Page 245 - Ship Construction.DJ Eyres 6Ed

Ch21-H8070.fm Page 234 Wednesday, October 18, 2006 6:59 AM

21
Aft End Structure

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Considerable attention is paid to the stern in order to improve flow into and
away from the propeller. The cruiser stern (see Figure 21.1) was for many
years the favoured stern type for ocean going ships, but today most of these
vessels have a transom stern (see Figure 21.2). A cruiser stern presents a
more pleasant profile and is hydrodynamically efficient, but the transom
stern offers a greater deck area aft, is a simpler construction, and can also
provide improved flow around the stern. It is interesting to note that the
Queen Mary 2 has a combined stern the upper part being of cruiser stern
form for seakeeping and the lower part being of transom form for efficient
hydrodynamics. ึ
Many forms of rudder are available and the type and form fitted is
intended to give the best manoeuvring characteristics. Both the shape of
the stern and the rudder type will dictate the form of the stern frame, and
this will be further influenced by the required propeller size. Of particular
importance at the after end are the arrangements which permit both the
propeller shaft and the rudder stock to pierce the intact watertight hull. The
safety of the ship may depend on these arrangements. Where more than
one screw propeller is to provide the thrust required to propel the ship, boss-
ings or ‘A’ brackets will be fitted to support the outboard shafts. Whilst most
ships are propeller driven other means of transmitting the power developed
by the propulsion machinery into a thrust to propel the ship are not uncom-
mon. Water jets for example are now a common feature on high speed pas-
senger and service craft.

Stern Construction
As the cruiser stern overhang may be subjected to large slamming forces a
substantial construction with adequate stiffening is required. Solid floors
are fitted at every frame space, and a heavy centre line girder is fitted right
aft at the shell and decks. The stern plating is stiffened by cant frames or
webs with short cant beams supporting the decks and led to the adjacent
heavy transverse deck beam. Further stiffening of the plating is provided, or