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

For racing I use the formula worked out by Colin Chapman- If it breaks it wasn't strong enough. If it doesn't break it was too heavy. He's probably outside your experience too....

 

I wanted to have a discussion about plywood vs. veneer in cold mold construction. A discussion is an exchange of knowledge, an argument is an exchange of ignorance. This is deteriorating to the latter.
I'll leave this discussion with that derivation.
Hull failure can come in many modes, but bending/buckling is a failure used in the design process. In the buckling mode particularly (F=Pi^2*E*I/(KL)^2 Where F is the critical load, E - Young's Modulus, I - Second moment of inertia, K - end condition, and L - length) the factors that would need considering would be Young's Modulus (E), Second area of inertia (I), and bulkhead/stringer spacing or L. Assuming bulkhead/stringer spacing to be constant, that leaves just E and I. It is the relation of ρ - Density that is difficult to include. analyzing just a small rectangular strip of the hull it can be concluded that the weight of the strip is M = ρ(lwh) If we are to set M, l, w of the strip as constant, we see that h α 1/ρ (height is inversely proportional to density) Since F α EI (Critical load is proportional to the sum of Young's Modulus and Second moment of Inertia) I =bh^3/12 for a Rectangular cross section, so I α bh^3 assuming a constant width strip again, and substituting h for 1/ρ we arrive at the final product. F α E/ρ^3 Critical load for buckling is proportional to Young's Modulus and inversely proportional to Density cubed. A similar derivation can be done to determine critical load for failure in bending to be proportional to Young's modulus and inversely proportional to Density squared.
This doesn't seem very intuitive, but this is why airplanes are built out of aluminum and not steel even though steel has a better Ultimate Strength to Density ratio. Using this as an example, if a steel column and aluminum column were put in a buckling situation where the columns were the same size, the steel would win. But if the columns are made to be the same weight, and length the aluminum column would be much wider for the same weight and therefore would be more resistant to buckling, and therefore would be able to handle a heavier load before failure.

 

they do make aircraft out of steel: the old "tube and rag" used steel tubing welded into a lightweight truss like frame. It is one of the lightest way to build an aircraft, but much more labor intensive. The former soviet union used to make their super sonic fighters out of steel because they could not produce quality aluminum and titanium alloys. they were were pretty good aircraft, the real problem with steel is it is difficult to keep corrosion under control, a very serious issue for aircraft structure because you can not afford any extra weight, and the steel would be quite thin to make it light enough.

Each material has very different properties, a good design will take these different properties to optimize the structural design, so you would NEVER use steel the same as you use AL or composite for that matter. it is difficult to make direct comparisons unless a complete design optimized for the properties of the material is done and than analyzed for weight, cost, etc.

I saw a performance comparison of a large yacht hull done in cold molded wood, composite, aluminum and steel, same shape hull but optimized for each of the materials. All were pretty similar in finished weight (not identical, but within about 4 percent), the wood was the least costly, but I am sure would take the most maintenance over its useful life. so other considerations have to be made to choose the material besides weight.

 

Cold molding does not need 1/8"peeled veneer, you can use sawn veneers that are the same thickness as the plywood you are likely to use, 1/4" or 3/8" are common to use. Down under back in the heyday of cold molding most boats were either 2 or 3 skins of sawn veneer, not many used 1/8' peeled veneer, whereas many boats here in the US used the thin veneers because everyone had drank the Gougeon koolaid, and used the method they pushed which of course consumes the most epoxy. Its been over 30 years since i last built a cold molded hull which was a 26ft sailboat, it was 2 layers of 3/16 sawn veneers which we resawed out of cypress 1x6 stock sold for fences. The boat was featured in issue#69 of "small craft advisor" magazine in 2011, still looking good. Veneers,sawn or peeled will be stronger but in most cases plywood will be strong enough. The only real advantage to using many layers of thin veneers is if you are building over a solid mold so you dont end up with stringers, which do
make it more difficult to keep the bilge clean.

Steve.

