If a wooden boatâ s planks and framing are sound, the next concern is the condition of the fastening system holding those planks together. But this is often difficult to determine. Nearly every owner wants to believe that his or her boat is fastened with silicone-bronze. Yet, the sad fact is that if the boat is more than 30 years old, she probably has galvanized steel fastenings. Until wooden boat building waned in the late 1960s, most boats were fastened with hot-dipped galvanized boat nails, or screwed together with galvanized steel screws. On the other hand, with the exception of military craft, most custom-designed yachts were fastened with bronze, rather than silicone-bronze, except in rare cases. The use of this marvelous alloy was much rarer in the 1950s, 1960s and 1970s than is generally thought. Silicone-bronze only became widely available in the late 1960s and early 1970s. Yet today, most new wooden boat builders learned their craft after silicone-bronze fastenings were readily available. Consequently, some donâ t know much about older iron and carbon steel fastening systems. In the resulting vacuum of expert knowledge, galvanic corrosion and â electrolysisâ are believed to be the culprits causing fastener deterioration, and so have become the subject of much myth and little understanding. Yet, galvanic action and stray current electrolysis have little, if anything, to do with fastener corrosion (see oxygen starvation sidebar). Corrosion is one of todayâ s most highly studied areas of metallurgy. On top of this is the availability of stainless steel fastenings. When you pull a fastening and ask a yard worker to identify the metal, he may quite easily mistake silicone-bronze for stainless steel (see the sidebar on materials for how to tell them apart). Rumors Abound One pervasive waterfront rumor is the myth that if the boat was originally fastened with galvanized steel or iron, then it is best to refasten using stainless steel. The myth says this will lessen the deterioration of the existing fastenings because the original fastenings and the new fastenings are both steel. Even if the fastener corrosion were actually due to galvanic corrosion of dissimilar metals, this still would not be true, because stainless steel is nobler than the ferrous steel. And the stainless would be cathodic in relation to the anodic ferrous. But, galvanic corrosion is not the problem with fasteners. In addition, stainless steel fastenings, which are installed below the waterline in salt water, shortly will begin to corrode from oxygen starvation. Stainless encased in wet wood in the presence of chloride (salt) makes the perfect condition for the onset of chloride crevice corrosion in the stainless. As a result, the Coast Guard will not allow stainless steel fastenings below the waterline in inspected passenger vessels (see sidebar on what the Coast Guard discovered). Inspections Not too long ago, the Coast Guard issued new regulations requiring fastener inspections at regular intervals for all inspected passenger vessels. The pleasureboat insurance industry then followed suit and began to require fastening inspections for all wooden pleasure vessels at regular intervals. HEAD: Pulling and Inspecting a Fastening How does this apply to 30-foot pleasureboats? Most underwriters specify that either six fasteners be removed and examined (three from each side of the vessel) or that three be removed from the entire vessel. Actually, this makes proportional sense if you consider that the Coast Guard requires eight per side on 100-foot passenger vessels. Frankly, with three fasteners pulled from different underwater areas, we really do have a significant â randomâ sample of the fasteners for a 30-foot boat; at 45 feet and above, six (three from each side) will provide the same representative sampling. From this sample we can make a fairly accurate determination of fastening conditions for the entire boat. Pull fastenings from the garboard strake immediately below the engine. Take another from the transom butts below the waterline, and a third from the stem planking butts (which usually is wetted by the bow wave). If the insurance company has specified a total of six, then repeat the process for both sides of the vessel. Also, look around for any started butts or planking and try to pull any fastenings that are in that weakened area. Next, examine the fastening for wastage near the center of the shank, where the joint between the plank and the frame occurs. This is where the water gets in and the oxygen starvation results. Look closely for â necking downâ of the shank in this region, and also for bright metal indicating acid attack. Note what you find, and then proceed to â clean upâ the fasteners using a wire brush. This knocks away all the loose material so the actual depth of any deterioration can be seen. Making the decision to refasten: First, examine the fastenersâ condition by pulling a representative sample. If any are more than 10 percent deteriorated, it is time to refasten. Second, look at the material and the age of the fastenings in light of the Coast Guardâ s published data from Baltimore (see Coast Guard sidebar).
Capt. Alan Hugenot, AMS