Years ago, in the midst of a difficult reframing job, I came across Barry Thomas’s excellent book, Building the Crosby Catboat. In it, Thomas describes his research into the Crosby method for fitting thick steam-bent frames into very tight bilges. He had had the good fortune to meet Horace Manley Crosby, Jr. aka “Bunk,” right when he needed him.
Jan AdkinsThe combination of an accurate adjustable template (a so-called “chain gauge”), a compression strap, and an adjustable bending table allows for quick and accurate reframing of even complex-shaped boats.
Thomas tells of Bunk showing him and his team the tools and method for picking up the shape of a frame using a wooden chain-like “timber mold,” transferring the shape to a bending jig, and bending the frame away from the boat with the aid of a compression strap. The frame was then installed in the boat cold after it had cooled and set to its curve.
Back in our shop, which was then in Kingston, Massachusetts, we set up a bending jig pretty much as Thomas describes. We bought a compression strap from Lee Valley Tools. And we built the chain gauge—our term for the Crosby “timber mold.” We then bent our very tortured frames to their shapes with perfect control. Knowing that one can always slightly relax an overbent frame, and that one cannot add more bend once the frame has steamed, we added some overbend here and there on our mold. It took a bit of experimentation to get the feel for it, but soon enough we found we could massage and relax these timbers to lie in our hull so sweetly that we could barely slip a piece paper between the frame and the planking. And that was without any fastenings installed. Our frames lay in the boat with no residual tension, and perfectly to form.
We’ve used our chain gauge, along with the bending jig, for nearly 20 years, and learned its limitations and benefits. We don’t use it for all the bending we do, but when we restored ST. LOUIS, a 36′ fantail Elco launch from 1896 (see WoodenBoat No. 289), this system played a major role in the reframing.
Jan AdkinsThe chain gauge is made of wooden “links” joined together with 1⁄4” machine screws.
Making and Using the Chain Gauge
For the chain gauge’s links, use a stable hardwood such as mahogany. The drawing above shows the dimensions of the links. In order to describe a fair curve, and mini-mize any jaggedness in the line, the centers of the bolts joining the links are not centered in the link, but are, rather, slightly outboard.
The friction of the bolts matters, so using nicely fin-ished smooth fastenings of stainless-steel or bronze helps the chain take and hold a shape. My bolts are 1⁄4-20 × 11⁄2″ round-head machine screws, with washers and Nylock nuts. My chain is about 9′ long, but we break it down, by removing a fastening, to fit a particular boat. Generally, we will set up our gauge to be longer than the longest frame, so we can bend a frame a foot or two longer than required.
We determine one bolt’s friction by trial and error, and then carefully set them all to the same light torque. We then lay the chain into the hull and work it into shape, softly pounding with the heel of a palm. If the bolt tension is just right, the links won’t bounce, but will instead settle into the hull and hold the shape when removed from the boat and taken to the bending table.
The Bending Table
The bending table consists of a plywood surface with a series of strategically located slots cut through it. Through each slot is bolted a clamping block, which can be slid along the slot until it aligns with the curve of a frame, and then fixed; to secure a clamping block, a nut beneath the table is tightened. The frame-to-be is bent and clamped to these blocks.
I admit it: we slapped my bending table together 15–20 years ago to suit one job. But it’s turned out to be a rugged and dependable jig, and we’ve used it ever since. It might not be pretty, but we’ve bent some hard pieces to it.
We made the table from two layers of ¾” fir plywood lagged to a pair of 2×6s. There wasn’t much science to laying out the slots. For that initial job 20 years ago, we were bending a series of S-shaped frames, so we laid out a series of slots that would more or less cross the frames’ curves at 90 degrees.
Jan AdkinsThe outer fibers of a bent frame experience enormous tension, which can result in failure. A compression strap reduces or eliminates this tension.
The Compression Strap
The art of steam-bending is compressing the wood on the inside of the bend, while allowing no damage-inducing tension on the outside. Tension is death to oak.
That’s where the compression strap comes in. It is a flexible, high-tensile-strength steel band that anchors to each end of a piece that’s to be bent, on the outer face of the curve, to keep that surface from stretching and breaking. The Lee Valley strap is predrilled with holes that allow fastening at each end; the company also sells specialized clamps: one that anchors of one end of the strap, and one that’s adjustable, allowing the strap to be tightened before bending. (This clamp kit, including 10′ of steel strap, is available from Lee Valley for $99; www.leevalley.com. That’s a pretty good deal, considering the cost of materials, but there are myriad shop-made ways to address the problem of compression straps. The tool also comes with fine instructions.)
Putting It All Together
Once we have picked up the frame shape, we lay the chain gauge on the bending table and adjust the table’s blocks to fit the captured curve. We generally add some overbend, particularly in the straight upper areas of the frame. With the aid of the compression strap, we are able to achieve good bends very quickly, with the timber still hot and sweating, and with minimal breakage.
One of the key benefits of our system is that we can place prebent frames into a weak structure; there is no need to reinforce the hull to resist the pounding and torquing required to force hot frames against ribbands. The action happens off the boat, on the jig, and the cold, curved frames are placed gently into the hull. There are thus no athwartship molds, no strongback, and no bracing to the walls, floors, and ceiling. The boat is fairly open, allowing easy access.
Another benefit of this system is it simplifies replicating the boat’s original shape. The Elco had not been built with molds, and she had never been perfectly symmetrical. Before reframing the boat, we had restored the original shape of the hull as best we understood it. Our bending system saved us the headache of trying to make molds to fit. Most important, it also assured that all the original joinery would fit back in with little fuss.
This bending system has also been useful in adding twist to a frame. I believe that in new construction, a builder should allow frames to run up the ribbands as they like, and let them cant at the ends, rather than insist on every frame being plumb. But in a restoration, we’re stuck with the original builder’s chosen frame path. In a beamy boat, the upper sections of the frames can have massive twist near the ends of the hull. We’ve used this system to pick up a shape, and then added significant overbend in the area of the twist. The compression strap is a key tool here. We’ve found that we can, in one motion, relax and twist those upper sections, thus getting the frames to sweetly lie against the ribbands or planking.
I made my bending tools in a quick-and-dirty way two decades ago, assuming there would be things I’d want to change about them. In reality, I’ve found them to work fine as is. I might have taken more care in making them had I thought they’d be permanent. But their longevity speaks to the simplicity and usefulness of this system—which does rely on the sensibility of the user. It’s not a paint-by-numbers process. One must develop a feel for bending and overbending, think about the end goal, and experiment in order to set the jig up and bend appropriately. The act of relaxing the frames into place will take a little practice.
Reuben Smith owns and operates Tumblehome Boatshop, in Warrensburg, New York, where he focuses on the high‐end restoration and new construction of historic and classic wooden boats.