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Hold It Together
by Capt. Alan Hugenot, AMS
What you need to know about boat fasteners: corrosion, regulations
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If a wooden boat’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 don’t know much about older iron and carbon steel fastening systems. In the resulting vacuum of expert knowledge, galvanic corrosion and “electrolysis” 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 today’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 “random” 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 “necking down” of the shank in this region, and also for bright metal indicating acid attack. Note what you find, and then proceed to “clean up” 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 fasteners’ 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 Guard’s published data from Baltimore (see Coast Guard sidebar).
HEAD: What the Coast Guard Knows About Fastenings
Beginning in 1993, the Coast Guard recorded the fastener history of all inspected wooden passenger vessels operating in the Baltimore area that were more than 15 years old (the average age was 29 years). The results are actually “fasten-ating” (excuse the pun), especially when you start thinking about the age of your boat and what she is fastened with.
*Galvanized fastenings: All vessels with galvanized fasteners required complete refastening. The average age of these vessels was 40 years.
*Mixed fastenings: Fifty percent of vessels with a mix of fasteners below the waterline required complete refastening. The average age of these vessels was 31 years.
*Bronze fastenings: Thirty-six percent of the vessels with bronze fasteners required complete refastening. The average age of these vessels was 31 years. However, most of these would have been fastened with naval bronze.
*Monel fastenings: None of the vessels with Monel fasteners required refastening. The average age was 25 years. The majority of these probably were not Monel, but were most likely silicone-bronze.
*Stainless steel fastenings: The results were inconclusive, due to the small number of boats in the sample, and the fact that these boats operated in brackish as opposed to salt water.
Resulting Laws
With that information, regulation number NVIC 7-95 was implemented, Guidance on Inspection Repair and Maintenance of Wooden Hulls. That regulation now requires that Inspected Small Passenger Vessels undergo random sampling of fasteners at varying intervals during the vessel’s life, beginning specifically in a vessel’s 10th year after construction, and occurring every five years thereafter (for vessels in saltwater service). In freshwater service, the regulations require sampling to begin in the 20th year (and occur every 10 years thereafter).
According to the Coast Guard, on small passenger vessels (65 to 150 feet), fasteners must be removed from the garboard seams, stem joints, plank ends in areas of bent planks, shaft logs and under engine beds where vibration is maximized. For passenger vessels, the Coast Guard inspector is required to pull a minimum of eight fasteners per side below the waterline.
All internal fasteners accessible to the inspector will also be examined. And, in the case of ferrous fasteners or stainless steel fasteners, an even more thorough examination is required.
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This article first appeared in the March 1, 2006 issue of Sea Magazine. All or parts of the information contained in this article might be outdated. |
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