Wood-Metal Composite

Wood shell and metal frames is the original form of composite boatbuilding, and it may have a future. Chris Barry, a naval architect with considerable experience design and building small craft, has an article on the topic in the April 2017 issue of (mt) aka Marine Technology, a SNAME publication.

Chris notes that wood with it's low density is a very efficient material for the shell/planking of the hull, but that metal is efficient for the frames. He argues that marrying the two together can result in a hull which is lightweight while much quicker and less expensive to build as a one-off than the usual resin-fiber type composite hulls once tooling is considered.

 

No two requirements are the same. Calling a material "efficient" is somewhat of a misnomer and tempered by the fact it is not efficient for the frames. Thus what is so "efficient" about it? Since it must be efficient in all applications to be 'efficient'! If it is not, and merely cherry picked, it is no different to any other material by that metric.

 

Let's reject that awful word "efficiency", how about "effective" ?

 

How about a frame out of this sort of metal framing system http://buildipedia.com/images/maste...g/Photo_Gallery_1/051000_metal-framing-12.jpg (you can nip the short side flanges and bend the member to desired degrees and run self-tapper to secure), covered by plywood for stitch&glue?

Run flathead screws from plywood to metal.

 

Chris Barry used "figures of merit" based on strength vs weight in the article rather than "efficient", so ignore my use of efficient in the first post - my mistake. His figure of merit for the shell plating/planking material is the square root of the yield strength divided by the density, which is based on the strength of a plate being proportional to the thickness of the plate squared multiplied by the yield strength. Wood planking with the same strength as metal will be thicker but lower weight. His figure of merit for the frame material is the yield strength divided by the density, with the assumption that the molding dimension (depth) of frames is usually constrained. Wood frames with the same strength and molded dimension as metal frames will have a much larger siding dimension (thickness) and be heavier. Wood is further disadvantaged by its shear strength limits which restricts the ability to use thin web sections for frames in wood.

Chris also talks in general terms about relative tooling and construction costs based on his experiences. His concluding paragraph is:

It is unlikely that composite wood construction [wood shell with metal framework] will be become the method of choice for most recreation or racing boats, but it has the potential to be an interesting niche construction technique for one-off or limited runs, especially for amateur builders. It might be worth considering.​

The article, is less than four pages long, and is not a comprehensive review of wood-metal composite construction. My posts here are intended to make the readers of this forum aware of the article, not present it's entire contents and conclusions. The article is titled "Composite Construction of Recreational Boats" and was the Marine Technology Note in the April 2017 issue of Marine Technology, aka (mt), a SNAME publication. For more information see the article.

 

So he is basically ignoring the standard measure of a material and its effectiveness that any structural engineer would use, that of its EI.

 

EI is an appropriate measure of material effectiveness when deflection is a primary design criteria. For situations where strength, not deflection is the primary design criteria then a measure utilizing yield stress or maximum allowable stress, not E (Young's modulus) is appropriate.

In the second paragraph of the article Chris says "One problem with many yacht is to produce a light enough, strong enough structure both for performance and for issues such as trailering. This is possible with modern composite construction, but at substantial cost to produce the molds." Chris' figures of merit appear to be based on the strength divided by weight with the loads being normal to the hull plating, the shell thickness open, and the molded depth of the framing being restricted.

Chris is far from the first to suggest that modern forms of wood construction such as cold-molding, strip planking, plywood or some combination can lead to a lighter hull shell than fiberglass or metal for recreational boats. Numerous custom high performance sail boats and power boats have been built with cold molded wood.

The short article mentions other factors which lead to Chris suggesting that the combination of wood planking/plating and metal frames be considered for one-off and limited production recreation where a strong enough, light enough structure is needed and cost is also a significant consideration. Anyone interested in more information should read the article.

 

Sorry, disagree totally.

1) Any bending moment equation being with the derivation of the bending moments with respect to EI d^2x/d^2y, the EI is already there.
2) The stress, in term sof strength is not applicable when the material is a low modulus, since deflections dominate. Again, hence the EI.

Most people think of steel and doing structural calculations in steel, a deflection check is almost never required, why, because the E is so high and the product EI, is similarly large. Its is only when using a low modulus material that the deflections become primary not secondary. Any calculation of stress with aluminium or composite must always be accompanied by a deflection check. The stress is usually fine, but the deflections, not so!

Thus, the EI the product is the important measure, as it takes into account materia,l E, and the stiffness, I, of the structural arrangement.

 

The problem is that in many cases the strength isn't the critical parameter, but the requirement is to resist buckling. In particular things like bulkheads may easily be strong enough in light weight aluminum, but in order to not have a problem with bucking you end up making them thicker than would be required if stress was the only criteria. That can often be mitigated by adding ribbing but that ups the weight and cost. He's right that in smaller craft where bucking isn't the key criteria that you might be able to use metal instead of composite and it would likely be an advantage, but in order to make it work it's not just as simple as a P/A or even a EI answer.

There's always different ways to skin a cat...

 

Well..if one wishes to be pedantic and to address structural design in its entirety .


There are 4 checks one must do when performing structural design.


1 failure from exceeding allowable stress limits – like yield strength

2 fatigue – from cyclic loading

3 Instability – such as buckling

4 Deformation – too much deflection


So one must review all. But over time and with experience one knows, when working with certain materials, some checks become less important than others. So each material has an order of priority of what checks to perform. It cannot be boiled down to a one liner, other than it is still about the EI. Since the EI influences all the above, except fatigue.

 

Aside from the engineering jargon i think upto 30,000 lb displacement should b very doable using metal framing and wood hull panels. How is it proposed to join the two materials

 

These types of composite structures have been tried many times and has proven problematic at best. Dissimilar material properties, fighting each other at the faying areas and corrosion have been the big issues. I once did a wooden hull shell with wooden frames, though steel gussets and reinforcements (braces, stringers, etc.) were incorporated into the structural elements to save weight. Saved about 10% maybe 15% over traditional wooden element weight, but was a interface nightmare within 5 years, in spite of encapsulation. Elongation modulus difficulties were a constant battle and though initially fast, maintenance was higher, because of the choices made.

I agree with Chris in that the premise seems reasonable over one off resin/fabric builds after tooling is considered, though I question the justification unless long term durability isn't a concern, such as in testbeds, prototypes, racers, etc., which is (I think) his quantification for this approuch.

 

Par, thats makes sense but still wonder if all frames, clamps, and stringers were metal with say a wooden 1×3 bolted to stringers/clamps to hang plywood on, that would be the critical interface, it would have to be able to accomodate the different movements for it to work.

 

Other than that junction the two materials would be completely isolated

 

As Par noted you still have differential thermal expansion to contend with. As things heat up and cool down the metal is expanding and contracting where the wood is not. This will tend to flex the fasteners between the wood and the metal and over time it will work itself loose. Once the fasteners move enough to get water intrusion around them they'll rot and come loose. If you look at lots of old wooden boats one of the key modes of failure is water getting around the fasteners and the wood rotting (and the screws rusting if they aren't SS or bronze) locally around the screws.. Paint isn't a good enough sealant. When you look at an old wooden boat and see small cracks above the screw heads, when you open it up and strip the paint you'll find rot under there. Seen it time and again.

If you are looking for the last increment in performance you can go to real high strength (and high stiffness) composites like carbon fiber and you'll likely do better than aluminum on both a strength and stiffness aspect and if you need section modulus you can use a core and you'll be better off than with metal an not have the disadvantages of trying to fasten it together or have the thermal expansion problems.