Replacing Keel Bolts

By Nick Bailey

Other than running aground, will you ever need to replace a keel bolt? If you keep your boat in fresh water, you aren't likely to have any problems for a very long time. Saltwater boats are another matter.
Consider these examples from Bristol Marine's service files: a 12-year-old racer-cruiser from a top-rated manufacturer was hauled out for some minor keel fairing. The owner also requested, as part of routine maintenance, that the keel bolts be tightened. Fifteen minutes into the job the technician returned and unceremoniously dropped a fractured 1-inch diameter stainless steel stud on my desk. Under very gentle torque, the bolt had snapped off deep inside the hull. Quick inspection confirmed that only 10 percent of the fracture face was freshly broken metal. The other 90 percent was badly corroded and had been for some time. A phone call to the owner confirmed that the boat had spent most of its life in saltwater.

On another occasion, a 15-year-old trailer boat was in for to have the fractured vermiculite and resin filler in the keel sump replaced. Upon removing the old filler to expose the bolts, we discovered some of the supposedly stainless bolts, buried in the cracked and soggy filler, were partly cut through due to corrosion. The exposed portion still looked nice and shiny. The boat had recently arrived from a saltwater location.
DAVID AIKENIntegral studs cast into keel.What happened here? In both examples, a process known as crevice corrosion destroyed the stainless-steel keel bolts.
WHAT LURKS UNSEEN
What makes crevice corrosion particularly nasty is that it only occurs in hidden and inaccessible locations, typically where keel bolts are buried in the hull. Small amounts of saltwater seep into contact with the bolts, stagnate and become de-oxygenated. Trouble begins when metal bolts are not in contact with oxygen. Stainless steel stays shiny and “stainless” under normal circumstances because it contains chromium, which forms a tough protective oxide when exposed to air and well-oxygenated water. Remove oxygen and the protective oxide coating breaks down, exposing metal to the water.
Saltwater provides the electrolyte required to set up a miniature galvanic cell, either on the surface, causing pitting corrosion, or inside a microscopic crack or crevice, causing the dreaded crevice corrosion. As long as stainless steel remains wet or even damp, corrosion proceeds unchecked, eating away at the metal. Crevice corrosion is surprisingly common in older salt-water boats and tends to be worse in tropical waters, due to lower oxygen content, higher salinity and the greater reactivity that warm temperatures provide.
DAVID AIKENEasy access, windows cast or cut into keel and filled with fairng putty have nuts and washers at bottom end.IDENTIFYING PROBLEMS
Most keel fastening systems are designed with a conservative safety factor and you can get away with one broken bolt. Real trouble begins when the next bolt goes. Various famous ocean racers, no doubt designed a little closer to the safety limit than your average family cruiser, have publicly demonstrated sailing without a keel. I don't recommend it.
There is no way to positively identify bolt condition without either removing the keel or the individual bolts. A broken or badly weakened bolt always becomes obvious when retorquing. It pops loose or turns indefinitely without tightening. Check with the manufacturer or your local yard to confirm the recommended maximum torque, but keep in mind that any 7/8 inches or larger diameter bolt requires tremendous leverage to the point where it's exceedingly difficult to over-torque.
DAVID AIKENHoles drilled and tapped into keep for threaded bolts have nuts and oversize washers or backing plates (preferred) on the top end.Another place to watch is the keel-hull joint. Anything much larger than a hairline crack here can be a sign of trouble, or at least may indicate it is time to tighten the bolts. This is best checked while the boat hangs in the slings. Once the boat is cradled and hull weight rests on the keel, any cracks tend to close up again. These signs may identify a broken bolt, but the only method to directly confirm if corrosion has begun an attack on the bolts is to expose them.
REPAIR STRATEGIES
Bolt configuration and ballast type largely dictate how to fix bolts that are severed or deeply pitted by corrosion. Keels with integral studs (Figure 1) are the worst case and the most common keel fastening method. The only practical option is to drop the keel (see sidebar) and, if there is enough room in the keel sump, install a supplementary, or sister, bolt.
This is done in one of two ways. Using an oxyacetylene torch, cut a window in the keel to access a nut (similar to Figure 2), then drill a hole down from above for a threaded rod or bolt. This method works best on lead keels. Alternatively, you can drill and tap the keel for a new sister fastener. This works best on cast iron. It may be possible to salvage the stud by threading on a heavy sleeve to bridge the damaged area and reinforce the bolt. This often requires excavating around the stud at the top of the keel to expose enough good threads to give any hope of success. It may also present difficulties refitting the keel due to the larger hole required in the hull. Most professional yards would do this sort of repair only on a “no guarantees” basis, as the sleeve will not be as strong as the original bolt.
DAVID AIKENExternal flange recessed or faired into hull has flat head bolts with nuts and oversize washers or backing plates on top.If you are faced with drilling or tapping large diameter holes, you will need to rent the appropriate power tools. However, the keel may need to be cradled and transported to a machine shop capable of handling large scale boring and cutting. Neither lead nor cast iron are easy materials to work with: lead jams and binds tools, cast iron is very hard and brittle.
Keel bolt configurations as shown in Figure 2, 3 and 4 can theoretically be removed, inspected and replaced. Because this is done one at a time, it may be practical to do this without dropping the keel. Haul out and cradling are still recommended to minimize the chance of disturbing the keel-hull bond. Aggressively caulk new bolts with a polyurethane or polysulphide sealant upon reinstallation. Boats with keel windows (Figure 2) or external flanges (Figure 4) have the bolt end hidden by fairing filler and may require lots of grinding and chiseling to expose the end. Broken bolts are more difficult to extract but can be knocked out of the external flange with a driving pin or, in the case of the window keel, driven down then cut off one piece at a time with a hacksaw (are we having fun yet?) An extractor tool designed for broken studs, called an Easyout, may also work with smaller bolts.
DAVID AIKENFull keel with external ballast converts to internal by adding a fiberglass laminate overlay.A broken tapped bolt (Figure 3) is very difficult to remove, particularly in cast-iron keels. The bolt is likely seized into place by corrosion. You may have to drill it out and retap the hole. I envision some difficulty getting the drill press down the companionway, so this one may also require keel removal and machine shop work as in keels with integral studs (Figure 1). For fiberglass boats with low-profile, full-keel configurations (Figure 5), it doesn't matter how the keel is fastened, because the repair approach is completely different. It may be more practical to laminate several layers of fiberglass to encase the keel and bond it to what becomes, in essence, a new part of the hull. This converts the boat from external ballast to internal. Many traditional full-keel glass boats are built this way and neatly sidestep the whole keel bolt corrosion issue.
Laminate thickness of the new keel should match the original hull thickness and overlap at least a foot or two onto the existing hull. As is normal practice, make sure the hull surface receiving the overlapping laminates is ground to bare glass and is clean and dry. If you are unfamiliar with glass lay up procedures, hire a professional.

A version of this story appeared in DIY Boat Owner magazine.