Composite wood / metal construction

For what it's worth, I recently gave a paper on construction of boat with wood shells over metal framing using CAD/CAM/CNC and other new technologies at the 2022 Chesapeake Sailing Yacht Symposium that was subsequently reprinted in the Journal of Sailing Technology. JST is open access so all of the papers can be downloaded for free. The composite construction paper is at:

CNC Enabled Wood/Metal Composite Construction of (Relatively) High Performance Sailing Yachts | Journal of Sailing Technology | OnePetro

 

Thank you for sharing that,I may or may not ever need the information but it may pave a way forward for some applications that others will benefit from.Truly in the spirit of this site to share our knowledge and experience and I hope to enjoy expanding my knowledge in 2024.

 

Thank you for posting that.
I've many times thought about that method of construction.
Interestingly, ~100 years ago or so the Herreshoff yard was doing that quite frequently in the building of medium>larger yachts.
The scheme at that time was to use "Bulb Angle" for frames.
The angle was heated in the furnace and would be pulled out on a steel floor that was perforated for hold-downs.
Men with hammers/bars would shape the hot metal to match a pattern, this work was called "Fettling", the men were Fettlers.
Today, if the frame stock were not too heavy one could cold-form the angle with a simple hydraulic press/rollers, a torch being handy for tighter curves.
An angle frame has a distinct advantage, a full length of flat surface that wedge shaped strips can be used on to match the angle of the planking.
Holes can be drilled in it and the first layer of planking goes on being fastened from the inside with screws.
After the fore-and-aft strips/planks are on then the cold-molded diagonal work is done.
At intervals the inside fasteners can be removed, and countersunk bolts can come-in from the outside to reinforce areas of high stress.
With flat frames cut from plate other ways must be found for fastening the shell and material wastage is a concern.
Flat frames also lack the rigidity that angle/bulb angle gives.

 

Interesting when new tools make "old" methods useful/competitive again. Would you please comment on design strategy to cope with the differences in thermal expansion coefficients in the materials involved?

 

I'm sure the Herreshoff yard knew much about thermal expansion/contraction, (they also built steam engines and boilers).
They also used bronze angle for frames in more "high end" construction, as well as bronze strapping on hulls to reduce wracking strains.
Thermal expansion?
Perhaps the engineers were not very worried about getting bogged down with what they may have considered to be inconsequential details, i.e., it's just a boat, not a spacecraft.

 

As rangebowdrie said:
Thermal expansion is negligible compared to moisture expansion. In addition, when using only wood, the difference in expansion between with the grain vs cross grain is similar to the difference between metal and wood, so there isn't much difference between metal frames and wood. Moisture expansion in wood outweighs thermal expansion to such an extent that it is impractical to measure thermal expansion. Since any temperature change will affect the moisture content, it is very hard to gather any meaningful data. Look at the wood handbook from the forest products lab. Wood also has highly anisotropic moisture expansion. Longitudinal expansion is rarely found because it is very small, while tangential and radial expansion are high. Holm Oak (random choice) for instance Shrinkage:Radial: 4.6%, Tangential: 8.4%, Volumetric: 13.0%, T/R Ratio: 1.8. Thermal expansion of metals commonly used is from 6 to 10 ppm/F (10 to 18 ppm/°C). Wood is also extremely anisotropic with respect to strength, roughly 10:1 for grain direction to perpendicular to grain.

 

Note that the paper itself and a lot of more interesting ones (especially on foiling under sail) are open access at www.onepetro.org/JST.

Mine is in the 2022 issue.

 

One point on this method is that curved members such as when frames can be CNC cut rather than formed. This is typical practice for most steel or aluminum boats, usually with the web games buit up out of smaller pieces and either lapped or seamed together.

 

The paper was given at CSYS 2022 in Annapolis. I suspect that they needed some comedy relief.

However, one of the organizers challenged me to design and build a boat using this method and bring it to the 2025 CSYS with a follow up paper. I may do just that, if I can convince my wife to let me use power tools.

The tentative design is "Runaway Bunny", a minimal overnight cruiser generally similar to a Dragon. (I had one for some years in San Francisco.)

With any luck I may be able get this underway and begin posting progress reports.