Page 192 - Practical Ship Design

158 Chapter 6

where
Capp is the resistance of appendages
AC is the roughness allowance which is discussed later in $6.2.6
Cair is the air resistance coefficient, for which there is the following approx-
imate formula:

Cair = 0.001 . AJS (6.8)
where A, is the projected cross sectional area of the ship above the waterline.
It should be noted that in association with C,, derived in this way the wetted
surface S in eq. (6.1) is the total wetted surface inclusive of the surface area of the
bilge keels, if fitted.
S(wetted) = S(naked) + S(bi1ge keels)

6.2.3 Discussion of the two treatments
The newer treatment is more scientific and is now in fairly general use in tank
testing. However, the use of the method for power estimation in advance of tank
testing presents some difficulty because there is still a lack of data in the new
format compared to the mass that exists in earlier formats.

6.2.4 Hull finish and the importance of skin friction resistance

An understanding of the importance of hull finish requires a knowledge of the
proportion of the total ship resistance which is frictional. Ideas on this have
changed considerably in recent years with first of all the change from the Froude
friction line to the ITTC line and then the introduction of the form factor. In
addition some, if not all, of the ship-model correlation factors that have been
applied to the total resistance ought almost certainly to have been applied to the
frictional resistance only.
In ITTC’78 practice the proportion of the viscous component is
(l+K)C, +AC
(6.9)
(l+K)C, +AC+C, +Cair
This may be compared with the former practice in which the frictional component
proportion was

(6.10)

It is important to note that the C, values in these two equations differ from one
another, indeed the biggest part of the difference between the two formulae occurs