Page 191 - Practical Ship Design

Powering I 157

The main component of the residuary resistance is the wave-making resistance,
but there are also smaller components stemming from eddy-making resistance and
from the resistance caused by the movement through the air of that part of the
model and of the ship, respectively, which is above the waterline.
For reasons that will be discussed later in this section, a change in the treatment
of resistance was made in 1957 when the basis for calculating the friction
coefficients was altered from the Froude line which had been used for many years
to the 1957 ITTC (International Towing Tank Conference) line.
The 1957 ITTC line is expressed by the formula:

C, = O.O75/(10g Rn - 2)2 (6.4)
R, = Reynolds' number (see 56.3, ix)
This change decreased the frictional component increasing the residuary component
C, correspondingly. Whilst it reduced the size of the ship-model correlation factor,
it did not entirely eliminate the need for this and did not improve the accuracy of
power estimation in the way that it had been expected to do and a further change
was made by ITTC in 1978 and indeed further changes proposed by Grigson are
discussed later.

6.2.2 The present day (ITTC'78) treatment oj resistance
The present day treatment recognises that the frictional resistances of both model
and ship differ from those of flat plates of the same length and area. The viscous
resistance coefficient (C"), as the frictional resistance of a shaped body is now
called, is increased over the frictional resistance coefficient of the corresponding
flat plate by a form factor K so that

(C,,,, is still based on eq. (6.4)).
The form factor can either be deduced from model experiments at very slow
speeds when C, is reduced to nearly zero, or in some tanks by the direct measure-
ment of C, from the energy delivered to the wave systems.
C, for the model is now calculated from the formula

As K tends to have a value of between 0.25 and 0.35 for most ship forms, a value of
C, calculated from eq. (6.6) is much smaller than one calculated from eq. (6.2)
The total resistance coefficient of a ship is now considered as made up of
C,, = (I + K> C,, + C, + Capp + AC + C,,, (6.7)