effective engineer. 



Shipworm and gribble damage 

 have plagued maritime enterprises 

 throughout recorded history, particu- 

 larly in warmer waters. Warships, 

 merchant craft and small coastal 

 trading vessels had short lives before 

 the 18th century. Two years without 

 care and the bottom timbers exposed to 

 the sea would be mined with tunnels. 



To take the bite out of these 

 borers, shipbuilders from the Colum- 

 bus era through the 1850s used wood 

 sheathing. Planks of pine or fir were 

 attached to the hull, with hair and pine 

 tar sandwiched between. 



In 1758, the first copper sheath- 

 ing was mounted to warships, using 

 tar paper over the wood sheathing. 

 Though costly, copper was so effective 

 in preventing teredo boring that by the 



1790s, most British naval ships were 

 sheathed with the metal. However, 

 only a few well-built merchant ships 

 could afford this kind of protection. 



Pitch and tar, the standard preven- 

 tive treatment for Colonial craft, pro- 

 tected the hull as long as no bare wood 

 showed. But the coatings had to be 

 reapplied often. 



To protect the keel timber, 

 mariners attached a worm shoe. This 

 sacrificial timber, which is still used 

 today, attracted shipworms away from 

 the keel. 



Portuguese and Japanese ship 

 owners, although worlds apart, 

 careened their vessels at the highest 

 tide and surrounded them with straw. 

 Then they set the straw afire. Their 

 hope was to boil or steam the teredos 

 to death in their tunnels without 

 destroying the boat. 



Baltimore shipwrights, using an 

 old Chinese treatment, brushed a 



bottom paint made of cayenne pepper 

 and shellac on their hulls. Geoffrey 

 Scofield, former curator of historical 

 maritime technology at the N.C. 

 Maritime Museum, said that pepper 

 was added to cheap paint and used as 

 a preventive in Caribbean islands. 

 These traditional methods are still 

 applied today. 



Georgia boatbuilders waited for 

 the cold. When night temperatures 

 were expected to drop below 26 F, 

 they hauled vessels from the water. A 

 freeze was considered successful if, in 

 the warming of morning, slime oozed 

 from hundreds of pinpoint holes in the 



hull. Then, each hole was laboriously 

 scraped and sealed. 



Of course, one of the oldest and 

 most energy-efficient ways to protect 

 a vessel was, and is, to sail up a 

 freshwater creek and dock there for 

 three or four weeks. Marine borers 

 have a low tolerance for fresh water. 

 However, they can seal themselves up 

 for weeks in poor environments. 

 Although most borers prefer a normal 

 seawater salinity of about 35 parts per 

 thousand, some species can survive at 

 least a month in brackish water of four 

 parts per thousand. For effective 

 treatment, mariners should locate 

 flowing rivers of fresh water, not tidal 

 currents in which the water frequently 

 contains salt. 



Today, the most effective and 

 controversial treatment for avoiding 

 shipworm damage is the use of anti- 

 fouling paints. Copper-based paints 

 release metal ions into the adjacent 

 water, preventing organisms from 

 surviving on the painted surface. But 

 copper, even in paint, is very expensive 

 and has a short life span. 



Shipyards were experimenting 

 with lead compounds by the 1700s. 

 Longer-lasting heavy metals, such as 

 arsenic, mercury and lead, were in- 

 corporated into paints. But these metals 

 are hazardous to workers preparing and 

 applying them and are now banned. 



Recently, extremely toxic 

 tributyltin (TBT) was incorporated into 

 anti-fouling paints. Tests showed that 

 TBT kept a hull free of surface 

 organisms and prevented shipworm 

 damage up to four years, but leaching 

 of TBT into estuarine water, even in 

 extremely low amounts (such as a few 

 parts per trillion), can harm crabs and 

 oysters. The use of these paints has 

 now been restricted. 



The biology of shipworms can 

 explain why wooden ships have 

 problems and how some cures work. 

 First, shipworms are bivalves, or two- 

 shelled mollusks, related to clams and 

 oysters. 



The animal consists of three basic 

 parts: valves, a wormlike body and 



18 MAY/JUNE 1993 



