines, V or with a small flat bottoms make good sail boats. There is now 70 years of experience of chine plywood, and all true NA have learnt the lesson...
Personal experience of sailing small coastal boats from the dinghy to the 40 feet fast coastal catamaran:
- Keep it simple and light.
- Enough sail for light wind (these boats are mainly used in summer), enough good upwind, enough fast downwind, not too wide. It must be fast enough and funny with safety.
- On small boats, for day sailing and for sleeping on board at mooring, a tent over the boat is far superior to crampy wet smelly berths in a far too small cabin. The tent is an old proven solution since thousands of years. yes it's that old.
- On small (and even big) monos a ballasted dinghy with centerboard, and swinging rudder is the best. You can go everywhere, tide beaches, rivers, hidden moorings, shallow draft places, it beaches and it's indecently easy to put on a trailer. Best that sails pretty well. Hundreds of thousand have been built since the 17th century in Netherlands and France. Yes it's that old, the modern ones appeared after WW2.
- Well designed light small monos can be insubmersible.
The tips and tricks list can be very long...

 

I think it comes from experience. Designs that have already been built a few times have had the chance to show off their quirks, and have those quirks addressed in design revisions, or even altogether abandonment of the design in favor of similar ones which behave better. Naval architecture is a complex field with centuries of experience to learn about.

If you enjoy finding things out for yourself, go for it. If you want a "good boat," you will likely get there a whole lot faster and cheaper if you stand on the shoulders of a few giants.

I designed up a 14' plywood rowboat/outboard powered runabout back in the mid '90s. Went so far as to build a 14" scale model out of cut posterboard and tape. As I was researching sources for the materials, etc. I figured I might as well check the open market and see the competition. For the projected cost of my wood and varnish (zero credit for labor), I purchased a brand new 14' aluminum hull with virtually all of the same characteristics I was looking for in my plywood build, except that it wasn't plywood: didn't look as nice, to my eye, and also lasted 20+ years stored out in the yard without a roof, still in very good shape when I sold it last year.

 

Well said Mike. NA is a profession, a true one. Unhappily there not a ruling of the profession in small yachts boats so everyone with a computer thinks they can design one when they are just drawing sketches. The results range from sometimes acceptable to often disastrous.
Curiously there are not forums of amateur surgery or dentistry...
In amateur planes you have some administrative ruling, and the Darwin's principle applies in total rigor: if the plane is bad you'll die. Very simple. So the bad designers are automatically eliminated or if they survive by extreme luck they leave the field for water color painting.
I have a friend who long time ago got the virus. He wanted to design his own boat, build it and go sailing around the world when retired. His experience of sailing was renting small boats in the summer...
Tired of listening his blabla, one day I took him in my office and I showed to him the thick pack of plans, and the booklet heavy as an bible of a Van de Stadt design for an amateur builder, my personal library of yachting design and technical books (about 200), my naval engineering paper library (a complete wall 6 meters by 2.40), and showed the excel sheets of stability calculations for a small fishing boat...He changed of color...After I took him to the shipyard I owned with a dear associate, besides my job of NE at the French Navy, and showed it to him. We were building a 40 feet fast fishing boat and my associate explained that a 60 feet sailing schooner was far more complicated.
He understood...He learnt to sail seriously in a excellent sailing school, and got an "diploma" of skipper. We went to Saint Malo where a 12 years old 40 feet cruising Wauquiez, a beauty very well equipped, was in sale for far less than the raw materials of the 60 feet dream. He tried it and bought it. Six months after his retirement he started with his wife a cruising around the world that lasted about ten years until they were too old and got fed up of turquoise waters, coco trees and the tropics, which are not paradises when you live out of the all included holidays club.
The example is too far fetched as Sobell is talking of a 20 feet cat boat. She talked also of knee problems that does not help. It's simpler to buy one, or to make one from good reputable plans and a kit. That helps a lot...Lofting and cutting the plywood means hours kneeded, unless an expensive investment in tables and other niceties.
The great success in France of the NA F. Vivier is that he sells good detailed plans of boats looking traditional but in reality with rather modern waterlines and there is an artisan who makes kits at a reasonable price. Result in the 2018 Morbihan meeting of old gaffers, there were 148 boats designed by him from the rowing and sail pram to the 28 feet, and the owners looked happy as I have been said.
I'll add my personal grain of salt, that will make scream a lot of people, traditional and cute doesn't mean always good boats, and fishing cat boats, made for and used by poor people, were not among the best and safer. Nobody cared of poor fishermen...A lot of sail traditional boats even yachts were "death traps", or at least not good performers.
Cat boats have major flaws, the first having the weight and efforts of the rigging at the worst place, the bow. Downwind in a breeze they have the center of sail far away from the boat inducing serious problems of directional stability, some being frankly dangerous.
In the 70s some NA tried the formula, as that gave a big advantage in rating in RORC and IOR rules, and none was convincing. Effraie by Phelippon in the quarter tonner made illusion once in very light conditions but did not had result after. But Cascade a "cat schooner" with no jib was a killer and the IOR changed the rules immediately, as Cascade was winning all the races. Easy to understand a schooner has not directional problems even with big mainsails and no jibs.
There are some cat cruisers with free standing masts and wishbone. I tried one, imported from the States, in Brittany in a normal breeze for us (a good 15 to 25 knots) and that was less than convincing. The mast pumped upwind in the waves, destroying the aerodynamics, the bow was too full and pushed a lot of water, and downwind that was rather delicate for a very deceiving speed compared to cruising boats of similar size.
Lone and great advantage; very simple, but visibly the boat was not designed for these conditions but was rather specialized for light wind and flat sea, because of its rigging and the waterlines forcefully drawn for the efforts induced by the rigging.
These flaws explain that most fishing sail boats at least in France, England and Holland used "normal" rigs, and a lot were schooners, maybe complicated, but sure, stable, and easy to adjust the amount of sail.

