FOULING OF SHIPS' BOTTOMS 217 



of fouling on each. It will be seen at a glance that passenger ships average a very 

 light amount of fouliag, while lightships and battleships show a very heavy growth. 

 The percentages given for each group do not show an exactly comparable relation- 

 ship, because data gathered from all the sources are iacluded. If one were able to 

 exclude all data from the Philadelphia Navy Yard, with its polluted, fresh-water 

 harbor, and also omit those ships that enter dry dock after an unusually short interval 

 (because of accident), the relative percentages in each group would show a steady 

 and proportionate increase in amount of fouling. 



However, in any chart of this kind more than one factor is represented. The 

 fact that the average docking interval for passenger ships is about 7 months, for 

 freighters about 8 months, for naval craft about 9 months, and for lightships about 

 11 months, must be taken into consideration. This factor, however, will be discussed 

 separately below. Kegardless of many of these compKcating factors, the uniform 

 difference in the amoimt of fouling is of real significance and, as will be shown, is 

 probably related more to the effect of the relative amount of time spent in port than 

 to any other one factor. 



Having seen that there is a significant difference in the amount of fouling on 

 ships belonging to the various groups, an analysis of some of the factors that deter- 

 mine this difference will be considered. Since the materials for construction are 

 comparable, the paints usually the same, and the environmental factors, such as 

 seasons, ports, and temperature, are similar in the main, the really significant dif- 

 ferences are clearly related to the different duties of these vessels, and this relation 

 to fouling can be analyzed by consideration of four main factors: (1) Hull design, 

 (2) speed of ship while cruising, (3) dry-docking period and use of intervening time, 

 and (4) the routes or waters cruised. 



HULL AND CONTOUR OP SHIP 



The construction of any ship plays a considerable part in the matter of fouliag. 

 The amount of fouling rarely is uniformly dense over the various portions of the hull. 

 This is due not only to differences in structural relations of the various parts of the 

 hull but to specific characteristics of the fouling organisms in attaching in definite 

 zones. Thus, we find that there is a very definite and clearly defined vertical grada- 

 tion noticeable in growths on ships' bottoms. Certain forms, Hke Enteromorpha 

 and some varieties of Balanus, are found characteristically in a rather narrow zone 

 around the vessel and extending from the water line to a depth of about 3 feet. 

 Hydroids, ascidians, and the stalked forms of barnacles are found rarely in this 

 zone. This, however, is the zone most commonly fouled, for in almost all classes 

 of lightly fouled vessels this was the only region fouled. Often it is covered with 

 a dense growth of algae, whose filaments often extend 5 to 6 inches. In such thickets 

 one often finds a bevy of animals, including such forms as amphipods, annehds, 

 isopods, and even canceroid crabs (probably Panopeus). Occasionally this algal 

 zone extended much deeper than usual. On several ships this growth extended 

 from the water line for fully 10 feet, almost to the bilge keels. It has been impossible 

 to correlate these few cases with any seasonal variation as suggested by Hentschel 

 (1923). 



