By Gail Cleere, Office of Naval Research
For as long as we’ve been building boats and putting them in the water, we’ve been battling those pesky little ocean critters that want to attach themselves to our boats for a free ride. The ubiquitous, determined barnacle — not to mention tubeworms, oysters, algae, and an array of other invertebrates — has long been the bane of many a fleet and flotilla. Pitch, copper sheaths, oils and gums, pesticides, silicone, arsenic… over the centuries all have been tried, and none have completely solved the problem.
And no wonder… the barnacle, for instance, is very good at what it does. This critter secretes a rapid underwater-curing cement that is among the most powerful natural glues known — with a tensile strength of 5,000 lbs per square inch and an adhesive strength that has been measured at 22 – 60 lbs per square inch. And that’s just barnacles. Blue mussels know how to make 21 different kinds of adhesives.
Attaching themselves to ship hulls, billions of crusty foulers cost the U.S. Navy over $50 million a year just in fuel costs due to drag. It’s estimated that a newly painted destroyer would lose 2 knots of speed every six months if not scraped and cleaned — and this doubles in tropical waters.
“It’s an age-old battle,” says Dr. Steve McElvany, ONR’s Program Manager for Environmental Quality, who also studies the mechanics of adhesion. “ONR is looking hard at the development of non-toxic, foul-release polymeric marine coatings.”
The problem is the toxicity of so many of the coatings that are used worldwide. “The old copper-based coatings are now known to be lethal to some marine organisms,” says Dr. Linda Chrisey, who as manager of Environmental and Marine Biotechnology at ONR, tries to understand which organisms settle on what surfaces, and why. “An environmentally concerned U.S. Navy never implemented the widespread use of the much more toxic tin-based paints on its ships, and has been using copper-based paints since the mid-1980’s, but that’s not true throughout the world.”
Meanwhile, Dr. Paul Armistead, manager of ONR’s Polymer Chemistry program is looking for ideas to outwit the offensive critters. One is a flexible coating — organisms might try to settle on a ship but wouldn’t be able to grip tightly, allowing the organisms to adhere when the ship is in port, but sloughing them off once the ship reached cruising speed. Yet another idea might be found in textured hull coatings, where the very nature of the shape of the coating (i.e., pattern dimensions and surface energies), might be repugnant to the fouling organisms.
“Nothing is quite as easy as it sounds,” says Armistead. “The silicone based paints are fragile and scrape easily, although we are seeking to improve this with research on nano-composite additives. The textured coatings look promising for repelling barnacles, but not other types of foulers. And some of the other coatings we’re looking at seem to repel the fouling invertebrates, but confound us by attracting seaweed.”
Nevertheless, the Navy knows how to stick to things, too. In a recent Broad Agency Announcement (BAA), ONR solicited proposals in basic and applied research in the age-old battle of the barnacle, and is evaluating those proposals now. We’ll keep you posted.