FOtTLING OF SHIPS' BOTTOMS 



219 



kills most of the living organisms attached to it in exposed places. As indicated 

 previously, the most usual place for fouling to be found on rapidly cruising vessels 

 (passenger ships) was in the groove made by the overlapping of the metal plates of 

 the hull. Here, then, is a case where the effect of friction through water is much 

 reduced or entirely absent, and a merely local growth of fouling results. The notice- 

 able absence of hydroids, tunicates, and other relatively soft-bodied organisms on 

 rapidly cruising vessels indicates that such forms probably can not withstand the 

 pressure, and consequently only shelly growths, such as barnacles and seruphds, 

 are found on such vessels. 



LENGTH OF PERIOD BETWEEN DRY DOCKINGS 



The amount of time spent in port, in relation to the amount of time spent 

 under way, obviously is related to the duty of the ship. It has long been known 



MONTHS 

 SPENT 



IN 

 PORT 



PERCENTAGE OF SHIPS (ACTUAL NUMBER INBRACKETS) 



10 20 30 40 50 60 70 80 90 100 



X 



16-20 



□ 



NO FOULING 



LIGHT FOULING 



m m 



MODERATE HEAVY 

 FOULING FOULING 



Fig. 25. — Relation between the degree of fouling and the amount of time spent in port between dry dockings 



that while idle in port boats frequently accumulate heavy growths of fouling; while 

 similar vessels, on the high seas during an equal period, remain relatively free from 

 fouling. In the past this fact has been associated more with the length of the 

 period that elapsed since the previous dry docking than with the relative amount 

 of cruising done during a given period, a relationship that is of secondary importance 

 only, as will be shown. 



From the records of the ships that have been considered in this study, it has 

 been estimated that passenger ships spend more than 60 per cent of their time 

 cruising, while freighters spend an average of about 40 per cent of their time on 

 the high seas. Naval craft vary greatly in this regard, but from the data given in 

 Table 1 it can be seen that destroyers spend about 30 per cent of their time cruising, 



