IDENTIFICATION OF MARINE GROWTHS. 5 



have caused severe fouling in almost fresh water {e.g., the river 

 Plate). 



(4) The Anti-Fouling Problem. 



The present-day method of combating fouling on the under- 

 water plating of ships is the use of anti-fouHng compositions applied 

 just prior to undocking. These compositions may contain as 

 poisons copper and mercury compounds and organic substances. 

 In water, these poisons leach out of the paint medium, so that a 

 more or less continuous supply of poison is available at the surface of 

 the paint film. The value of an anti-fouhng composition depends 

 on the rate of leaching of the toxic ingredients, which in turn is 

 a function of the type of medium employed to bind the paint and 

 of the percentage of toxic ingredients present. All anti-fouling 

 compositions thus have a limited life, which for most commercial 

 products is of the order of 4-12 months. Thus, periodical renewals 

 of anti-fouUng compositions are determined by this hmited life. 



The actual anti-fouling mechanism is being actively investigated 

 at the present time. Available evidence indicates that the toxicity 

 of a surface has httle effect on the growth of an organism once 

 attachment has taken place, so that the main line of attack lies in 

 the prevention of the initial attachment. The constituents of the 

 anti-fouhng composition may thus include substances which exert a 

 repellent or lethal action on the larvae and spores, or alternatively 

 substances which prevent the adhesion or setting of the cementing 

 substances produced by many of the organisms for effecting the 

 attachment, it is also possible to eliminate attachment by exfoUa- 

 tion, i.e., the continual shedding of the surface layer of a suitably 

 compounded paint. 



A brief summary of the Sub-Committee's investigations which 

 are now in progress on this matter will be appropriate. Tests are 

 being made on formulated anti-fouHng compositions containing 

 inorganic and organic poisons. It has been found that mercury is 

 approximately twice as effective as copper against weed fouling 

 (weight for weight) and three times as effective against barnacle 

 fouling. Inorganically combined arsenic is almost ineffective. 

 Although many organic poisons have been found which are very 

 much more potent than copper and mercury when in solution in 

 sea water, unfortunately in most cases this toxicity is difficult to 

 bring into effect when the organic poison is incorporated in a paint 

 film. There are, however, certain promising developments in this 

 connection. A new technique has been developed for studying 

 the leaching rate of poisons from paint films immersed in sea 

 water. 



An important aspect which is receiving consideration is the paint 

 medium in which the poisons are incorporated, due regard being paid 

 to the conflicting requirements that the poisons shall be readily and 

 continuously available whilst the coating shall be as durable as 



