A steam box for bending wood for skin-on-frame canoe construction should serve it's purpose and need not be elaborately built, as it will swell over time under the effects of temperature changes and humidity and should be considered more of a consumable item. A 12mm thick textured coated board withstands the effects of steam, moisture and heat well, making it a good choice for building a steam box. It doesn't require any sophisticated joining techniques. Plates sawn at right angles and joined with wood screws make a very useful steam box for bending wood.
Dimensions
The steam box should be slightly longer than the longest rib, wide enough to accommodate several ribs next to each other and not very high, in order to keep the internal volume low. For example, a steam box with internal dimensions of 150 x 30 x 6 cm is suitable for strips of 140 cm in length. In addition, the steam box should be equipped with a grid of wooden slats that "holds up" the ribs so that their underside is also steamed. Do not use metal bars or grids, as wooden strips tend to turn black when in contact with metals and moisture. It is recommended that the strips are located just below the top of the steam box, as this is where the steam has the optimum temperature.
Flap
A flap can be used to close the chamber, but it should not seal the steam chamber too tightly so that there is always a certain "flow" of steam. A wet cloth placed over the opening works just as well as a flap. We have equipped our steam chamber with a flap made of textured wooden board, which is attached with two hinges. A weight automatically closes the flap after a strip is taken out. This is very practical, because when bending wood, every second after taking out a strip counts, so you don't have to worry about closing the box.
The steam generator
In addition to commercial steam generators, home-made steam generators also perform well, for example, a cooking pot on a gas stove. The cooking pot is connected to the underside of the steam box with, for example, an aluminum flex pipe. The tube should be as short as possible and have a large cross-section of about 10cm, otherwise the amount of steam will not be sufficient. We do not recommend the use of electric stoves or kettles, as water can drip from the steam box or water can condense on the tube and there is a risk of electric shock.
Temperature
A meat thermometer is an excellent way to check the temperature. To do this, you can drill small holes in the steambox and insert the thermometer. The optimum temperature for steam bending wood is 95-100°C.
Refilling water
Also make sure you can easily control the amount of water in the pot and easily refill water. Refilling larger amounts of water should be avoided, as the water will then stop boiling and the steam temperature in the chamber will drop too much. Therefore, refill smaller amounts of water more often.
We hope that our tips helped you and wish you a lot of fun building your skin-on-frame boat and especially with the craft of steam bending!
