Page 203 - Introduction to Naval Architecture

RESISTANCE 189

relatively small, say 10 to 15 per cent of the hull resistance, and errors
in their assessment are not likely to be critical.

Wind resistance

In conditions of no natural wind the air resistance is likely to be small
in relation to the water resistance. When a wind is blowing the fore and
aft resistance force will depend upon its direction and speed. If coming
from directly ahead the relative velocity will be the sum of wind and
ship speed. The resistance force will be proportional to the square of
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this relative velocity. Work at the National Physical Laboratory
introduced the concept of an ahead resistance coefficient (ARC) defined
by:

where V R is the relative velocity and A T is the transverse cross section
area.
For a tanker, the ARC values ranged from 0.7 in the light condition
to 0.85 in the loaded condition and were sensibly steady for winds from
ahead and up to 50° off the bow. For winds astern and up to 40° off the
stern the values were -0.6 to -0.7. Between 50° off the bow and 40° off
the stern the ARC values varied approximately linearly. Two cargo ships
showed similar trends but the ARC values were about 0.1 less. The
figures allowed for the wind's velocity gradient with height. Because of
this ARC values for small ships would be relatively greater and if the
velocity was only due to ship speed they would also be greater. Data is
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also available for wind forces on moored ships.

CALCULATION OF RESISTANCE

Having discussed the general nature of the resistance forces a ship
experiences and the various formulations for frictional resistance it is
necessary to apply this knowledge to derive the resistance of a ship. The
model, or data obtained from model experiments, is still the principal
method used. The principle followed is that stated by Froude. That is,
the ship resistance can be obtained from that of the model by:

(1) measuring the total model resistance by running it at the
corresponding Froude number;
(2) calculating the frictional resistance of the model and subtracting
this from the total leaving the residuary resistance;