Page 112 - Introduction to Naval Architecture

THE ENVIRONMENT 99
discontinued. Sewage presents some special problems. It can be heat
treated and then burnt. It can be treated by chemicals but the residues
have still to be disposed of. The most common system is to use treatment
plant in which bacteria are used to break the sewage down. Because the
bacteria will die if they are not given enough 'food', action must be taken
if the throughput of the system falls below about 25 per cent of capacity,
as when, perhaps, in port. There is usually quite a wide fluctuation in
loading over a typical 24 hour day. Some ships, typically ferries, prefer to
use holding tanks to hold the sewage until it can be discharged in port.
In warships the average daily arisings from garbage amount to 0.9kg
per person food waste and 1.4kg per person other garbage. It is dealt
with by a combination of incinerators, pulpers, shredders and
compactors.

SUMMARY

The interactions between the ship and the environment in which it
operates have been outlined. The greatest impacts of the environment
on the ship arise from the wind, waves and temperature. The apparently
confused ocean surface can be represented by the summation of a large
number of individually small amplitude regular waves. The energy
spectrum concept is useful in representing the irregular sea surface.
Formulations of such spectra have been given and sources of statistical
wave data discussed. The ship motions and hull stresses induced by these
waves are discussed in later chapters. The need for the ship to avoid
polluting its environment is a matter of growing concern and is
increasingly the subject of national and international regulation.

References
1. Froude, R. E. (1905) Model experiments on hollow versus straight lines in still water
and among artificial waves. TINA.
2. Milne-Thomson, L. M. (1949) Theoretical hydrodynamics. Macmillan.
3. Lamb, H. (1965) Hydrodynamics, Cambridge University Press.
4. Hogben, N. (1995) Increases in wave heights over the North Atlantic: A review of the
evidence and some implications for the naval architect. TRINA.
5. Seakeeping and Weather. RINA International Conference, London, 1995.
6. Hogben, N. and Lumb, F. E. (1967) Ocean wave statistics. HMSO.
7. Hogben, N,, Dacunha, N. M. C. and Oliver, G. F. (1986) Global wave statistics. British
Maritime Technology Ltd.