Pat Chapman, proprietor of McFarland Lake Canoe Company offers the following tips for steambending ribs when building a new canoe or replacing broken ribs.
There are a number of considerations to take into account when steambending wood.
Type of wood
How the wood was dried
Alternatives to steambending
Type of wood - Wood from different tree species don't all respond the same when you try to bend them. Some take to steambending well, while others will break before bending significantly. That's why Northern white cedar is the top choice for ribs in a traditional wood/canvas canoe. It is strong, light, and bends exceptionally well after only a short period of steaming. Other woods will work though, so if you can't find a source for white cedar, you can use alternatives. Other cedars, including Port Orford, and Alaska yellow work well. I have also seen Western red cedar used for ribs, but this is less common. A good replacement is Sitka spruce, although it doesn't bend quite as easily and is heavier than the cedars. Some of the hardwoods such as oak or ash will work, but will result in a much heavier boat than is considered desirable. Some of the extra weight may be reduced by planing the thickness down, but ribs too thin will not offer sufficient strength. Wood for the ribs should be free of knots and the grain should have a long runout to avoid splitting when bending. Sand the ribs smooth so that there are no slivers to encourage splitting.
Wood thickness - The general rule for steambending any wood is "an hour per inch". Regardless of the width of your stock you should plan on steaming for one hour for each inch of thickness of your wood. This is just a starting point, however, and depends on how "green" the wood is and the type of wood you're using. You'll have to try some experimentation to determine what's needed in your
particular case. Have some extra scraps in your steambox to test their pliability before bending the real ribs.
How the wood was dried - Kiln dried wood is difficult to steambend, regardless of how long it's steamed or how thick the wood is. Air dried, or better yet, freshly cut wood will bend much easier. In my experience, air dried white cedar yields to steambending methods particularly well. I have not worked with kiln dried white cedar to be able to comment on its properties, however. When possible,
though, avoid using kiln dried woods for steambending.
Steam generation - There are as many methods of generating steam and containing it in a steambox as there are boatbuilders and furniture makers! It doesn't make much difference what you use provided you generate sufficient quantities of steam that is hot enough and that doesn't result in the destruction of your steambox (or shop!). You'll need to provide a source of heat (coffee maker, hotplate, campstove, wood or charcoal fire, or propane fish cooker), a water tank (pressure cooker without lid weight, or recycled 5 gallon can), hose to convey steam to the steambox (radiator hose is ideal) and some sort of steambox (dryer vent hose, black plastic pipe, or wooden box). To get your wood hot enough you'll have to keep the steam as close to the boiling point of water (220 degrees F) as possible, so the bigger the better when deciding on your heat source. I used to use an old Coleman campstove with a pressure cooker to hold the water, but found it barely yielded enough steam for ribs. I now use a propane-fired fish cooker with a 5 gallon can and I have plenty of hot steam for bending ribs for a whole new canoe. Make sure that you allow plenty of ventilation for the steam to escape your steambox so you don't build up pressure in the box. For our needs, steam under pressure is not only unnecessary, but it's dangerous as well. If you use plastic pipe for a steambox, be sure the complete length is supported because the heat will soften it and may cause it to collapse. An oven or meat thermometer inserted in the box is helpful to monitor the temperature - it is surprising how much the temperature fluctuates depending on the volume of steam added and how often you open the box.
Alternatives to steambending - You may not need to use steam to bend ribs or other framing members. There are some effective alternatives and it just depends on your needs. Boiling, soaking, and laminating are all useful.
Boiling - This will work well provided you have a container large enough to hold the stock you'll be bending. I've used an iron pipe with one end capped stood on end over a heat source for boiling stem stock. This worked well with two exceptions. Water tended to boil out of the open end of the pipe, sometimes violently and dangerously. The iron reacted with the oak I was using and turned the wood a metallic black. This was only a surface staining and was removed by sanding the wood, but was disturbing when I first saw it! Be cautious when using this method.
Soaking - Soaking is similar to boiling, but is safer to use. This method entails wrapping the rib stock in a towel or other fabric that will hold water and pouring boiling water over the whole setup. Keep the soaking the towel with boiling water until the wood is softened enough for bending - somewhere around 10 minutes should do. If all you have is a couple of ribs to bend, this is a great method.
Laminating - Laminating is not usually used for bending ribs, but is very useful for bending stems and gunwales with lots of upsweep. This entails glueing thin strips together and bending the combined strips over a form. The advantage of laminating is that there is no springback of the wood after bending and it is very strong. Depending on where you place the laminated piece, the laminations may be visible, so you should decide whether that is important to you.