Page 81 - Practical Ship Design

Setting Design Requirements 51

An approximate critical stowage rate can be synthesised as follows:

cargo dwt total dwt
-----Xp-- -
cargo dwt - total dwt displt
Cargo S.G. = -
cargo vol cargo vol total vol
x __~
total vol displt
If we make:

Cargo dwt =K, and total dwt = K,
total dwt displt

Drafddepth ratio TID = K3

Cargo vol =K, and C (at depth D) =K,
total vol Cb(at draft T)

1.025K,K2K,
Cargo density (S.G.) =
K4K5
Putting some fairly arbitrary figures to this equation

K, = 0.90; K2 = 0.70 (see Fig. 3.3)

K, = 0.73 (see Fig. 3.10)

K, = 0.58 (see Fig. 3.5) and K, = 1.05

1.025 x 0.90 x 0.70 x 0.73
Cargo density = = 0.77 or 1.29 m’ltonne
0.58 x 1.05

The ship in this example will be weight critical if the cargo that it is designed to
carry has a cargo density of more than 0.77 or stows at less than 1.29 m3/tonne and
volume critical if the cargo is lighter.
Other requirements which may make, or help to make, weight the critical factor
can be the need for particularly heavy construction or particularly heavy items of
outfit; and/or the inclusion of a severe limitation on draft.