8 FOULING OF ships' BOTTOMS : 



fouling paints. The most usual forms occur in patches or clumps 

 of knobbed stalks, which are much stiffer than those of fouling sea- 

 weeds, so that they stand out from the hull surface even when in 

 dry -dock. The tresses of seaweeds almost always collapse against 

 the surface when the water is withdrawn. One of the commonest 

 forms of hydroid in British waters {Tubular ia. Fig. 10) has a very 

 striking bright pink head to the stalk. These heads disappear 

 under unfavourable conditions {e.g., fresh water, low tem.perature, 

 &c.), but the stalks persist and form characteristic stiff, wiry grey 

 clumps. 



The soft-bodied forms include the sponges (Figs. 19 and 26), 

 sea-squirts (Figs. 20 and 2o) and sea-anemones (Fig. 27). 



(6) The Identification of Foulinc Organisms. 



It is impossible to gi-ve an absolutely hard and fast rule for the 

 identification of any particular type of fouling organism. The key 

 in Section (7) gives a reasonably accurate method of establishing 

 the principal groups ¥/ithout going into detail, which can be seen only 

 under a hand- lens or microscope. Whenever doubtful cases occur, 

 a small sample can readily be preserved in water to which a few 

 drops of formalin have been added. The Marine Corrosion Sub- 

 Committee will gladly examine and identify any specimens sent 

 to them, and will supply tubes of formalin to anyoae wishing to send 

 samples ; correspondence on this subject should be addressed to the 

 Secretary of The Iron and Steel Institute. 



In using the dry-docking report, the following general remarks 

 should be borne in mind. Seavjeeds are almost always confined to 

 the water-line area ; if a plant-like growth occurs over the whole or 

 the darker parts of the hull it is probably a hydroid. A green colour 

 is an almost certain mark of a seaweed, but the latter may also be 

 red or brown. A red seaweed is usually of a fairly rich deep colour, 

 "the stalk as well as the " branches " being all of one colour. The 

 red colour of some hydroids is limited to the knobbed head alone ; 

 the stalks (which maj^ not bear any head at all) are usually greyish, 

 brown or white. A diatom slime may look like a patch of brown 

 oil; it is not, however, greasy, and washes off the fingers easily 

 with water ; when dry it often appears as a grev or greenish-grey 

 film. 



The recognition of the various kinds of plant growths, both 

 diatoms and seaweed, that occur on fouled vessels is most satis- 

 factorily accomplished by the use of the microscope, a magnification of 

 100 diameters being adequate for this purpose. Some seaweeds are, 

 however, difficult to distinguish from one another except by an expert. 

 Some identification is possible by examining plant growths with a 

 good hand-lens ; for this purpose it is advisable to spread them out 

 thinly on the fingers or in water between two small sheets of glass, 

 examining them against a strong light. 



