Sourcing timber from faraway forests has a lengthy history. More than 4,000 years ago, Lebanon exported cedar (Cedrus libani) to Egypt, nearly extirpating the tree that still adorns the Lebanese national flag. The Roman Empire transported silver fir (Abies alba) from Central Europe across the Alps to Rome, and beginning in the 13th century the Hanseatic League shipped Baltic timber to Central European ports.
Transatlantic trade in timber ramped up in the 1600s and 1700s as British forests became ravaged by years of wars. Great Britain then targeted the woods of its far-flung colonies, including white pine (Pinus strobus) in North America and mahoganies in West Africa (Khaya) and the Caribbean (Swietenia species). By the 19th century, Norway’s shipping fleet added the East Indies to the international timber trade, targeting teak (Tectona grandis) forests.
But a truly global timber trade that included multitudes of tree species began in earnest just after World War II and continues to expand. Boatbuilders are now faced with a bewildering list of imported woods to choose from.
Recently, Scott Gifford, the director of the Henry B. DuPont Preservation Shipyard at Mystic Seaport Museum in Connecticut, wrote to ask about some issues he was encountering while working on the replica ship SUSAN CONSTANT from the Jamestown Settlement historic site in Virginia. “The vessel,” he reported, “was built in 1991 and the bottom was framed with purpleheart, planked with Hymenaea courbaril, and fastened with HD [hot-dipped] galvanized square spikes. We are finding rot directly behind the fastening heads and inboard of the traditional cotton and oakum seam caulking, along the plank edges. Rot is also prevalent at the end-grain of the butt joints, inboard of the seam caulking and below the heads of the fastenings. The fastenings, for the most part, are still holding the galvanizing. The degradation around the fastenings looks a lot like iron sickness…but is very localized. These findings imply that the rot is potentially initiating from inside the vessel. After mapping out the affected areas, it is apparent that the planking is mostly compromised from the waterline down to the turn of the bilge. At this point, the planking below the turn of the bilge shows little to no signs of decay. The vessel is docked far enough up the James River that she sits in fresh water that can get quite warm in the summertime. The hold of the vessel is treated regularly with a borate-salt solution. Individually, these findings do not seem out of normal, given the age of the vessel. But I am just curious if the warm, fresh water should be a consideration in deciding what species of wood to replace the bottom planking with. I don’t have a lot of experience with vessels this size that live year-round in fresh water. I would like to use white oak, with durability, availability, and bending capabilities being the driving variables.”
Where the decay got its start is not possible to know. Fungal spores are everywhere. The reason the decay appears to originate from the inside of the vessel is because that is where the right balance of low light, sufficient moisture, and oxygen exist. The outer portion of the planking is oxygen-starved due to moisture saturation and is exposed to bright sunlight. If I had to hazard a guess, I would say the decay began at the caulking seams; the photos that Scott sent support this contention.
A Tale of Two Trees
Purpleheart (Peltogyne spp.) has a considerable history as a boatbuilding wood, in part due to its high resistance to decay fungi and marine borers. A dense, strong wood, it excels as a framing timber. It grows naturally in Central and South American rain forests with the center of its range in the Brazilian Amazon.
Courtesy of Mystic Seaport MuseumThis segment of courbaril planking displays a sound exterior surface. Pale colored end-grain, with a slight pinkish cast, is indicative of sapwood.
Courbaril (Hymenaea courbaril), also known by the common names of algarrobo, jatoba, locust, and more recently Brazilian cherry, grows on a variety of sites from tropical wet to subtropical moist—and even, occasionally, in dry forest zones—at a range of altitudes. Since at least the 1990s, it has been grown as a plantation species in Puerto Rico, Trinidad, Tobago, and more recently Brazil. Large quantities of Brazilian cherry are exported as flooring lumber. Many references say that courbaril is decay-resistant, but one source adds the caveat that the wood is resistant to the attack of white-rot fungi but less resistant to fungi that cause brown rot.
Courtesy of Mystic Seaport MuseumExtensive brown rot, initiating from the upper plank edge, can be seen in this inner surface of courbaril planking.
