Cost Of Traditional Wood Build Vs Various Modern Techniques

Tad,

It is my belief, and has been for decades, that plywood is stronger than a piece of wood of the same type and thickness...as has/is been taught in boatbuilding schools here, and from practical experience.

waterproof glued laminate veneers (plywood), i believe act as structural beams against each other, the thicker the laminate the deeper the beams and thus the greater strength


......am I totally wrong here.......

 

Plywood
Plywood is made by slicing wood into thin veneers and gluing the veneers together to form a sheet. By alternating the grain direction for successive layers, the sheet has similar areas of grain in the direction of the width of the sheet and the length of the sheet. Therefore, plywood sheets have properties that are nearly the same in both directions in the plane of the sheet, making it ideal as a membrane. By using thin slices of timber in the manufacture of these products, the size and influence of any natural characteristic is limited to the thickness of the veneer.
Characteristics:
High strength and stiffness to weight ratios - make its handling and installation very cost effective in applications such as residential and commercial flooring, shear-walls and diaphragms, formwork and webbed beams.
Surface dimensional stability - the cross-laminated construction of plywood ensures that sheet sizes remain relatively stable under changes of temperature and moisture, making it particularly suited to formwork applications.
Panel shear - approximately double that of solid timber due to its cross laminated structure, making it suitable for use in gussets for portal frames, webs of fabricated beams and thin plywood bracing panels.
Workability – plywood is easily nailed close to edges, which is important when used with relatively slender timber backing members, or as bracing, webs and gussets in portal frame construction.
Impact resistance – plywood, as with all timber products, has the ability to accommodate the occasional short-term load up to twice the design load. This property is advantageous for applications such as loading docks where vehicle impact can be expected or in buildings subject to seismic activity or cyclonic winds.
A variety of plywoods are made with the following variations:
Strength of the material
Thickness of the material
Glues – interior, exterior, marine
Quality of finishes of faces
Species of face and other plies

from
http://www.timber.org.au/ntep/menu.asp?id=104

sort of agrees with me, but it is an Aussie site, so what would they know anyhow.

 

Not totally wrong.... but partly.

"Strong" is a fuzzy metric.

Plywood is stronger in that it is more puncture proof, and more flexible

Its closer to being an isometric material... so it "may" be the case that if you average the strength over all directions it may be overall stronger than a solid sheet, but not as strong as the bias direction of that sheet.

Or another way to think about it...

If plywood was directionally stronger.... why wouldn't we use it for beams?

Ok.. now before I get ahead of myself... a Laminate is not the same as a Ply (in the common use of the word)

In an engineered Laminate all the "plys" are on the same bias. in a "plywood" they are in cross bias.

If you built a beam with cross biased wood, like a plywood, it would be substantially weaker than a straight laminate or solid piece of wood.

 

This is the important thing to take away from it.

Alas.... plywood is crap in tension (ok, crapish). Basically with a plywood bodied boat you are relying more on the framing of the boat, and the skin acting as a membrane.

Whereas in a cold mouldled boat, it is performing double duty.

This is why cold moulded boats are usually lighter strength for strength.


EDIT:
Not that I am hating on plywood, just as with any material, it has its caveats.

 

Moar

I was just pondering...

It may help to think of both solid lumber and plywood.... both as made from the same material (wood), but just with a different structural geometry.

To compound things though, plywood and laminates are sort of like wood fiber based sandwich composites....

Epoxy is a better bond than the glue in plywood usually though... so there is that.

When you make a stripped plank boat with diagonal bias, you are basically making your own plywood, but with better more unified wood and less glue/epoxy weight.

Richard (Apex1) has suggested a novel idea to use a light weight inner wood core, and a denser stronger laminate on the inside and outside such that it'd be analogous to a foam-composite sandwich or all-natural Lindsay Lord.

Cutts Patent Method is also a good read.

 

On plywood strength....

From Wood Handbook - Wood As An Engineering Material - Chapter 10, pg 11

http://www.woodweb.com/Resources/wood_eng_handbook/Ch10.pdf

"It is obvious from its construction that a strip of plywood cannot be as strong in tension, compression, or bending, as a strip of solid wood the same size. Those layers having their grain direction oriented 90 deg. to the direction of stress can contribute only a fraction of the strength contributed by the corresponding areas of a solid strip because they are stressed perpendicular to the grain."