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Weight is a major design consideration for most sportfishing boats. I would like to see the performance comparison you are referring to. Sounds fishy to me. If steel boats were only 4% heavier than composite boats I don't think we would see way fewer composite boats, and probably from steel race boats.

Weight is very important in a planing boat. A 10% reduction in weight can mean a 10% reduction in horsepower (with the same speed), which in some cases can mean another 1-5% reduction in weight (engine and mechanics as well as fuel for constant range) and reduction in horsepower ,etc, etc. Snowball effect... reduction in cost. Only problem is reducing weight is rarely cheap. I understand there are other factors that must be included in the design spiral, but dropping weight in a sportfishing boat is almost always helpful, and if it can be done without much additional cost... that is a winner.

I really like what you are saying. I guess I've always assumed the maximum veneer thickness to be 1/8" because that is the thickest that most veneer facilities will cut slice veneers. How do you finish the surface on the veneers after sawing? With a sander? Do you get consistent thicknesses of veneers? Do you run into difficulties laying thicker veneers in areas of lots of compound curvature? I think veneers would approach these difficulties much quicker because of their extra stiffness.

I'm sure plywood is strong enough in any properly designed scantling, but the question is can it be done with veneers, to produce a lighter product at a comparable price.

 

In the example of the 26ft sailboat we built years ago we bought 1x6 cypress planks and split them in half on the table saw by making 2 passes and then ran them through the planer, so yes, you end up with consistent thickness. These days these little lunchbox planers are available everywhere for not much money. There are always choices to make when selecting material, some woods are stiffer than others this is where experience comes in. Im not sure how much difference in weight you would get assuming a 2 or 3 skin hull but if the scantling were exactly the same and you were simply substituting sawn veneer for plywood the sawn veneer would be cheaper and stiffer. What boat are you looking to build?

Steve.

 

Ok i just went back and looked at your first post, on boats like those beautiful powerboats you can often use much wider planks in places, which plywood allows you to do. Ive only built sailboats which are much more shapely and need narrower planks so sawn veneers work well.

Steve.

 

I do not remember where I saw it, it was likely somewhere on this forum as an attachment to a posting. I may have seen it in a textbook like "Elements of Yacht Design", but I do not remember. It was for a fairly large yacht, like 60 ft. Results might be very different for say a 24 ft or a 120 ft yacht. And it just depends to what basis you are comparing. If you are optimizing strength to weight, most common building materials come out very similar. The only time you get any significant advantage is when you go to very costly composites like graphite.

Often steel hulls are overbuilt to compensate for corrosion, and often on a cruising sailboat extra weight is desirable to improve sea-keeping. that would not be a design optimized for best strength to weight ratio.

 

The Gougeon Brothers, and I believe Oyster on this forum can confirm, say that re-sawing, and planing your own veneers can end up costing more that purchasing pre-sliced veneers. But veneer facilities (according to the Gougeons) can't slice veneers thicker than 1/8" So it seems like its the only option. The labor is something one can't really get around. If the boards are not planed to a consistent the extra cost will still be there in the form of extra epoxy.
I think the Custom Carolina is a unique planking situation in that there are large areas with almost not curvature at all, and then some areas in the bow flare with very strong compound curvature.
I guess my goal with using Veneer over plywood wouldn't be to get a stiffer product, but rather a product of the same stiffness that is lighter.

 

Petros, Please read chaper 43 "Even from Cheese Whiz" of Dave Gerr's "The Nature of Boats" It does a good job explaining the weights, advantages and disadvantages of various styles of boat construction. I'm sorry but the article you are referring to might have had a different design criteria or you have been badly mislead.

 

The veneers are only a small part of the cost of building the hull, 1/8" peeled veneers require more man hours of spiling, a lot more epoxy (which is why the Gougeon book pushes it of course) and a bunch of man hours in the mold rather than the hulls structure. Really we are talking 2 very different cold molding methods, the carolina boats i believe are thicker veneers over a permenant structure so the peeled veneer argument is not really relavent. The choice is between plywood ripped into strips or solid sawn veneers, if your goal is lighter for the same stiffness i dont think there would be much if any difference assuming the same number of layers but there will be labor savings using plywood as you only have to rip the strips and will be able to use wider strips in places saving on spiling.

