Page 13 - Introduction to Marine Engineering

2 Ships and machinery

have various cargo holds which are usually the full width of the vessel
and formed by transverse bulkheads along the ship's length. Cargo-
handling equipment will be arranged on deck and there will be large
hatch openings closed with steel hatch covers. The accommodation areas
in each of these ship types will be sufficient to meet the requirements for
the ship's crew, provide a navigating bridge area and a communications
centre. The machinery space size will be decided by the particular
machinery installed and the auxiliary equipment necessary. A passenger
ship, however, would have a large accommodation area, since this might
be considered the 'cargo space'. Machinery space requirements will
probably be larger because of air conditioning equipment, stabilisers and
other passenger related equipment.

Machinery

Arrangement
Three principal types of machinery installation are to be found at sea
today. Their individual merits change with technological advances and
improvements and economic factors such as the change in oil prices. It is
intended therefore only to describe the layouts from an engineering
point of view. The three layouts involve the use of direct-coupled
slow-speed diesel engines, medium-speed diesels with a gearbox, and the
steam turbine with a gearbox drive to the propeller.
A propeller, in order to operate efficiently, must rotate at a relatively
low speed. Thus, regardless of the rotational speed of the prime mover,
the propeller shaft must rotate at about 80 to 100 rev/min. The
slow-speed diesel engine rotates at this low speed and the crankshaft is
thus directly coupled to the propeller shafting. The medium-speed
diesei engine operates in the range 250—750 rev/min and cannot
therefore be dircci'f coupled to the propeller shaft. A gearbox is used to
provide a low-speed drive for the propeller shaft. The steam turbine
rotates at a very high speed, in the order of 6000 rev/min. Again, a
gearbox must be used to provide a low-speed drive for the propeller
shaft,

Slow-speed diesel

A cutaway drawing of a complete ship is shown in Figure I.I. Here, in
addition to the machinery space, can be seen the structure of the hull,
the cargo tank areas together with the cargo piping and the deck
machinery. The compact, complicated nature of the machinery
installation can clearly be seen, with the two major items being the main
engine and the cargo heating boiler.