From my brain to the world

I want to build a small catamaran 16' to 20' nothing too difficult right? I like the formula 16 and 18 designs with the sharp pointed bows. I know I can buy plans, but that is not nearly as much fun as designing it from the ground up.

My problem is I can't seem to get the design from my mind into the real world. I figure the best way to construct this would be to use the "tortured ply" method, but in order to do that I need to know what shape to cut the plywood to make the hull sides.

I can sketch a pretty good plan and profile view onto graph paper or I can use AutoCAD at the office to do the same (I have never used the 3D options in AutoCAD). But I am still not sure how to convert that very curvy 3D shape to a flat 2D panel. How does everyone else do this? I have seen that some software can be used (Rhino is one I saw) to design the 3D hull shape and then create a template to cut, but there are other ways, since these boat have been around longer than computers.

By the way, I am very technical and hold a degree in engineering. I am sure I could use any computer program out there, but would like to know all my options.

Tom

 

Tom; You are tempting the fates when, as a newbie, you decide that it would be fun to design your own. You have the education, computer skills, and intellectual capacity to do the design work. BUT you do not have the experience that would enable you to avoid all the niggling little detail failures that lie in wait for the beginning designer/builder. Take these remarks as a word to the wise.

Best advice is to use plans for a proven design. If you want to entertain the tortured ply scheme, you might look at the Tornado class boat. The class has plans for that method. The Tornado is one helluva boat, proven year after year. The prospect of improving on that boat is..............not likely.

 

Read Gougeon Brothers wooden boat building book, there is a chapter on tortured ply. All opinion I have read suggets that CAD programmes are not yet able to predict the compounding properties of plywood yet (Given the limited use of this method I doubt if there is much pressure on them to do it in the near future either.) As the ply is stretching and displacing in more than one plane, the CAD programme cannot resolve a flat plate development as it would need to take into account the elacticity and dimensional stability of the plywood as well as the displacement forces.

In the book they recomend the use of scale models built from model aircraft ply to develop the shapes and describe how to take the measurements to full scale. There is also advice on how to complete the build. While it sounds like trial and error (it is), they do give guidelines for influencing the shape. Tornado cats used to be built by this method. Keep in mind that the stress in the ply generated from the build must be added to the stress generated during sailing. So you potentially require more reinforcent or a degree of overbuilding to compensate for this.

Have a look at Cat designer Richard Woods web site, I have noticed that he also posts here now and then. One of his plans may be of interest as he uses this method.

http://www.sailingcatamarans.com/

There are a number of blogs floating around where guys have constructed moths and International Canoes using this method with some modifications, and the addition of modern composite reinforcements.

Good luck

 

Its the easiest thing in the world.

You draw your elevation, you draw your plan, you draw 2 or 3 chines in plan and elevation.

You make some arbitrary station reference lines, and plot the xyz co-ordinates from the plan and elevation.

You feed the chine and station co-ordinates into your Freeship or Delftship with a text file, and presto a hull appears.

If you paid for your Delftship, it will produce your side plate 'plots'

If you export as a 3d cad file, your evaluation copy of Rhino will tell you with color coding if the hull sides are developable, but using the Sam Devlin book ( cheap) you can do the same with a scale model ( as previously mentioned) quite easily.

Of course, you may have 'glossed over' things like displacement, weight tables, all the things that make up a successful design.

There is nothing like a build of a proven design to get started, as you will learn a lot of what you have to know before you can think about a completely fresh start.

 

Making the jump from 2D to 3D

First, I would look for plans that already have offsets - paying $50 to $100 for plans is money well spent. Even the cheapest boat, and that's all I have ever built, is going to cost a couple hundred dollars.

A cat, with tramp, Sail and rigging is going to cost well north of $2000 if it costs a nickle. So 100 dollars for plans is cheap insurance on the rest of your money.

There are a lot of used cats out there in the size you are talking about. I would Buy a dog and fix it.

That said, if you have 3D AutoCAD or Rhino at your disposal (I use the latter) there are commands to develop a mesh surface. In other words, roll them out in a single plane.

