FOITLING OF SHIPS' BOTTOMS 215 



one-haK of 1 per cent per day for periods as long as three months." This would mean 

 an increase in resistance of almost 50 per cent by the end of this period, while "exam- 

 ination of the bottoms of the vessels in dock revealed no apparent fouling." That 

 such practical tests are fully in accord with theory, as based upon experimental data, 

 is shown by the additional studies of McEntee (1915) on the use of graphite, soaps, 

 and oils as a coating for the wetted surfaces of a model ship. He found that all of 

 these produced greater resistance than a smooth, shellacked sm-face. 



For an analysis of the resistance of ships, the work of Hovgaard (1908) is one of 

 the more recent, while a very excellent bibliography on this subject is given by Rigg 



(1915). 



FACTORS THAT DETERMINE FOULING 



The factors that determine the presence and the amount of fouling on a given 

 vessel are very numerous and variable. The major factors, however, may be classi- 

 fied with some degree of accuracy. The season of the year, the weather, and the 

 temperature of the water constitute one group of factors. The condition of the water 

 in various harbors, both as to salt content and pollution, also affects fouling. The 

 contour of the ship, which is correlated with the duty and speed of the vessel, and 

 also the waters cruised, all affect the amoimt of fouling. The length of time between 

 successive dry dockings and the proportion of this time spent in cruising or in port 

 are very important factors. The nature of the material of which the ship's bottom 

 is made, as well as the paints or other materials that protect it, also are of importance. 

 Inasmuch as life is more abundant and rapid in its growth in tropical regions, it 

 follows that boats that travel in tropical waters become more heavily fouled and in a 

 shorter time than do similar vessels in more temperate latitudes. Likewise, ships in 

 port during the spring and summer show heavier growths than those that are idle in 

 port during the autumn and winter. 



It will be impossible to consider aU of the factors that condition fouling in aU its 

 variations, but the following pages will be devoted to a discussion of some of the major 

 ones, with special reference to the effectiveness of paints, both as regards their 

 poisonous properties and their protective properties from a biological consideration 

 of the reactions to them of the larvee of the various forms that cause fouling. 



We shall discuss the relation of fouling to (1) duty, racluding the factor of 

 "dry-docking period"; (2) seasons; (3) fresh waters; (4) paints and surface film; 

 and (5) fight and color. 



RELATION OF DUTY OF SHIP TO FOULING 



The "duty" of a ship determines, in large measure, the amount of fouling that 

 will accumulate on its bottom. This is due to several factors, which include the 

 effect of hull contour, of relatively much or little time spent in port, of the ship's 

 speed while cruisiag, and, finally, the effect of the waters cruised. 



By examining Table 1 it wfil be noted that there is a marked difference in the 

 amount of fouling on ships belonging to different classes; i. e., having different 

 duties. Thus, it was found that passenger ships with regular schedules were by far 

 the least foul of any group. This applies not only to vessels plying between America 

 and Em-ope, but to those carrying trade from New York to South American ports 

 as weU, and can be stated as a general rule. 



