Steaming Ribs

We are now steam bending the ribs that fit inside the stringers. You can see the steam box in the background. If you look out the window you will also see snowbanks that reach up to half the window height on the left side, indications of the snowiest winter on record here in Boston. 

Monfort's design calls for 3/16"x1/2" but we were given scrap quarter sawn oak from a toboggan building project already cut to 5/16" depth. The wood is a little harder to bend because of its relatively low moisture content, but with the ribs in place, the hull is very stiff. 

We soak the ribs for several days in a set of chambers made from PVC drainage pipe cut to 45" - just a little taller than the 42" ribs. With a cap on top of the PVC tube, the ribs sit suspended in the water which we fill to the brim. The buoyancy of the wood should put the surface of the wood under pressure forcing as much moisture into the wood fibers as possible. 

What seems most important in the bending process is to get the wood as hot as possible. We use the kind of thermometer sold for outdoor grills poked down into the steamer to make sure that we have reached close to 212 degrees before we put the wood in the chamber. The general rule for steaming wood is an hour per inch of thickness. 20 minutes works well for our ribs. 

Here is the method we have used to bend the ribs:

1. Rib positions are carefully marked on the outside of the stringers by placing a piece of surveyor's tape over the hull and marking the positions on all stringers and gunwales. That allows us to quickly get the rib into position on both sides.

2. After 20 minutes at 212 degrees, two benders are ready at the box with heavy gloves. One opens and shuts door to conserve heat, while the other fits a very bendable wooden strap to the back of the rib. We use a piece of fir cut to 1/8" depth as a strap rather than metal because we worry a metal strap would wick away the heat quickly. We start with the fir strap in the steamer and draw it out with the first rib. 

3. Both benders race to the hull knowing bending time is one minute. 

4. We place the rib and strap above the hull and bend down with pressure on the strap from both sides to keep the rib's outside edge from splintering as that side of the rib gets stretched. Quickly but gently we bend the rib to take the shape of the hull.

5. The first bender then passes the rib inside the hull and we gently squeeze clamp it at the points of maximum curve while pushing up from the ends of the rib to make sure it meets all stringers and the keelson. If the rib is sufficiently hot, it should go in quite easily - at least during that first 60 seconds. You will feel it stiffening very quickly. 

Monfort's "clamp its" work well in securing the rib to the stringers because they can be set up beforehand on the stringers and then slid over the rib. But with practice, you can also quickly secure the rib with cable ties before it cools.

The boat on the left has all but its end ribs in place, drying out before we will glue them in place. While you could laminate ribs or probably get the bend with a boiling or soaking, steaming the wood is very exciting - and a great place to learn about wood with both the successes and failures. If a rib breaks, examine the point of failure carefully and you will probably find that the wood grain has "run out" - the grain veering off toward the edge just at the break or you may find a splintery edge that should have been sanded before steaming. Small knots or other irregularities in the grain will also cause breaks. Save the broken pieces if they are long enough as they can be used later as cutwaters, cross braces for sailing rigs, or deck ribs if you sew spray covers for the ends of the canoe.