 

Its of my opinion and observation that a single sheet of plywood splinters while solid plank of the same thickness fractures given the same impact. Of course there are more varibles in solid planking as to how extreme the plank fractures related to grain orientation.

 

There are very good reasons why cold-molded hulls are not built with the lamination's running 0-90 to the centerline (as a plywood skin would be). Rather they are oriented at +45/-45 and up to perhaps +60/-60. First off it's easier to bend the veneers with some longitudinal orientation. But also you are lending greater longitudinal strength to the skin by splitting the difference across the panels rather than having half your material (at 90 to CL) spanning the entire 1/2 beam.

Some North Carolina sportfisher builders used to rip plywood into wide strips and cold-mold diagonally with them....I always thought this method a bit nuts as half of every plank was little short fibers connected to nothing. But it did build thickness in complex shapes very quickly and efficiently.....with heavy sheathing they are tough boats...not the lightest...but tough.

Varying species and grain orientation...essentially building custom plywood...has long been the major asset in building a cold-molded hull. We built huge sail boats (to 154') using six layers of planking and little framing. First and last layer (skin) are 7/8" Douglas Fir at zero to centerline, inside are 4 diagonal layers of 7/16" red cedar, with fir or mahogany substituted in high load areas. The inner and outer layers run fore and aft because the largest loads seen by a modern sailing vessel are longitudinal bending from the rig headstay and backstays.

 

The North Carolina builders build hulls that varys in strength in the sides while creating a solid hull unlike typical round bellied sailboats which for the most part are convex which increases the strength but also can be built using more suttle angles. The "S" frame seems to be exclusive the region even though there are some splattered about doing simular hulls.


Not withstanding the angles, the planks need to be shaped at the same time from its straight sides so they fit with minimum fill. While the larger ones need less, the intermidiate ones requires more attention to this detail.

I feel that the method and builders unigue to the area are unlike any other part of the world. Numerous old timey builders will build the hull strip planked and then stabilize the hulls with plywood, one even uses plywood exclusively inside with overlays of the strips and then glasses the outside.

 

I work with both all the time and I'd give solid wood the prize anyday not only for impact but for dead loading as well the ability to carry a live load and for compression longitudinally

plywood works well in sheer and makes great sheer panels as well as covers large areas quickly but for all around strength I'd cast my vote for solid timbers any day

my two cents

B

 

Plus.... plywood is expensive by weight compared to raw lumber.

Solid pieces of wood only get more expensive when they are exceptionally large/engineered types.

Of course if you are making strip planks or laminates than this isn't and issue.. Your main cost will most likely be epoxy.

 

Sure.

And TAD made the right point!
We know why we don´t use ply in a hull layup. A layer of strip, 3 or more layers of + 45° / - 45° veneers provide the fibre orientation we need. Ply is giving the wrong fibre orientation in 50% of the material.

Regards
Richard

 

Its really all about the strength. A glass fiber (or carbon or whatever) also needs to be primarily laid up on the diagonals. For example:

http://experiencethis.mst.edu/steel_bridge/

 

Interesting side note about the Kurt Hughes cylinder mold plywood hulls:

Since they use 3 sheets of 3mm ply, you have much less than 50% of the fiber orientation in the wrong direction.

The 3mm ply has 3 layers. 2 layers go in the same (0 deg) direction, while the other layer goes 90 deg.

This means when you laminate the 3 sheets together forming the hull, you get 6 layers of fibers in the 0 deg direction and only 3 layers of fibers in the 90 deg direction.

Thought that might be worth mentioning in this discussion because not all plywood configurations are isotropic.

If you use 3mm and laminate, you can double the number of fibers going in the desired direction.

 

Err.... Ummmnnn.....

Wouldn't you want your glass fibers primarily going bow to stern?

The "diagonals" in your bridge truss are there mainly to keep your longitudinals from tweaking.

Diagonals on a bridge are nice because they prevent shear between the various longitudinals much better than say an H/boxed pattern