making a scale cardboard model

I make a model in Rhino3d, check for fairness and then unroll the panel and print them. Then I laminate the papers and cut out the panels. Then you tape them edge to edge. It is surprisingly accurate!

 

A critical skill in boatbuilding, design is the use of a long flexible batten to create an elegant shape. Many times Ill see a modern computer drawn yacht and its " lines" just dont look beautiful...they dont look hand made.

 

Any sheet material, unless slit or cut into ribbons, will tend to assume the shape of some combination of general cones or general cylinders when bent. There is a more mathematical definition of developability which will permit some slight departures from this, but only slight. Use of a computer program that will take a chine line and a keel (or rabbet) line and make the surface between them developable, without conically matching the two curves, strikes me as lazy and not good design practice. One can shape and build fast and excellent boats with conics.

But is it possible to simply build a model using sheet material, then unwrap it and measure the panels, without reference to geometric precision? The answer is yes. Sheets will bend the same way at a small scale they bend at a large scale. I believe this is how the Mistral Moths in my photos (post #74) were originally designed: by bending plywood around stations taken from another design, a Duflos Moth. Mistrals have since proven FASTER than Duflos (in part because the construction is lighter weight).

 

Makes sense to me: design with a computer if you're comfortable with computers, unwrap the panels, then check it by building a model before building full scale.

 

I do like to do half modes for simple boats. The only way to compete price-wise is to have very little waste. If you are making a hundred of something, you can usually also find a use for scrap. If you are making just one, scrap is figured as waste. Halfhulls let you reduce the waste factor. My designs of ply boats tend to shrink as the design progresses in order to improve nesting. When I finally layout panels for cutting, all the pieces are tangent, And my waste factor is on the order of two percent on paper. I built a 9.5 foot dingy from three sheets of ply and glass and foam. It weighed 84 pounds and I carried the sawdust, scrap ply, glassing waste, rags and whatnot off in a five gallon pail when I was done. About five pounds in all. That includes gloves, mixing containers, disposable rollers, everything.

I use a good painted 3/4 ply board about 4 foot by 2 foot and ink in my stations and draw the profile view right on the board. I print out stations (or copy with zoom/reduction from paper designs) and use foam board or modeling supplies to get out the frames. These were all laid out on stock to prove nesting arrangement. I also make sure the grain direction and good side of the ply is identified, It's way too easy to end up with a lovely layout and then discover some piece has the wrong side up.

The actual hull plating is usually the least of my worries, but you can prove you have the correct bow in the frames. I sometimes tape spaghetti to the plate to check my focal points. If I am concerned about the plating, I determine the scale of the model by matching the bending moment of the model plating to the proposed hull's plating, so my scales are usually weird, but the printer doesn't care.

Heres one where I was interested in the hull plating.

 

Well I downloaded a free copy of Delftship this morning and had a play around designing my own boat. The hull looked pretty much identical to a Bateau Harbour Master displacment hull, which was one of my first choices back when I started researching building a boat over a year ago!



By using this hull, the only changes required would be to raise the height of the bow by having the sides taper up maybe 6 to 8 inches higher than standard, and fitting a smaller cabin further forward where the lower section of the existing cabin sits. The new cabin only needs to be big enough for a seat and the controls.
Anyway rather than take the dangerous step of designing my own hull from scratch, I think using an existing hull design would be safer, then building the cabin to my own requirments. As the boat will be in the water most of the year, and the harbour empties at low tide leaving the boats sitting on the deck, would it be wise or even possible to fit some sort of outer keel to keep the hull off the bottom? many of the local boats have small bilge keel type fins and a steel strip running the full length of the keel to protect the hull when sitting on the sand and shingle.

 

A computer won’t ensure the boat looks nice or even works right. I’ve never heard of one refusing to print a design because it’s ugly, won’t go together, or will sail like a dog. It’s like any other tool, obedient to a fault.

I suspect half-models are becoming an art form now although many still use them for their original purpose. I have nothing but respect for those few that still do that, but most are now factory-made, and sold already finished as wall decorations. As a (retired) techy my instincts are to pull up a computer rather than a pencil or knife, and FreeShip allows me to view a model at a variety of distances from any angle, although the “lighting” is poor and the result looks crummy on a wall.

John: the problem in designing your own hull is lack of experience, and the risk of build difficulty and physical injury rank high. To a certain extent one can overcome these challenges by importing a similar existing design - if available - into the software and comparing its hydrostatic and hydrodynamic characteristics with your proposed design. For my early canoe designs I did just that. However, if there are major differences you’ll need knowhow to understand their significance too, and you may not wish to invest the time and effort.

Seems like an existing design is the way to go. That is the standard advice from the experts here, and it is the right route for most. Of course there are a few who - like myself - are here to learn the how and why of it, but wisdom comes with a price tag. I think you have made the smart decision.

 

Now that you have a nice design and Since you wish to modify the existing plan...build a model.!!!


Many of those stock plans are cleverly drawn to minimize material waste. It possible that by adding one inch to the design you will no longer get a plank out of a standard sheet of plywood.

That boat looks like it has three sheet plywood panel topsides ...careful that by adding an inch you make it a 4 sheet panel ... or a whole pile of expensive ply will be wasted.
A model will help you see panel size..length...compared to 4 x 8 stock.

 

cheers ancient kayaker, i'd love to build my own boat from my own design, but you have to be realistic when your safety is at stake, i think using a proven design by someone with a lot more skill and experience is the way to go.

 

Cheers Michael, good point, some sort of mockup could save a lot of wasted plywood, and i'll have a nice model to display in the livingroom! if the wife lets me

 

Good time to start your materials search. Stuff like Biaxial cloth tape and non blushing epoxy can be difficult to source locally. A word of advice is to buy a little extra to account for learning curve waste.

 

The fiberglass factory I used to work for is only ten miles up the road, so can get all the materials i'll need, might even get it at cost price!

 

Nice work!
There are others using Rhino and cardboard for non-marine applications.
Here is an interesting offering from a US architect:
http://www.wired.com/design/2012/11/cardboard-towers/

 

Plywood can be made into an oval too. I suppose you could break that into cones, but why bother?

 

You can skin a basketball with plywood if you want, but diagonally molding compounds, doesn't best take advantage of plywood and you'd be better off using veneers, to increase strength for the same weight.

When bending sheet goods, you naturally get some compound in the panels. The edges being in more tension, cause the expanses of panel between them to compound to a degree. A simple test to show this is; clamp a sheet of plywood along it's short edge to a table, then weight the opposite end to make a simple curve. If you restrain the full edge that's hanging free, it'll cause a cup to form in the middle of the panel. Trust me, just place a straight edge on the panel perpendicular to the curve. Logic suggests it should be a straight line, but it's not and will have a significant hump in the middle of the panel. This compounding is predicable to a degree and anyone that's forced a taped seam build, against a straight edged bulkhead knows what I mean.