The ribs of a wooden boat are aptly named since they provide the strength and resiliency to maintain the shape of the hull, exactly like the ribs of your chest. Broken, cracked or split ribs weaken the framework that supports the planking, resulting in leaks and even hull failure. You may strengthen the ribs in your boat by fastening a new piece alongside the broken rib, a technique called "sistering" or "twinning". Serious boat builders agree that the broken ribs should be removed and replaced with new ribs and new through-hull fastenings.
Look for damage to the ribs using a bright light. Notice areas that are discoloured, water-damaged or rotted. Poke the wood with an ice pick to locate soft places. Mark ribs that must be repaired or replaced.
Examine the hull shape carefully, looking for warping or twisting. Calculate the cost of the two methods of repair: sistering or replacing the damaged ribs. Include the personal cost in time and physical effort, and ponder whether the boat is really worth the trouble and expense.
Remove any decking, seats or other obstructions so that you have a clear area for work. Make cardboard patterns for each of the ribs you intend to repair or replace. Clean the work area thoroughly and allow time for the wood to dry completely.
Rip lengths of straight-grained white oak to the same dimensions as the ribs, and long enough to extend well past the damaged areas. Soften the new parts in a steamer (a piece of PVC pipe and a teakettle work well). Scrape old varnish, paint and dirt from the damaged ribs, and clean the work area carefully.
Fill voids in the damaged ribs with epoxy paste, being careful not to join the ribs to the planking. Allow that repair to harden completely.
Measure up from the keel and determine the locations for new fasteners. Mark the newly-steamed sister rib, and drill holes to accept stainless steel through-bolts. While a helper flexes the new rib in place alongside the damaged rib, drill laterally through the pilot holes and through the old rib. Start at the keel and work your way up, flexing and securing with bolts and washers as you go.
Finish with paint or varnish to match the rest of the interior and to protect the new wood from damage.
Locate the damaged ribs and mark their locations on the outside of the hull. Scrape or sand the finish to find the hull fasteners for each rib. Drill out and remove the fasteners for one rib only, and remove that rib. Carefully scrape and clean the inside planking, fill the screw holes with epoxy paste and allow that to cure.
Cut, shape, steam and bend a replacement rib using measurements from the old rib. Starting at the keel force the new piece in position while a helper drills and inserts a new stainless-steel screw from the outside of the planking. Bend further and add more fasteners until the new rib is securely installed. Repeat the procedure for each damaged rib.
Fill the screw holes flush with the hull surface, sand the entire hull, then prime and finish.
When replacing ribs, remove and replace them one at a time to maintain the shape of the hull.
Wear eye protection when sanding, sawing, drilling or handling epoxy. Wear thick gloves when handling steamed ribs.