Although wood and water don’t always mix well, their union is sometimes unavoidable, particularly in marine applications. Like structures on land, boats, docks, and piles, are subject to deterioration from water—yet they also have an additional enemy that awaits them in the briny deep. Marine borers.
Damage by marine-boring organisms to wood structures in salt or brackish water is practically a worldwide problem. The rapidity of attack depends on local conditions and the types of borers present. Along the Pacific, Gulf and South Atlantic Coasts of the United States, attack is rapid, and untreated wood may be completely destroyed in a year or less. Along the coast of New England, the rate of attack is slower because of cold water temperatures, but is still sufficiently rapid to require the protection of wood where long life is desired.
Shipworms are the most destructive of the marine borers. They are mollusks of various species that superficially are worm-like in form. The group includes several species of Teredo and several species of Bankia, which are especially damaging. These mollusks are readily distinguishable on close observation but are all very similar in most respects.
Upon finding suitable wood, shipworms quickly bury themselves in the material. A pair of boring shells on the heard grows rapidly in size as the boring progresses, while the tail part (siphon) remains at the original entrance. Thus, the animal grows in length and diameter within the wood but remains a prisoner in its burrow, which it lines with a shell-like deposit. The entrance holes never grow large, but the interior of the wood may be completely honeycombed and ruined.
Wood barges have been constructed with planking or sheathing pressure-treated with creosote to protect the hull from marine borers, and the results have been favorable. Although coal-tar creosote is an effective preservative for protecting wood against marine borers in areas of moderate borer hazard, it has disadvantages.
Creosote adds considerably to the weight of the boat hull and its odor is objectionable to boat crews. In addition, the anti-fouling paints that are used in marine applications are difficult to apply over creosoted wood.
Anti-fouling paints that contain copper protect boat hulls against marine-borer attack, but the production continues only while the coating remains unbroken. Because it is a difficult to maintain an unbroken coating of anti-fouling paint, the U.S. Navy has found it desirable to impregnate the hull planking of some wood boats with certain copper-containing preservatives.
Such preservatives, when applied with high retentions, have some effectiveness against marine borers and should help to protect the hull of a boat during intervals between renewals of the anti-fouling coating. These copper preservatives do not provide protection equivalent to that furnished by coal-tar creosote; their effectiveness in protecting boats is therefore best assured if the boats are dry docked at regular and frequent intervals and the anti-fouling coating maintained. The leach-resistant wood preservatives containing copper arsenates have show superior performance to creosote in tests conducted in areas of severe borer hazard.
Plywood as well as plank hulls can be protected by preservative treatment. The plywood hull presents a surface that can be covered successfully with a protective membrane of reinforced plastic laminate. Such coverings should not be attempted on wood that has been treated with a preservative carried in oil, because the bond will be unsatisfactory.
Part of the job of researchers at the Forest Product Laboratory’s (FPL) Durability and Wood Protection Unit, is to investigate the best ways to mitigate the effects of destructive pests like ship worms, so that wood products maintain long service lives. For more information, please see Chapter 14 of the FPL’s Wood Handbook: Wood as an Engineering Material.