Page 160 - Practical Ship Design

Weight-Based Designs 127

4.8.2 A first approximation of the lightship VCG and LCG
A first estimate of the VCG is generally made by the use of the ratio VCGID. This
is simple but crude. Provided the data from the basis ship has been established by a
reliable inclining experiment, the two ships are of the same type and there are no
obvious differences which could be expected to alter the ratio, this crude method
can give quite a good answer.
If there are obvious differences, corrections can be made. Items fitted to the
basis ship which do not appear on the new ship should be deducted at their basis
ship centres before the ratio is used with items which only appear on the new ship
being added at their centres after the proportioning to the new ship depth.
Although this method is crude, it does have some advantages. The basis VCG is
a proven figure from the inclining experiment of an actual ship whereas a calcu-
lated VCG is derived from many weight and centre of gravity assumptions in
which there may be both errors and omissions particularly in calculations made
early in the design process. So, even when detailed calculations have been made it
is still no bad idea to compare the answer from these with that given by the
approximate method and try to rationalise the difference if any -and if this proves
difficult, further detailed checks may be wise!

4.8.3 A second approximation
Intermediate between the method outlined above and calculations based on detailed
weights and centres is the Volume-Density method presented in the Watson-
Gilfillan 1976 R.I.N.A. paper. In the simplest version of this method the VCG (and
LCG) are calculated using the volumes and volumetric centroids of the various
spaces which make up the ship. In a more sophisticated version of the method each
volume is multiplied be a density value and each volumetric centroid is corrected
by a factor to correlate with the centre of gravity position.
If the densities and the centroidNCG ratios selected for each compartment are
100% correct, the calculation becomes a completely accurate weight calculation.
Whilst this is unlikely to be the case, intelligent selection of these factors based on
an analysis of ships for which accurate information is available can enable this
method to give quite good results. It may be worth noting that the accuracy of the
final centre of gravity depends on the relative values of the densities used rather
than on their absolute values.
A standard sheet, again readily convertible to a spreadsheet, which can be used
for this type of calculation is presented as Fig. 4.21 and should be used in
association with the following notes.
- Lines 1 to 8 give the input of dimensions and various hydrostatic particulars.
- Lines 9 to 16 deal with the calculation of the weight and centres of gravity of
the superstructure.