Effects of Bilge Stringers
The concentrated stresses in bent frames from contact with stringers are responsible for a great many broken frames and deformed boats. In my opinion, bilge stringers a vestigial holdover from the days of sailing ships and large schooners can be eliminated in moderate-sized yachts, at least in many cases. Stringers are often too stiff and bear on a very small area of the frame. When the rest of the boat flexes, the part of the frame under the stringer is held rigid and is likely to break. Frames are further weakened at this critical intersection by the large through-bolts that are frequently used to fasten stringers in place.

Where bilge stringers are used, they should never be more than about 15% thicker than the planking and under no circumstances thicker than the molded dimension of the frame. These rules apply to seat risers in small boats, as well. In addition, stringers should never be narrower than three times their thickness. They should be carefully rounded on the contact face so that they fit well against each frame over their full width. If bilge stringers are through-fastened, those fastenings should follow the guidelines for plank fastenings set down earlier. It is better to screw-fasten bilge stringers than to use large through-bolts (thus avoiding another fastening through the “tension” part of the frame), although bolts may be necessary in large boats.

Split Frame Heels
Improper size and placement of floor-to-frame bolts can cause split and broken frame heels. As a general rule, the diameter of floor-to-frame fastenings should not exceed one-fifth the molded dimension of the frame, and whenever possible these fastenings should be kept on center or a little toward the compression side (the side toward the concave face) of the frame. Floorboard support beams should be notched over the frames and bear against them, rather than depend entirely upon their fastenings for support. Engine beds cause similar problems and sheer clamps often break frames up high and aft where only a corner of the clamp contacts the frame.

Shear Failures
Clean, square breaks across a frame in line with a plank seam generally indicate shear failure. Often caused by external damage, such breaks are frequently seen just above the tops of the floor timbers in boats that have run aground fast and hard. In this situation, the keel and lower part of the boat come to an abrupt stop while the part of the boat above the floor timbers tries to keep going; when the planks slide by each other a bit, the frames snap off. Shear failure is also caused by the flexing of the hull under twisting loads. In this case, the failures are usually found within a diagonal swath of hull between the chain plates and the center of the ballast, where the torsional loads are the greatest. A loosely caulked single-planked hull is most prone to this problem; it rarely occurs in double planked or diagonally strapped hulls.

Strategies of Frame Repair
The first step is to find out in detail just what’s wrong and to try to figure out how it got that way, so the repair can be made without a reoccurrence. Whether the boat is damaged by collision, weakened by previous repairs, or just plain old and tired, a complete and detailed survey of the whole boat not just the frames is in order. Since the ultimate decision about how to fix the frames depends as much on the overall condition of the rest of the boat as it does on the frames themselves, be thorough. Professional help is probably more valuable to you now than during any other stage of the job, and unless you are intimately familiar with wooden boat construction, ask an experienced boat carpenter or a good