Page 116 - Practical Ship Design

Weight-Based Designs 83

where
I, and h, = length and height of full width erections, and
I, and h, = length and height of houses.
If a numeral of this sort was being devised for the specific purpose of steel-
weight estimation, the constants would undoubtedly have been different, but the
availability of a great deal of data collected over many years in the E form has
become, for the author, a major influence in retaining its use.

4.2.2 Invoiced or net weight
The question of whether to plot invoiced or net weights is worthy of some debate.
The net weight is that initially arrived at by detailed calculations based on ship’s
plans and it is the weight that is required for the deadweight calculation. The
invoiced weight, on the other hand, is the weight recorded in a shipyard’s steel order
books and the one used for cost estimates. In earlier days the invoiced weight was the
one that was known more accurately and data was generally presented in this way.
More recently, with shipyards ordering much of their steel in standard plates for
stock, the reliability of invoiced records have declined, whilst with prefabricated
units now often being weighed, net steel-weight records have improved in accuracy
and seem the better choice nowadays.

4.2.3 The effect of the block coefficient on steel-weight
Since the parameter E attaches no significance to the fullness of the ship, a factor
which clearly has an appreciable effect on the steel-weight, all steel-weights are
corrected to a standard block coefficient before plotting. By the same token all
steel-weights read from the graph must be corrected from the standard to the
desired block coefficient.
The standard block was set at C,’ = 0.70 measured at 0.80.
Corrections to the steel-weight for variations in C, from the standard 0.70 value
can be made using the following approximate relationship:

W, = W,,[ 1 + O.O5(C,’ - 0.70)] (4.2)

where
W, = steel-weight for actual C,‘ at 0.80, and
W,i = steel-weight at C,,’ = 0.70 as plottedlifted from graph.
In this case C,,‘ has been taken at 0.80, because the available data is on this basis.
C,,’ at 0.80 can be calculated from the known value of C, at the load draft using
the formula given as eq. (3.10) or read from Fig. 3.6.