Plywood Vs. Veneer in Cold Molding of boats with Compound Curvature

I couldn't find a good thread on this. If one already exists please redirect me. Thanks.
Anyways, I was reading this article http://www.powerandmotoryacht.com/node/143212 and it says "The basic process for Howell includes applying three layers of marine-grade okume plywood"
I thought I remember reading in "The Gougeon Brothers on Boat Construction" that single veneers were much stronger than plywood? Especially in bow sections were there are compound curvatures and small strips much be used. Because half of the fibers are aligned in the wrong direction?
Does anyone know the cost benefit on this? In that article they are talking about a very high performance boat, where a small decrease in weight could be huge.

Are their any Custom Carolina Builders that still use veneers?

 

Plywood veneers build up bulk faster than individual layers of veneers. Yes, veneers are the lighter, stronger, stiffer way to go, but you can fit two layers of 1/4", high quality plywood and have a 1/2" hull or you can fit 4 layers of 1/8" veneers and have a lot more fitting and gluing labor to absorb. Fitting veneers is tedious and requires a fair bit of skill. 4 layers of this, versus two with plywood and the cost effectiveness becomes obvious. Now, on a racer, with every ounce accounted for, you'll employ the veneers, but if the design's SOR can afford it, the laminated plywood hull shell will save huge amounts of labor and a fair bit in goo too.

 

Tragic.
So I guess to answer my own question, any builder would build from veneers if you wanted them to, but you are going to pay for it?
Personally, I would prefer 3 layers of 4mm Western Red Cedar or Sitka Spruce Veneers over 2 layers of 1/4" Mahogany Ply.

 

I resawed my own white cedar veneers from solid stock planking, which does have some waste in that approach. Of course the labor does come with some costs if you are a numbers cruncher along with some additional planing thats involved so that you can get unform thicknesses. Make sure your wood is not green though.

 

I think I'd rather pay someone to do it for me.

Unrelated, I think I just fell in love with Paulownia. From what I've been reading they make it sound like a miracle material. Stronger and lighter than anything in the universe. haha

 

Look for some Spanish Cedar if you wish for a better alternative along the eastern seaboard. There are outlets that do sell solid veneers of the species.

 

I'll second the Spanish cedar. Molded veneers are a good way to go, but again, it's labor intensive. If building a molded boat, I prefer Ashcroft method and it's usually thin plywood, so I can pick up cross grain strength. The nice thing about the Ashcroft method is you can lay down both layers at once, which means you don't have to plank the hull for each layer, repeatedly, like double or triple diagonal molding requires. Of course Ashcroft can use veneers too.

If you have to pay to have stock milled, then labor costs as well as veneer count will be a concern. This is another advantage of plywood layers.

 

Paulownia is being grown in GA, but I don't see anyone in the US manufacturing Veneers. I like the idea of light wood cores, because of the benefits of adhesion, shear, and fatigue over foam.

 

Were veneers ever used for double diagonal planked, cold molded, custom Carolina sportfishing boats? (say that five times fast... haha)
From reading "Carolina Flare" it seems like there was a point in the recent past where some boat builders transitioned from Plank on Frame to Cold Molded with ply. But at that point plywood had advanced far enough that the Custom Carolina's never really had a period where cold molding with veneers was necessary. Juniper planks were used from strip planking plank on frame boats, but I haven't seen any evidence that juniper veneers were ever used for cold molding boats. It seems like cold molding was popular in other parts of the US and in other areas of the industry (i.e. sailing vessels) long before in was introduced to the Carolinas. And at that time, quality plywood wasn't available, so veneers were very popular and hence are still somewhat popular today.
I'm thinking that if the cold molding started earlier in the Carolinas, builders would have been forced to use juniper veneers, and they would have realized that this was actually a better product than the plywood cold molding. Now the idea of juniper veneer cold molding sounds so untypical that it is avoided.

 

The problem with using Juniper for veneers is the labor involved to create the veneers. Personally I have a 24 foot hull right now that I sawed and resawed venners and laminated the entire hull with it. There would also be a huge amount of waste if you went that way too. . There is however an electric boat guy downeast that uses it. In the good ole days flitch cut lumber was readily avaliable too. These days some of the outlets are selling kiln dried lumber, which after sawn can become brittle too. Picking the grain is essential for a usuable veneer.

 

Mike, I have had problems with brittle juniper. Cypress seems to take bending much better. Both air dry well and quickly so I would look for that. They take up moisture quickly also. I don't know of any local NC builders of sportfishermen that do laminating with other than plywood though. As you said, the labor and waste of material makes that unattractive from a cost measure.

 

The other part of the race boat equation is the weight of the epoxy used for the laminations. 2 is half the weight of 4. When the Gougeons built the 60' Newick trimaran racer Rogue Wave they went to 2 layers of ply for this reason as well as the labor savings.

 

This still goes back to the shape for a certain span or location in certain shapely hulls. So the broadbrush of reducing the total amount of laminates does not really apply across the board. Consider the foward most portion of the bow flare in the smaller carolina hulls as a good example. While you can angle thicker veneers and gain some ability to layup thicker veneers, your seams also will need to be hand fitted more in compound curve locations.

 

Definitely fewer seams to fit with less laminations....The hull shape does define what you can do though. Tight curves mean more laminations to get the skin thickness required.

 

But with the added cost, comes a better product.

I think a lot of the cost is going to depend on what type of plywood and where one is getting it from. My preferred plywood retailer in my area gets $75 for 1/4" Merranti Lloyds Approved 5 ply. That works out to $9.38/BF. I've seen White cedar as low as $1/BF. Granted this was 4/4 and not veneers, but I can't imagine it should cost more than $3-$4/BF. Is there really 2-3 times as much waste as plywood? Labor on the other hand is almost unavoidable.

I have trouble following this logic... If plywood has its own glue/resin to laminate its veneers and makes a product that is lighter than a laminate made using veneers and West System. Then the only deduction would be that the glue used by plywood manufacturers was a better product than West System for producing high strength to weight ratios. I don't see why the Gougeon's would endorse this. Again I understand the labor savings.

To simplify and complicate things even more... Assuming a planking strip as a simply supported beam (which is a grossly over generalized assumption.) A plywood strip is going to have roughly 50% of its grain going in the wrong direction. Depending on the wood used, there is going to be a relation between elastic modulus parallel to the grain and perpendicular. For cedar, and other woods of similar densities, it is around 10:1. So making more assumptions, we can say that the stiffness of a beam of of plywood is going to be roughly 55% that of a beam of solid wood with the grain parallel ([10E +E]/2 = 5.5E.) So It can be said that a solid veneer boat would be 80% stiffer than a plywood boat. Or a solid veneer boat could have a hull that is 18% thinner and have the same stiffness as a plywood boat. Or a wood that had a 45% lower elastic modulus could be used which would correlate to a wood of about 15-20% lower density (If such hypothetical wood existed.) this is all very simplified, and over generalized and obviously stiffness is not the only property that matters for hull construction, but I think these figures help to show what actually happens with the changing of orientation of veneers.