4-82	NICHOLLS'S  SEAMANSHIP  AND  NAUTICAL KNOWLEDGE
W S is  the skin surface area
L the length of the ship
d   the mean draught
Cb the ship's block coefficient
B the  breadth
This equation is used in some calculations dealing with the fricfcional
resistance offered by the surface area of the under water body of a
vessel. It may also be used to estimate the quantity of paint required
to cover an area of shell plating from a given draught downwards
by dividing this surface area by the known covering capacity of the
paint.
The coefficient of a vessel is not always known and in such cases
the manufacturers of special paints adopt empirical equations based
on the'results of experience and the average covering power of their
own products, when they wish to estimate the quantity to send to the
dry dock. Here is one such equation:—
Cwts=j£ (B-\-d) expressed in square yards and divided by 350
square yards per cwt.
Such equations can only give a rough approximation as the covering
power of patent compositions depends upon the < consistency of the
paint, the temperature, the manner in which it is applied and the
character of the coated surface. Allowance has also to be made for
considerable waste owing to the awkwardness of working under the
ship and the haste due to the economic urgency of getting .the vessel
out of dry dock as quickly as possible.
Example.—Ship 400 ft. long, 54 ft. beam, block coefficient=*8.
Find the amount of antifouling composition required to cover the
bottom up to 15 ft. draught, assuming that 1 cwt. of composition
covers approximately 3000 sq. ft.
(i) WS=L(
=400 {(1-7x15)+(-8X54)}
=400  (25-5+43-2)
=27,480 sq. ft.
27480
•'•cwts' = 300P" "" 9'16 cwt'
Scantlings.—The term "scantlings" often appears in descriptions of
ship construction, and it would be as well to clear it off here by defining
"scantling" as the term used to indicate the sizes of the different compon-
ent parts of a ship. The sizes are given in the Rules and Regulations oi
27600
cwts. ~   oT^wT     — 9*2 cwt.
(ii) Surface Area = L (B+d)
= 400 (69)
= 27,600 sq. ft.