Steve.

 

First off when you say peeled veneers do you mean rotary peeled? Because those are not preferred for boatbuilding. Slice veneers from quarter sawn logs should be used because of the early, late season growth rings of the lumber.

There are many different forms of wooden boat building and several different forms of cold molding. The two styles used for building Carolina sportfishing boats are Plank of Frame and double diagonal plywood Cold molding. Juniper planks are usually used for the traditional plank on frame. In this building style, the frame does contribute a lot to the structure of the boat. The Cold molded boats are built over a jig which contains some permanent components, bulkhead, stringers, keel, chines, etc and some components that will be removed after the hull is laid, "temporary bulkheads" if you will.

As for plywood strips being just as stiff as sawn veneer strips of the same size, I'm sorry but this is far from the truth. To experience this for yourself go to home depot and buy some plywood and some doug fir and rip to the same dimensions and try flexing. Depending on the orientation of the veneers in the plywood, the number of plys, the type of glue and the species of wood. A plywood strip is going to be 35-65% as stiff is a veneer strip. Put another way, the veneer strip is going to be 55-300% stiffer than the plywood. This is no small amount. Since buckling and bending are the major failure modes, stiffness is very important. How a lighter product is achieved is by one or both of the following. Woods of a lighter density. (ie Western Red cedar Veneers are going to have the same or better stiffness than Mahogany plywood strips of the same thickness, but weight about 5 lb/cu.ft. less.) Or Thinner layers. (i.e. 1/8 veneer strips will have the same stiffness as 5/32 plywood strips for the same material and will weigh less accordingly)

*Saving time laying strips. In areas where wide strips can be used, there is little spiling needed.
True... Bottom line Veneers are going to take a lot more labor.

 

I dont think i said that plywood strips were as stiff as sawn veneers of the same thickness, of course they are not, in fact i said the opposite in post #36. To reduce the weight while maintaining the same stiffness you would need to reduce the thickness of the veneers. There is always a trade off however as a thicker skin will have better impact resistance.

Steve.

 

Okay... yeah i think we are in agreement. My personal preference wouldn't be to go with a thinner layer, but rather a layer of the same thickness, but of a wood of lighter density ie Western Red cedar, Atlantic White Cedar, or Paulownia. I can't seem to find any calculations for minimum bending radius that would include Young's Modulus, Tensile Strength and Thickness. Something of the sort would be useful for maximizing thickness and minimizing the number of layers.

 

Some of the math in using plywood is actually a bit more complicated than what has been suggested. I'm not advocating one construction over another because both have merits. On a solid veneer boat it is easy to see where the strength is going.

For Multihulls the planks can be much wider 8- 10" in areas, and the split resistance of ply as well as the availability of larger widths makes this possible as has been pointed out. With half the grains running crosswise ply is indeed less stiff along the length allowing a thicker plank to take the same curve as a thinner solid veneer. The internal ply construction now comes into play, the fiber orientation that runs cross wise is now running fore and aft at a shallow angle joined by the "butt blocks" of the second plank layer in double diagonal construction. This helps the fore and aft stiffness of the entire structure and is more effective with the wider plank widths allowed by ply. The second ply skin of course has the same thing happening with the internal layer becoming the "butt blocks'. Perhaps not as effective as an unbroken fiber length but far from being just fill.

To determine actual real world strength hull section mockups, say 10' or so would need to be destruction tested for deflection, impact resistance, wracking etc....as mentioned there are different kinds of strength and the loads imposed by sailing on a moving surface are by no means simple.

On a tangent ply lapstrake construction allows thinner planks to be used with greater strength because of the split resistance. The vertical internal plies there help the task of the frames helping keep the supporting structure light.