Page 213 - Introduction to Naval Architecture

RESISTANCE 199

apparent amplitude per 50mm. This mean apparent amplitude per
50mm gauge length is the standard parameter used in the UK to
represent hull roughness. Roughness can be considered under three
headings:

(1) Structural roughness. This depends upon the design and method of
construction. In a riveted ship the plate overlaps and edges and
the rivet heads constituted roughness. These are avoided in
modern welded construction but in welded hulls the plating
exhibits a waviness between frames, particularly in thin plating,
and this is also a form of roughness.
(2) Corrosion. Steel corrodes in sea water creating a roughened
surface. Modern painting systems are reasonably effective in
reducing corrosion all the while the coating remains intact. If it is
abraded in one area then corrosion is concentrated at that spot
and pitting can be severe. This is bad from the structural point of
view as well as for frictional resistance. Building ships on covered
slipways and early plate treatments to reduce corrosion both
reduce the initial hull roughness on completion of build. To
reduce corrosion during build and in operation, many ships are
now fitted with cathodic protection systems, either active or
passive. These are discussed briefly under structure.
(3) Fouling. Marine organisms such as weed and barnacles can attach
themselves to the hull. This would represent a very severe
roughening if steps were not taken to prevent it. Traditionally the
underwater hull has been coated with anti-fouling compositions.
Early treatments contained toxic materials such as compounds of
mercury or copper which leached out into the water and
prevented the marine growth taking a hold on the hull.
Unfortunately these compounds also pollute the general ocean
and other treatments are now used. Fouling is very dependent
upon the time a ship spends in port relative to its time at sea, and
the ocean areas in which it operates. Fouling increases more
rapidly in port and in warmer waters. In the Lucy Ashton towing
trials it was found that the frictional resistance increased by about
5 per cent over 40 days, that is by about | of 1 per cent per day. This
is a common allowance applied to a ship's frictional resistance to
allow for time out of dock.

For an operator the deterioration of the hull surface with time results
in a slower speed for a given power or more power being needed for
a given speed. This increases running costs which must be set against
the costs of docking, cleaning off the underwater hull and applying
new coatings.