Page 256 - Ship Construction.DJ Eyres 6Ed

Ch21-H8070.fm Page 245 Wednesday, October 18, 2006 6:59 AM
Aft End Structure 245
speeds are shown to have benefited by a decrease in resistance when bossings
have been fitted rather than ‘A’ brackets. However large liners of more
recent design have in some instances had extended shafts solely supported by
‘A’ brackets of improved design.

CONSTRUCTION OF BOSSING AND ‘ A’ BRACKETS The shaped
frames and plating forming the bossing terminate in a casting known as the
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‘spectacle frame’ which provides the aftermost bearing for the shaft. This
may be cast or fabricated and forms a box-like section athwartships which is
rigidly connected to heavy plate floors. The arms carrying the shafts extend
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from this section which may be split in two or more parts in some instances
to aid alignment when it is erected (see Figure 21.7).
‘A’ brackets may be cast, or fabricated, particular attention being paid to
the strut section to avoid increases in resistance and cavitation. The con-
nections to the main hull are of particular importance since considerable
rigidity of the structure is required. Although on smaller vessels the upper
palms may simply be welded to a reinforcing pad at the shell, on larger
vessels the upper ends of the struts enter the main hull and are connected
to a heavy floor with additional local stiffening (Figure 21.7).
ึ
Propellers

Ship propellers may have from three to six similar blades, the number being
consistent with the design requirements. It is important that the propeller is
adequately immersed at the service drafts and that there are good clear-
ances between its working diameter and the surrounding hull structure. The
bore of the propeller boss is tapered to fit the tail shaft and the propeller
may be keyed onto this shaft; a large locking nut is then fitted to secure the
propeller on the shaft. For securing the propeller a patent nut with a built
in hydraulic jack providing a frictional grip between the propeller and tail
shaft is available. This ‘Pilgrim nut’ may also be used with keyless bore
propellers. A fairing cone is provided to cover the securing nut.

CONTROLLABLE PITCH PROPELLERS These are propellers in which
the blades are separately mounted on the boss, and in which the pitch of the
blades can be changed, and even reversed, by means of a mechanism in the
boss, whilst the propeller is running. The pitch is mechanically or electro-
mechanically adjusted to allow the engines’ full power to be absorbed under
different conditions of operation. It is incorrect to refer to such a propeller
as a variable pitch propeller since virtually all merchant ship propellers
have a fixed pitch variation from blade root to blade tip.
Propellers of this type are often found on diesel-engined tugs and
trawlers where the propeller pitch may be changed to allow the full torque