White-rot fungi attack the lignin in wood, exposing stringy white cellulose, while brown-rot fungi digest cellulose, leaving a brown, crumbly, lignin-rich residue. In boats, brown rot is by far the prevalent form of decay, and this is what we see in the two photos.
The variance in reported decay-resistance for courbaril may be linked to the variety of sites that this species can occupy, both in natural forests and plantations. Climate and soil differences can affect the development of wood durability. In past columns, I have cited examples, such as teak that is planted in Central America and lacks the durability properties of its Southeast Asian indigenous cousins. When soil and climate favor more rapid growth, and this is goaded by wide plantation spacing, heartwood production can be starved of the precursors needed to synthesize fungal-inhibiting chemicals.
So, while purpleheart and courbaril at first glance appear to be similarly adaptable for boat construction, a deeper dive reveals some significant differences. Knowing the source of a wood and whether it grew in a natural forest or a plantation might not mean the difference between “the best of woods” and “the worst of woods,” but it should improve one’s chances of finding an appropriate species, keeping in mind that any previously untried exotic wood carries risks.
Finally, no matter what wood species is chosen, a thorough examination of the delivered timber can avoid later problems. Courbaril heartwood is known to have a color that ranges from bright orange to reddish brown when fresh, darkening to deep reddish brown over time. The sapwood is whitish to pale gray with, sometimes, a pinkish hue. At least some of the wood in the photos appears, to me, to be sapwood—which for any species lacks decay resistance.
Testing Conundrums
About 154,500 species of fungi have been identified, globally, and of these approximately 6,300 decay wood. In North America, around 1,700 species have been associated with wood rot. Yet standardized methods for determining heartwood durability in laboratory tests normally expose wood to only two to six wood-rotting fungal species in a petri-dish culture. In field tests, wooden stakes are exposed to only the soil fungi present at a particular site. I know of no tests designed to determine species of decay fungi regularly infesting boats. The closest approximation can be found with wooden poles set in marine environments to test for marine borer activity but usually not fungal decay. Furthermore, decay tests are often performed in the country of origin not in export localities.
To this paucity of useful data, we need to add the many situations that boats encounter, from cold marine to warm fresh waters. It is well-known that marine salts inhibit the growth of many decay fungi, and these fungi display maximum growth at temperatures between 60°F (15°C) and 104°F (40°C). James River freshwater temperatures can range from 70°F to 80°F in the summer; and air temperatures often reach 90°F or above. These are ideal conditions for active fungal growth, especially near the waterline where moisture, oxygen, and temperature reach a convergence that provides ideal wood-decay conditions. Local woods with known reputations for rot-resistance in boats would be the conservative option for replanking this vessel.
Global CO2 Balance
On the subject of wood decay, it is worth noting how wood-rotting fungi affect global carbon emissions, a topic receiving close scrutiny these days. While living trees capture CO2 and store carbon in wood, once a tree dies, that stored carbon will remain in storage in protected structures, furniture, or boats for lengthy periods of time; but it may be degraded rapidly by brown or white rot fungi when conditions are ideal—like the forest floor in the wet tropics. But here the story gets a bit more complicated. Brown-rot fungi, by degrading cellulose and leaving lignin intact, often release less than 2 percent of stored carbon to the atmosphere while white-rot fungi, by degrading lignin to get to the cellulose, can release 20 percent to 25 percent of stored carbon.
We know that brown-rot fungi are the dominant source of decay in softwood conifer forests, while both brown- and white-rot fungi attack and decay hardwoods. Any shift in the management of the world’s forests that reduces conifer forests in favor of tropical hardwood forests could enhance the release of greenhouse
emissions. Recent research has revealed that some tropical hardwood forests may even now be releasing more CO2 than they are capturing and storing. As wooden boat owners, I guess we can pat ourselves on the back for mostly storing carbon, and even when rot attacks only releasing about 2 percent of that storage pool. 
Dr. Richard Jagels is an emeritus professor of forest biology at the University of Maine, Orono. Please send correspondence to Dr. Jagels by mail to the care of WoodenBoat, or via e-mail to Senior Editor Tom Jackson, tom@woodenboat.com.