You can also build your frames and hold the plywood skin (or pattern material) in place and trace the outline of the panel before you cut it. As long as your design doesn't ask the plywood to do something it can't do, like bend in two direction at once, this will yield very good results.

Russ

 

I am not opposed to plans, and the cost is not an issue, I just like the idea of saying I designed and built my own boat. Perhaps you all are correct. Maybe for a first attempt I should just go with a set of plans and use that as a learning experience for my next build.

Thanks for the information guys

Tom

 

Never having even built a boat before, but having some knowledge of boat design and some mechanical aptitude, I designed a boat on Prochine from New Wave Systems and built it. Yes, I made some blunders along the way. Some of my ideas didn't work out and I had to try something else. Yes, spent twice as much money as it would have to just go out and buy a boat, but, as I can see you get, that isn't the point. I didn't want what I could buy. Not only did I want something out of my own head, I had the conceit to think I could do it better, and maybe I did. That is yet to be seen. Yes, design your own boat. But be flexible and I hope you have deep pockets.

 

My pontoon boat that I built ,..went from my head to the water at the cost of $4500.00 +/- I made every piece ,including the pontoons,..which are basically flat bottom ,..about a 4 degree "V" with an added skag down the center for directional stability ,..the reason for the almost flat bottom ,..displacement ,..I didnt want to displace much water ,..all said and done,..the boat alone when floating sets in the water about an inch of displacement,..with me my wife and my daughter onboard it displaces about 4 inches,..which works very well for the shallow waters that we fish

I have it posted in "boat building" under River Pirate ,..there is one photo of it in the post

 

Concur river runner..
One more pitfall in this route is the ease you can modify and change your plans so they never become ready thou the boat might..

 

In tortured plywood construction the surfaces are not developable, so as JRD mentioned conventional CAD tools to unroll a developable surface don't work.

 

I was toying with the idea of a rigid hull cataraft and I too was thinking tortured plywood. What I planned on doing is making some paper models, then moving on to thin pressed wood models. When I got the shape about right, I'd take the model apart and lay the pannels out on graph paper to figure out a table of rough offsets. When I got everything looking good on the final plywood, I'd cut out the two panels and start putting them together. When those were tortured into shape, I'd lay something over them and trace around the edge of the hull to get the shape of the deck and transom. I was thinking of using a similar process to make a cheap, easy drift boat out of regular lumber yard plywood.

 

If an amateur wanted to play with boat design (after doing the reading and looking at similar boats) it seems to me that you do a lot worse than designing and building a daysailer. If the OP uses a beach cat rig and blades, it might not even be too expensive. If worst comes to worst, he can throw out his hulls and put the rig back on the donor boat. I'd say go for it.

This is not what I would say to someone who wanted to go out of sight of land, though. That would be crazy.

 

By default, tortured ply has to be able to be 'developed'

Sure, not in the same way as hard chine, but they must be able to be created from sheets of ply, which is a 2 dimensional material

The two programs, Delftship and Freeship will 'unroll' a curved hull, though they cant tell you if you have exceeded the plywood 'torture level'.

http://forum.woodenboat.com/showthread.php?129760-tortured-compounded-plywood-design-software

The other related method is constant radius design, where the curved sections interact with each other

eg

http://www.amateurboatbuilding.com/articles/design/radius_chine/index.html

This link is talking about doing it with steel, but the geometry considerations are the same.

Every 'tortured ply' hull ( take the Tornado or Mosquito catamaran)
http://home.vicnet.net.au/~mosquito/documents/building.php

... ensures that the chines that the plywood 'rotates' around, are almost straight along the hull, and the radius of the sides doesn not exceed the 'bendability' of the ply.

Some material pretesting will provide you with the material limitations of the latter - but in essence, the hull design is just a conservative chine exercise.

 

some photos of my home built mini pontoon with built in trailer

 

Can you do air shockers to make he boat dance on water too ?

They look like they would suit the 'fluffy dice' mindset.