Page 85 - Introduction to Naval Architecture

72 FLOTATION AND STABILITY

Angle of heel due to turning
When a ship is turning under the action of its rudder, the rudder holds
the hull at an angle of attack relative to the direction of advance. The
hydrodynamic force on the hull, due to this angle, acts towards the
centre of the turning circle causing the ship to turn. Under the action
of the rudder and hull forces the ship will heel to an angle that can be
determined in a similar way to the above.

STABILITY STANDARDS

It has been demonstrated how a ship's transverse stability can be
defined and calculated. Whilst the longitudinal stability can be
evaluated according to the same principles, it is not critical for normal
ship forms as the longitudinal stability is so much greater than the
transverse. This may not be true for unconventional forms such as off-
shore platforms. The stability of planing craft, hydrofoils and surface
effect craft also require special analysis because the forces supporting
the weight of the craft, which will determine their stability, are at least
partly dynamic in origin. In what follows attention is focused on
transverse stability of intact conventional monohulls. Stability in the
damaged state will be dealt with later.
The designer must decide very early on in the design process what
level of stability needs to be provided. Clearly some stability is needed
or else the ship will not float upright, but loll to one side or the other.
In theory a very small positive metacentric height would be enough to
avoid this. In practice more is needed to allow for differing loading
conditions, bad weather, growth in the ship during service and so on.
If the ship is to operate in very cold areas, allowance must be made for
possible icing up of superstructure, masts and rigging.
The designer, then, must decide what eventualities to allow for in
designing the ship and the level of stability needed to cope with each.
Typically modern ships are designed to cope with:

(1) the action of winds, up to say 100 kts;
(2) the action of waves in rolling a ship;
(3) the heel generated in a high speed turn;
(4) lifting heavy weights over the side, noting that the weight is
effectively acting at the point of suspension;
(5) the crowding of passengers to one side.
1
Standards for USN warships have been stated as have the standards
2 3
adopted by Japan and for passenger ships . These last may be
summarized as: