Page 193 - Ship Construction.DJ Eyres 6Ed

Ch17-H8070.fm Page 182 Wednesday, October 18, 2006 6:57 AM
182 Ship Construction
STRENGTHENING FOR NAVIGATION IN ICE If a vessel is to be
assigned a special features notation for navigation in first year ice (see
Chapter 4) the additional strengthening required involves primarily an
increase in plate thickness and frame scantlings in the waterline region and
the bottom forward, and may require some modifications and strengthen-
ing at the stem, stern, rudder and bossings, etc.
A main ice belt zone is defined which extends above the ice load water-
--- ใช้เพื่อการศึกษาเท่านั้น---
line (i.e. normally the summer load waterline) and below the ice light
waterline (i.e. lightest waterline ship navigates ice in). The extent of this
zone depends on the ice class assigned.
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
The shell plating thickness in this zone is greater than on a conventional
ship and increases with severity of ice class and with position from aft to
forward. For the more severe ice conditions the thickness of the side shell
is also increased below the main ice belt zone for at least 40 per cent of the
length from forward.
Transverse main and intermediate frames of heavier scantlings are fitted
in way of the main ice belt zone. Where the shell is longitudinally framed
longitudinals of increased scantlings are fitted in way of the ice belt zone.
Both transverse and longitudinal frame scantling requirements are depen-
dent on the severity of ice class assigned and distance of frame from for-
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ward. Transverse ice framing is supported by ice stringers and decks, and
longitudinal framing by web frames the scantlings of which are increased
with severity of ice class and distance from forward.
Strengthening for addition of ‘Icebreaker’ notation to ship type notation
and assignment of special features notation for navigation in multi-year ice
and the associated requirements for plating and framing at the bow and
stern are too extensive to be covered adequately in this text.

Bilge Keel

Most ships are fitted with some form of bilge keel the prime function of
which is to help damp the rolling motion of the vessel. Other relatively
minor advantages of the bilge keel are protection for the bilge on ground-
ing, and increased longitudinal strength at the bilge.
The damping action provided by the bilge keel is relatively small but
effective, and virtually without cost after the construction of the ship. It is
carefully positioned on the ship so as to avoid excessive drag when the ship
is under way; and to achieve a minimum drag, various positions of the bilge
keel may be tested on the ship model used to predict power requirements.
This bilge keel then generally runs over the midship portion of the hull, often
extending further aft than forward of amidships and being virtually perpen-
dicular to the turn of the bilge.
There are many forms of bilge keel construction, and some quite elabo-
rate arrangements have been adopted in an attempt to improve the damping