Page 211 - Practical Ship Design

6.8 GULDHAMMER AND HARVALD’S METHOD

A more modern powering method which avoids some of the difficulties mentioned
earlier in this section was published by Guldhammer and Harvald in Ship
Resistance - Eflect of Form and Principal Dimensions (1974 Akademisk Forlag,
Copenhagen). This defines the total resistance R, as:

R, = C, . 112 . p . S . V2 (6.39)

and
C, = c, + c,

where
C, = total resistance coefficient
C, = residual resistance coefficient
C, = frictional resistance coefficient
p = mass density = r/g
V = velocity
S = wetted surface
(all in SI units).
The C, value is based on the 1957 ITTC Friction line. A pt of this based on
ship length L for a range of speeds from 0.1 m/s to 20 m/s (approx 0.2 to 39 knots)
is reproduced in Fig. 6.2.
It is worth noting that the difference between the C, values for two Reynolds’
numbers can be used to correct a C, value from one ship length to another (see
$7. I).
These C, values do not allow for a form factor, the use of which was not adopted
by the ITTC until 1978.
A correction for the increased resistance resulting from any appendages is
made by increasing C, proportionally to the increased wetted surface due to the
appendages.
C,’ = c, x S‘IS (6.40)

The C, values are based on vessels with a standard position of LCB, a standard
BIT value of 2.5, normal shaped sections and a moderate cruiser stern. They are
plotted for a number of values of L/A* 1/3 ranging from 4.0 to 8.0 by 0.5 steps. Each
graph is on a base of Froude number and is plotted for a range of prismatic
coefficients from 0.50 to 0.80 by 0.01 steps.
The graph of C, is intended to correspond to an LCB position close to optimum.
A graph of this standard (optimum) LCB position on a base of F, is given, together
with the correction to be applied if the actual LCB is forward of the standard.
Interestingly, there is no correction for the LCB being aft of the standard position.