Page 24 - Introduction to Naval Architecture

12 DEFINITION AND REGULATION

will indicate V-sections. A low value of midship section coefficient
indicates a high rise of floor with rounded bilges. It will be associated
with a higher prismatic coefficient.

Displacement and tonnage

Displacement
A ship's displacement significandy influences its behaviour at sea.
Displacement is a force and is expressed in newtons but the term mass
displacement can also be used.

Deadweight
Although influencing its behaviour, displacement is not a direct
measure of a ship's carrying capacity, that is, its earning power. To
measure capacity deadweight and tonnage are used.
The deadweight, or deadmass in terms of mass, is the difference
between the load displacement up to the minimum permitted
freeboard and the lightweight or light displacement. The lightweight is
the weight of the hull and machinery so the deadweight includes the
cargo, fuel, water, crew and effects. The term cargo deadweight is used for
the cargo alone. A table of deadweight against draught, for fresh and
salt water, is often provided to a ship's master in the form of a deadweight
scale.

Tonnage
Ton is derived from tun, which was a wine cask. The number of tuns a
ship could carry was a measure of its capacity. Thus tonnage is a
volume measure, not a weight measure, and for many years the
standard ton was taken as 100 cubic feet. Two 'tonnages' are of
interest to the international community - one to represent the overall
size of a vessel and one to represent its carrying capacity. The former
can be regarded as a measure of the difficulty of handling and
berthing and the latter of earning ability. Because of differences
between systems adopted by different countries, in making allowances
say for machinery spaces, etc., there were many anomalies. Sister ships
could have different tonnages merely because they flew different flags.
It was to remove these anomalies and establish an internationally
approved system that the International Convention on Tonnage
1
Measurement of Ships, was adopted in 1969 . It came into force in
1982 and became fully operative in 1994. The Convention was held
under the auspices of the International Maritime Organisation to