Page 224 - Introduction to Marine Engineering

Shafting and propellers 205
Older designs, usually associated with sea water lubricated stern
bearings, made use of a conventional stuffing box and gland at the after
bulkhead. Oil-lubricated stern bearings use either lip or radial face seals
or a combination of the two.
Lip seals are shaped rings of material with a projecting lip or edge
which is held in contact with a shaft to prevent oil leakage or water entry.
A number of lip seals are usually fitted depending upon the particular
application.
Face seals use a pair of mating radial faces to seal against leakage. One
face is stationary and the other rotates. The rotating face of the after seal
is usually secured to the propeller boss. The stationary face of the
forward or inboard seal is the after bulkhead. A spring arrangement
forces the stationary and rotating faces together.

Shafting
There may be one or more sections of intermediate shafting between the
thrust shaft and the tailshaft, depending upon the machinery space
location. All shafting is manufactured from solid forged ingot steel with
integral flanged couplings. The shafting sections are joined by solid
forged steel fitted bolts.
The intermediate shafting has flanges at each end and may be
increased in diameter where it is supported by bearings.
The propeller shaft or tailshaft has a flanged face where it joins the
intermediate shafting. The other end is tapered to suit a similar taper on
the propeller boss. The tapered end will also be threaded to take a nut
which holds the propeller in place.

Propeller
The propeller consists of a boss with several blades of helicoidal form
attached to it. When rotated it 'screws' or thrusts its way through the
water by giving momentum to the column of water passing through it.
The thrust is transmitted along the shafting to the thrust block and
finally to the ship's structure.
A solid fixed-pitch propeller is shown in Figure 11.5. Although
usually described as fixed, the pitch does vary with increasing radius
from the boss. The pitch at any point is fixed, however, and for
calculation purposes a mean or average value is used.
A propeller which turns clockwise when viewed from aft is considered
right-handed and most single-screw ships have right-handed propellers.
A twin-screw ship will usually have a right-handed starboard propeller
and a left-handed port propeller.