Page 134 - NAVAL SCIENCE 3 TEXTBOOK

SHIP CONSTRUCTION AND [)A~.IAGE CON THOL 139

However it is produced, propeller shafts carry the power to
the propellers. They run from the reduction gears through long
Propulsion
, ___ , power wi.ltertight spaces called shaft alleys in the very bottom of the ship.
SIJrter coupling Propellers drive the ship. Aircraft carriers and many cruisers have
~==~:;:::==I Turbine four propellers. 1vlost destroyers have two propellers, but many
r
ShJfi
newer ones have only one. They are mriable-pitch propellers, the
blades of which can be rotated on the hub ("0 provide more or less
,\uno.\pheric Exhamt to bite into the water for additional control of the ship's speed or to
air illtah ;Hnl05pherc
provide reverse thrust.
Basic parts of a gas turbine.

S hipbuilding
The gas turbine has several advantages over a conventional
Almost all large ships constructed in shipyards throughout the
steam plant. It is more compact, lighter, and easier to maintain and
world today are built in dry docks. The dry docks have a number
repair. Because it has a spark ignition system, much like a car, it
of large blocks on the bottom upon which the hull is assembled.
can go from "cold iron" shutdown to fully ready to turn the shaft in
The blocks under the ship are high enough so workers can work
only one minute, in contrast to the several hours of warm-up time
under the hull while the ship is being built. As the hull is built, scaf-
required to bring a steam engine on line.
folding is raised along the sides to make construction easier.
\,Vith a nuclear power plant, the primary system is a circulat-
ing water cycle. This consists of the reactor, loops or piping, pri-
mary coolant pumps, alld steam generators. Heat produced in the
reactor by nuclear fission is transferred to the circulating primary
coolant water, which is pressurized to prevent it from boiling. This
water is then pumped by the primar}' coolant pumps through the
steam generator, where steam to run the turbines is produced, and
then back into the reactor, where it can then be reheated for the
next cycle. The steam produced in the generator and used to run
the turbines circulates in a separate loop outside the reactor, to
avoid problems with radioactivity.
Because the generation of nuclear power does not require oxy-
gen, submarines can operate underwater for extended periods of
time. Since there are high levels of radioactivity in the reactor dur-
ing operation, no one is permitted to enter the reactor compart-
ment. Heavy shielding around the reactor protects the crew so
well that they receive less radiation than they would from natural
sources ashore.

Schematic showing steam flow from a nuclear reactor to the ship's
propulsion unit. Nuclear fuel (uranium) in the reactor heats the water Today large portions of ships are often built in subassembly bays
to produce steam, much like burning fuel oil does in a conventional away from the site where the main part of the ship is being built.
boiler. The rest of the propulsion system, with turbines, condenser, Here shipbuilders at Bath Iron Works Corporation, Bath, Maine, posi-
and reduction gears, is essentially the same. tion the 97-ton upper bow on a new merchant ship.