Repairs to steam-bent frames are often within the capabilities of boat owners who have a basic knowledge of carpentry and who have access to a few portable power tools and some basic stationary machinery. However, most amateur frame-repair jobs are not as effective as they might be, more often because owners don’t know what to do than because they lack the woodworking skill. Before examining what to do to solve framing problems, however, let’s look at what causes those problems in the first place.

Causes of Failure
The greatest cause of the failure of bent frames, in my opinion, is inadequate structural design, usually in at least one of four areas: (1) There is insufficient cross-sectional area to carry the transverse loads in the hull; (2) the fastenings are either too large for the sided dimension of the frame or too close to the edge of the frame; (3) the frames weaken due to being bent to too severe a curve; (4) internal longitudinal members, such as bilge stringers and sheer clamps, are improperly designed.

Tensile Cracks and Breaks
Tensile cracks and breaks are the most common frame problem. The cracks usually start at the convex face and extend inward to about the middle of the frame, although sometimes the frame is completely severed. When the cracks are small and numerous and occur in more than just one frame, the problem is caused either by excessive severity of bend or by fastenings being placed too close to the edge of the frame. The same problem often occurs in the reverse curve near frame heels, this time on the face of the frame toward the inside of the boat. In this situation, the frame may split rather than crack. Excessive severity, of bend is usually the culprit here, although improper placement of floor-to-frame fastenings can encourage failure of frame heels as well. Severity of bend is the ratio of the radius of curvature of the bend at any given point to the molded dimension of the frame at the same point (the radius is measured to the inside of the curve, or concave face, of the frame).

When the severity of bend is between 20 and 14, the frame is slightly weakened in its ability to carry tension; but unless subjected to an excessive or concentrated stress, most of these gently bent frames perform satisfactorily. When the severity is 14 to 12, the chance of frame breakage increases considerably, all other things being equal, and when the severity is 12 to 10, you can be assured of considerable premature breakage. When the severity is less than 10, the frame is nearly broken by the act of bending it, and the added stress from service aboard a boat will surely cause it to fail. To put it another way, just because the frame didn’t break upon steam-bending is no indication that it will not fail later on. If the severity of curvature is less than 14 for a solid frame, you can ease the severity by slitting or kerfing the frame to form two layers before bending.

Cracks due to severe bending almost always occur at fastenings, and if the fastenings are not selected or put in properly, the chance of frame breakage is much higher. The center of any fastening hole in a frame bent more severely than 20 should be no closer to the edge of the frame than two-and-a- half times the shank diameter of the fastening. This means that if the fastenings are not staggered, the sided dimensions of the frame must be at least five times the fastening diameter, and, if staggered, five times plus the stagger. For bends with severity less than 14, the distance from hole to frame edge should be approximately three times the fastening diameter.

When multiple cracking occurs, the frame often lasts for many years without further damage or serious loss of shape, especially when the hull is double planked. In single-planked hulls, these cracks usually develop slowly into more serious fractures, accompanied by hard spots in the frame’s curvature.

Complete breaks in frames may be due to tensile loads beyond the capability of the frames or to normal loads on a frame already cracked and weakened. These breaks tend to cluster in adjacent frames. Here’s a possible sequence of events: One frame breaks, and the loads it once carried fall on its neighboring frames, perhaps causing them to fail as well. The hull develops a hard spot at the break and starts to leak, since the frames are no longer able to resist the compression on the caulking seam. In an attempt to correct the problem, an owner may recaulk the area, putting even more stress on the neighboring frames, and refasten the planks, further weakening the frames until they break. In this fashion, the problem continues to spread along the seam and the boat progressively loses its shape.

Tensile breaks are a particular characteristic of single-planked mahogany hulls, because mahogany has a high compressive strength and thus swells powerfully when it takes on moisture after the boat is launched. This problem is often accentuated by the low moisture content of the mahogany planking stock. It’s common that mahogany used for planking is a lot drier than other planking materials, and thus it swells more when the boat is launched. If a mahogany hull is even slightly overcaulked, or if the frames are not of sufficient cross section, you’re in for problems that generally don’t occur in hulls planked with softer, more forgiving wood. The spacing and scantlings for frames for mahogany should not be less than those given in Nevins’s scantling rule and particular care must be taken to avoid severe bends and improper fastenings