July 23, 2012
by Mike Elliott, Kettle River Canoes
One aspect of canoe restoration that seems to confound many people is the process of bending wood. It is puzzling, and sometimes terrifying, until you know what is happening in the wood itself.
Wood fibres are made up of countless cells that have a tough exterior wall made of cellulose which cannot be bent or stretched. Therefore, when a piece of wood bends, it is the space between the cells that is either compressed or lengthened. A compound called lignin holds the wood fibres together. When it becomes hot (close to the boiling point of water – about 200°F or about 93°C), it is liquefied thus allowing the cells in the wood fibres to move. The cells on the inside of a curve are forced closer together while the cells along the outside of the curve are pulled further apart. When the wood cools, the lignin solidifies again to hold the new shape.
To bring the internal temperature of the wood to 93°C, two things are required: Water and heat. Heat is used to heat water molecules in the wood. They heat the lignin which then liquefies and allows the wood to bend.
Normally, there is not enough water in the wood to heat the lignin effectively. So, the first step in the wood-bending process is to raise the moisture content of the wood. This is achieved by soaking the wood long enough to saturate the wood. The time required depends on the wood species and the thickness of the piece. I don’t bother measuring the moisture content. Instead, I just go on experience. For example, a piece of Red Cedar rib stock 3/8” thick requires about 2 to 3 days of soaking time while a piece of White Ash stem stock 7/8” thick requires about 6 to 7 days. A piece of Red Cedar planking stock 5/32” thick requires less than one hour of soaking time.
Once the wood has been soaked, the wood is heated with steam or near-boiling water. The amount of heat required depends on the thickness of the wood. The Red Cedar rib stock is steamed for about 45 minutes while the White Ash stem stock is steamed for about 65 minutes. Meanwhile, the Red Cedar planking stock requires about15 minutes of steam. Too little heat and the wood will break, too much and the fibres are cooked and start to break down. The over-cooked wood then crumples on the inside curve when bent.
A key element in this process is the steam generation itself. It takes a lot of steam to create the heat required to liquefy the lignin. I use a 2 gallon (8 litre) pot over a propane stove.
My steam box (inside dimensions – 12” high by 12” wide by 7’ long) sits directly above it (balanced horizontally) with a firm seal made of plywood rings.
Each piece of wood in the box must have lots of air around it to allow the steam to heat each piece evenly. A series dowels creates a number of shelves in the box.
In the “old days” many of the canoe factories were heated with large boiler systems. These boilers were also set up to deliver steam into large steam chambers for wood bending. In many cases, the steam could be held under about 5 psi pressure in these chambers. These “pressure cookers” could reduce the amount of time required to soak as well as heat the wood quite considerably – about 6 hours soaking instead of 48 and about 15 minutes steaming instead of 45.
With the heated wood pliable and ready to bend, it is removed from the steam box and bent into shape immediately. The “working time” is about one minute. If the bending can be achieved in the first 30 seconds, that is ideal.
When I am bending a thicker piece of wood (more than ½ inch thick), I use a thin strip of hardwood – usually ash – as a backing strip for the work. It helps keep the integrity of the piece while it is bending. Make sure your wood is as close to perfectly straight grain as possible. If the grain slants, the outer edge of the wood grain will “run out” and it is more likely that the wood will split or break when bent.
Once the wood is bent into the desired shape, it is important to keep it in the form for at least 3 days. This allows the wood to dry and thereby retain its new shape. When I am bending stems, I let the stock dry for at least a week to reduce the amount of “spring-back” in the piece.
Sometimes, you must construct and use custom forms for the specific bend required. One example are the gunwales of the Chestnut or Peterborough Pleasure canoes. A quick look at the sheer line reveals a sharp curve about 18” from the stem-end at each end of the canoe. This bend is too sharp to be done when the new wood is dry. It must be bent prior to installation. To do this, you need custom forms for the job. I’m sure there are as many types of steam-bending forms as there are canoe builders. Here is the system I use.
I take the shape of the form directly from the original outwale and transfer it on to a piece of ¾” plywood that is 8”x24”. For four bending forms, I use eight pieces of 8”x24” plywood. Although the outwale is ¾” wide, the form is 1½” wide. The extra width makes it easier to work quickly when bending the hot wood. The curve in the form must be greater than the final curve desired. Compensate for a certain amount of “spring-back” in the new wood once it has been bent and dried. By adding about 2” more curve over the 18” of outwale than in the final curve required. Cut the first piece of plywood and use it as a template for the other seven pieces.
Cut strips of ¾” plywood 2½” wide. These will be the braces for the holding points along the length of the form. For the outwale forms, I use three holding points. In this series of photos, I numbered the stations starting with the first is at the end of the form that is 8” wide. The third is at the end of the curve in the form while the second is placed 18” from the third holding point. The second point is where most of the curve in the outwale occurs.
Each holding point will consist of a dowel and wedge system held in the braces. A ¾” hardwood dowel is placed in a 7/8” hole in each set of braces. The dowel is located so that the outwale (1-1/8” high) can fit between the top of the form and the dowel. There must also be enough room to allow a hardwood wedge to fit between the outwale and the dowel. Each brace piece extends at least 3¼” beyond the top edge of the form. The centre point for a 7/8” hole is placed about 1-7/8” above the top of the form. Attach each of the braces on one side of the bending form at their intended locations using 2” deck screws.
Now, flip the form over and place the braces in their intended positions. Insert the dowel into the holes and make sure that the dowel fits properly. It should fit easily into the holes on each side of the form and be located with enough space to accommodate the outwale and the wedge.
Cut the new outwales as exact copies of the originals. You will need four pieces to create two outwales. Determine which pieces will fit together to form each outwale and then mark the ends to identify them for bending. Soak all four ends for about 72 hours. I use a piece of 4” ABS pipe 7’ long as my soaking tube.
When ready to bend, remove one of the outwale pieces and lock it into the form at location #3 with the dowel and wedge. Pour boiling water over the outwale between the braces #2 and #3. Immediately, bend the outwale on to the form until you can lock it into place at brace #2 with a dowel and wedge. Repeat the heating and bending process between braces #2 and #1. The entire process is very quick. The dowel and wedge system allows this to happen with a minimum of delay.
Once all four outwale pieces are bent, set them aside to dry. The results vary with each piece of wood. Some hold the bend with very little “spring-back” while other pieces straighten out a fair bit. In any case, the wood can be adjusted easily to fit the canoe.
For new stems, I bend the stock (7/8″ thick by 2″ wide) first and then slice it on the table saw to make two stems that are then shaped to fit the canoe. It is all a bit complicated with lots of trial and error along the way.
At times like this, I think back to my years training to become a Fencing Master. My mentor was Zbigniew Skrudlik, who had trained the Polish Olympic Men’s Foil Team to 2 gold medals at the 1972 Games in Munich. As I was struggling for months to learn a particularly subtle and elegant action, I complained about my troubles in mastering something so difficult. He replied, “If it was easy, then everybody would be doing it. And if you can only do what everyone else can do, where is the advantage in that?”