 

Just saying there are hundreds of Marshall cat boats with more than 800 18 ft Sanderling models built. Some of the used boats sell for ten grand or less. There are always used boats on the market many with trailers and if you live in USA not hard to come by. I own one now. I have been involved with a couple of custom builds and it is unlikely you will match economy of buying a used boat. If its a boat you need to build why not do a garage project with a smaller kit boat.

 

I like your drawing. But if this is to be a flat bottom boat it will have definite problems. The worst of which is that it will quickly lose way when coming about. This is especially true in choppy water. When a boat loses way when coming about, it falls back on its old tack. A light, beamy center boarder I once owned had this fault. It ended up on the rocks during its first sail for this reason. So after my first sail, I had to learn gel coat repair. I later learned that I had to back the jib if I wanted to come about in anything but light conditions. I soon became an expert at this technique. What I did was hold the jib on its old tack, so it was sheeted upwind. Once it filled in this condition, it pushed the bow onto the new tack. I then had to scramble to get the jib sheeted for the new tack before the boat fell off the wind too far. With a jib, this was easy to do. My buddy and I got so good at this that we sailed upwind through a very narrow channel. People came out of their houses to watch.

With a catboat, you will probably have to back the main. And even then your boat, with a much fuller bow, may lose way before getting onto the new tack. For this reason, working catboats tended to be quite heavy for their length. They also had either "V" or round bottoms, as those tended to lose way later than flat ones. Even then, according to Howard Chapelle, it was common practice to not only back the mainsail but to raise the centerboard too while changing tacks.

A very light catboat will likely not work well. And a more normal one, with an 8 ft Beam, is likely to displace around 3,000 lbs. Do you really want a boat which is this heavy? If you do, I suggest you go with a "V" bottom, with the "V" carried up to almost the water line at the deepest section. This will have less initial stability but will have considerably less drag, and certainly pound less when coming about in a chop. Going with less Beam and greater Length helps to limit some of these vices. You could go for a 6 ft Beam, instead of an 8 ft one, and set the mast a little further aft. Then the bow would be a bit sharper. You could also shave off the bottom of the "V" bottom with a narrow flat section, which could house the centerboard slot. This would ease construction despite adding an extra panel to the bottom. This 6 ft Beam version would displace around 1,900 lbs, which seems a lot more reasonable.

 

As alluded to above it can be tricky to get the right combination of design and build factors with a cat boat. Many weaknesses in this area can be corrected with a multiple sail rig not so in a cat. I am on my second cat boat, both from well tried design and build sources, Legnos and Marshal, and these boats can still be tricky. If you are really set on building your own I suggest you do a sloop rigged Cat. In this configuration the mast is moved back slightly and or a bowsprit is used to fly a small jib-let. Both Marshall and Legnos had versions of this type of Cat-Boat. If trailering and shallow water are not in the equation I would suggest a deeper keel and rudder.