Page 205 - Practical Ship Design

Powering I 171

Substituting the above formula for (c= and @ together with the formula for P,
given in eq. 6.14 gives

R 1000 ~113 A2/3vk3 75
D=--I- ~~
V:
A (0.7355~0.5846)~ IOOOR, 0.5144Vk

75
D= = 580
(0.7355~ 0.5846), x 0.5144

(0 is used in the Froude treatment of skin friction for the correction from model
to ship size or vice versa or between two ship sizes L, and L,.

6.4 EFFECTIVE HORSEPOWER CALCULATION METHODS IN GENERAL

Designers have available to them several methods for estimating effective horse-
power. The older methods have, however, become outdated in recent years for a
number of reasons:

(i) the increased dimensions and in particular the increased length of modern
ships;

(ii) the greatly improved smoothness of the hull resulting from the change
from riveted to welded construction, from the use of shop-blasted and
primed instead of “weathered” plates and from the use of modem paints;

(iii) changes in the design of ship lines which have come from years of tank
testing and have greatly improved the performance of modem ships;
(iv) the tank test results which formed the basis of the older methods were
recorded prior to the universal adoption of trip wires or studs to eliminate
laminar flow;
(v) the fact that the friction line used for the extrapolation from model to ship
size has changed with the earliest data still in use being based on the
Froude line and later results being calculated using the ITTC 57 line. More
recently there has been the further change bringing in the form factor. This
means that 0 values ought to be annotated as Froude or ITTC’57, whilst
C, and EHP values have the further possible annotation of ITTC’78 indi-