Page 161 - Practical Ship Design

128 Chapter 4

Opposite page: Fig. 4.21. Calculation sheet for lightship centres of gravity using volume density
method.
~ ~ ~ ~ ~~ ~ ~ ~~~~

- Column 1 gives the mean height to the deck on which each erection tier is
built.
- Column 2 gives the tween deck height.
- Column 3 gives the factor relating the VCG of each layer to its centroid.
- Column 4 gives the VCG of each layer above the deck on which it is built.
- Column 5 gives the volume of each erection.
- Column 6 the density and column 7 the weight.
- Column 8 gives the VCG of each erection above the base.
- Column 9 gives the vertical moment.
- Columns 10, 11 and 12 deal with the corresponding LCG and moments.

The table at the bottom left deals with:
- the mean depth corrected for sheer and camber;
- the total hull volume;
- the hull VCB at the mean depth;
- the hull LCB as a percentage of the ship’s length.
Below this are noted the R,, K, and Fg, values used, or analysed.
- Line 17 gives the hull weight, centres and moments.
- Lines 18 to 24 concludes the calculation.
Some values which may be used in the calculations are:
- R, for accommodation constructed in steel and fitted out to normal cargo ship
standards appears to have a fairly constant value of about 0.13 tonnes/m3;
- K, for accommodation of this type generally has a value of about 0.6;
- R, varies not only with ship type, but also with ship size and should be
assessed with care.
Where the hull below No. 1 deck and the superstructure both contain accom-
modation as on a passenger ship it may be reasonable to make R, = R, = 1.00 at
lines 1 and 8 and apply a correction factor to the weight obtained at line 10 to give
the corrected hull weight at line 12.
With R,, K, and R, known, the value of K, for a suitable basis ship can be
determined by analysis starting at both top and bottom of the table. Using a suitable
value of K, from a good basis ship, the light ship VCG of a new design can be
calculated.
As noted above, this method can also be used to establish the LCG position,
with that of the hull being assumed to be largely determined by LCB position at
mean depth.