Plywood vs. Double Diagonal Cold Molding

Guys,
Aside from the direction of fiber orientation in/on a hull, is there much material strength difference between quality plywood, & double diagonal cold molding? And if so, why?
And regarding this question, I'm referring to ply which is properly fastened & joined. With quality scarfs, not butt blocks. That & the ply is a quality hardwood, of the non-checking variety. IE. not Doug Fir.

And it's not the primary reason I'm asking, but it also seems that double diagonal molding is held in higher regard & has a better resale value than plywood. Other than the fact that I know that some/a lot of boats are made with crap plywood, I'm in the dark on this one as well.
Assuming of course that the plywood boat is built in accordance to plans provided by a reputable designer.

 

Material strength difference depends only on the mechanical properties of each material. Hull resistance depends on the structure to be designed in each case as each material requires a different construction system.
What you should consider is that you can never get the same hull shapes with plywood than with double diagonal cold molding method.

 

Very true, you can get some 3D curvature with cold moulding, unlike ply. Also some construction calls for both methods on the same hull. Used to be quite common, with cold moulded lower part of hull blending or creating a chine where the ply takes over on theupper part. So horses for courses...

 

Cold molding is plywood. On a flat or developable surface, they are the same thing. Cold molding is generally considered superior because of the inherent strength advantage of compound surfaces. This can result in lighter and stiffer structures requiring less interior structure. I knew a fellow who built a racing sailboat in cold molded cedar that is normally built in plywood because all surfaces are developable. Turned out heavier and more costly and way-way more effort with no performance advantages. The structure at hand will dictate which is best.

 

tom28571, in my opinion compound surfaces need not be, per se, more resistant than others. Against them is the fact that in the case of heterogeneous materials, it is necessary to study what proportion each component collaborates to the strength of the assembly, ie to do a study of the stresses absorbed by each layer or material.
Although as you very well say, "... on a flat or developable surface, they are the same thing....", I think we all understand that working with plywood is not what is commonly known as "cold molding".
By the way, a flat surface is a developable surface per se.
I do not know exactly what you mean by "Cold molding is plywood" but, in my opinion, cold molding is NOT plywood.

 

Plywood is multiple thin layers of wood glued together. The grain direction of the layers alternate in most plywood.

Cold molding is multiple layers of wood glued together. The grain direction of the layers alternate in most cold molding.

The is also cold molding using multiple layers of plywood. Typically the seams between adjacent pieces of plywood run in the same direction to simplify construction.

 

Some very expensive boats are currently built with plywood. Several layers of narrow strips of plywood (cold molding) are used in areas where compound curvature is needed.

Photo gallery of an 84' "Carolina flare" boat under construction by Jarrett Bay near Beaufort, NC. http://www.jarrettbay.com/carolina-construction/custom-yachts/hull-60/60-construction-gallery/

Description of the construction method:

In the tradition of Carolina custom construction, safety, durability, and the confidence that comes from both are built into your Jarrett Bay. Our method is an epoxy-composite process, generally referred to as cold molding.

Materials selection is paramount. Your yacht’s bottom is sheathed in three layers of fir plywood, and the topsides with three layers of ¼” Okoume ply made from tropical hardwood. Fir is tough and impact resistant. Okoume’s ability to conform to sensuous curves and accept a high-grade finish is unequaled. Both are marine grade, meaning the veneers are glued with waterproof adhesive and there are no voids in the interior of the panels which can create hidden weaknesses. This “planking” is laid on to the mold forms and ribbands with a proprietary epoxy formula–epoxy being stronger than the wood itself once cured. The whole is sheathed in knitted fiberglass cloth, and also wet-out with epoxy. Knitted fabrics distribute stresses more equally and provide a superior resin-to-‘glass ratio than woven material. This method creates a stiff, lightweight structure that stands up to high-performance running for the long haul. http://www.jarrettbay.com/why-jarrett-bay/rugged-construction/

 

Slide show of another builder using cold molded plywood: http://www.baylissboatworks.com/built-for-you/the-process/

 

OK, if you are right, I made a very silly mistake. In that case some of my statements are incorrect. According to what you say, I should say that plywood is cold molding.

 

Cold molding is gluing veneers at room temperature. There is hot molding which is not in use any more. It used to be popular before low pressure/low temperature adhesive existed. Plywood still uses high pressure and temperature adhesives, so it could be considered hot molding. They usually are melamine or phenolic.

 

Well, I do not know enough to comment. Please make clear among you two and let´s see if we know what is cold molding and what is not. Thank you both for your explanations.

 

I guess I wonder what you are understanding from my post which is I think is correct. When I say that plywood is cold molding, I mean that they are both laminated of thin layers of wood glued together. Any problem with that? Only difference is that the glue in plywood is usually exothermically heat cured.

There may be some instance where a compound panel of the same material, thickness and weight as a flat plywood panel is not stiffer but I can't think of one. A flat panel can bend more easily and thus absorb a point load (puncture resistance) better than a compound panel. If that is what you are talking about, there is a small point there.

"I think we all understand that working with plywood is not what is commonly known as "cold molding".

Actually most all of the cold molding I see in this area (and there is a lot of it) is built with thin layers of plywood. That may be considered getting picky, but then so were you.

Edited: Yeah TANSL, I should have read the whole thing before answering. Jarrett Bay is about 10 miles from me and maybe dozens of other builders who work in cold molding plywood on some very elegant and very expensive yachts are scattered along the NC coastal area. I crawled over the 70 foot Carolina sport fisherman (actually a luxury yacht) last year and was properly impressed, especially with the two separate wine cellars. One for cheaper wines less than $100 and the other for the expensive ones. Did I say that? Stopped for gas last week and took a brief look at the 90+ footer under construction now. Who will be next with a 100' + extravagant palace? Only thing I know is that the owners must be in the 1% group.

 

I have built a few boat in the Outer Banks in cold molded plywood. Even when we started using Baltec Duracore on the sides, the bottoms were always plywood core. Seems like the trend now is for plywood bottoms and foam cored sides.

 

The real difference with a plywood build over a molded build is, the molded build permits fiber orientation in preferred and desirable directions, while a plywood build forces the 0/90 relationship, which adds to weight for the same modulus.

Plywood is essentially a molded build, though several layers at once are arranged. You can plywood mold any hull shape that can be cold veneer molded and most that are hot veneer molded. Agreed typical plywood builds use the panel size to advantage, more so than the veneer orientation, but this doesn't have to be or is always the case. You can special order veneer orientation as you like.

One of my favorite build methods is Ashcroft molded, using thin plywood. On small craft this is typically two layers of 1/8" plywood, ripped to widths that will make the curves and laid, in the same direction as the previous layer. This allows the planking process to progress with both skins at the same time. Of course, the 0/90 fiber orientation isn't as helpful as a double diagonal 1/8" veneers molded build, but you do gain in planking speed and the same thickness plywood (0/90) skin is inherently weaker then a true molded build. From a business point of view, it's often easier to absorb slightly more in weight and materials, than labor installing more costly materials, that happen to be lighter, so a balance needs to be realized during the design's development, to access the value and need for a certain engineering approach, with the whole of the project on the table.

 

What do you guys think of Dudley Dixs' "radius chine" method of plywood building? I like it enough to have ordered study plans for the "Didi 34", but what do I know?