Long term stress after bending wood?

[Alan Peck]

In bending wood and plywood (not steam bending)I have had to apply considerable force; brute strengh, clamps, etc.

Once the clamps are removed either glue or screws (or both) hold the wood in place. My question is: After a period of time does the wood finally relax or is the boat always trying to tear itself apart and return to the unbent shape.

I guess epoxy has good long term strength, but not forever I assume.

Any thoughts?

[NorthLight]

That's a good question, because I've tortured 4mm Okoume plywood and 1" Mahoghany rails in my small boats. My guess would be that there will always be some stress in the wood as the fibers in a dry piece of wood won't slip like they would if they were steamed. I know from experience though, that the wood will have some permanent bend in it after it's held in a bent position for a long time.

[gary porter]

Alan, if you laminate thinner strips and glue with epoxy then the end product will tend to relax slightly but can be made to fit easily such as in making a midship frame or a stem. It does take a lot of pressure and a good form. Its easier if you use a second clamp and alternate the pressure rather than just bending the wood with brute force. Some folks use straps and such as well. On some boats, the bow of a Tolman is a good example, the full thickness of plywood cannot be easily tortured into that shape and would probably not stay in that shape very well. In that case we make the initial shape with thinner plywood then laminate another thinner sheet on top to achive the final thickness.
What are you building??
Garu

[dmede]

I bent in some coaming pieces for my canoe recently. While working on the fit I would have them clamped in place for several days to a week and then pull them to do more work. There was a lot of strees involved in getting the peice tight to the sheer clamp but once pulled it retained a good deal of the bent shape.

So I would say the wood definetly relaxes into its new shape even if unsteamed. Though I'm sure the wood type, dimensions and degree of bend will all effect to what extent it will relax and how well the fasteners will hold in that shape.

dave

[Dan McCosh]

Strictly from experience in undoing old stuff, the wood does take a shape, but not completely. A bowmaker would be able to give better data.

[Sam Desmet]

Duplicating cherry outwhales (am I spelling that right?) on an old Adirondack guide boat caused me to cold mold/bend the strips at a rather harsh angle front and back. Two years later the same boat is back with a foot long section of my cherry outwhale missing, along w/ the lock, both of which were later recovered... But I digress.

The cherry took on the bend of the hull with only mechanical fastening.

[Canoeyawl]

Wood has an engineering property called “creep” …the molecules will move internally and assume the new curve. This is why we don’t want to leave a wooden boat resting on the planking or the spars not well supported in a straight line.
Many other materials also have this property rope and lines are included.

[Dave Fleming]

We used the term 'memory' for wood that has taken on a shape from being fitted or bent.

[Canoeyawl]

I knew a boat builder that used sawhorses and weights to put boards “In Training”…sometimes for months before using/bending them. They would take a pretty good set making the task easier.