you
as we know that the propeller and rudder enable the ship to move in a
three-dimensional space. In other words the captain alters the ship's
orientation around its own center of gravity giving it the necessary
movements. ship motions are defined by the six degrees of freedom that a ship,
boat, or any other craft can experience. Degrees of freedom refers to the number
of basic ways a rigid object can move through a three-dimensional space. In
other words degrees of freedom defines the number of directions a body can move
Before understanding the ship motions, first, let us get a clearer picture of
the reference axes that are used to define the ship's motion. The vertical or
z-axis also known as yaw axis is an imaginary line running vertically
through the ship and through its center of mass. The transverse or Y axis also
known as lateral axis or pitch axis is an imaginary line running horizontally
across the ship and through the center of mass
the longitudinal or x-axis also known as Roll axis is an imaginary line running
horizontally through the length of the ship through its centre of mass and
parallel to the waterline. The movements of the ship can be divided into two
sets - rotational motion and translational motion. The rotational
motion occurs if every particle in the body moves in a circle about a single
line and a translational motion occurs if all points of a moving body move
uniformly in the same line or direction
which is referred to as the up-and-down rotation of the vessel about its
transverse or y-axis. A deviation from normal on this axis is referred to as
trim or out of trim. Rolling is the tilting rotation of the vessel about its
longitudinal or x-axis. An offset or deviation from normal on this axis is
referred to as list. Yawing is defined as the turning rotation
of a vessel about its vertical or z-axis. An offset or deviation from normal on
this axis is referred to as deviation or set. Heave is the linear vertical or
up-and-down motion of the ship
excessive downward heave can swamp a ship. Swaying motion is the leaning transverse or
side-to-side motion of the vessel. Surge is the linear longitudinal bow and stern
motion imparted by maritime conditions. These kinds of motions along the axis
can sometimes be harmful to the ship and can result in stresses that act upon the
hull. panting refers to the continuous
bellowing in and bellowing out nature of the ship's hull plating due to variable
water pressure of waves. This effect is accentuated in the forward region when
the ship surges headway through. The Ship bow region is the most affected area
where the entire vessel encounters the wave system for the first time although
panting is still said to exist throughout the entire length of the hull
the effect dies away as the wave system at the bow starts losing its energy
from the bow shoulder onwards. The pounding induces forces due to the
slamming motion of the ship triggered by heaving or high pitching motions. This
situation is further aggravated in case of empty or light ballasted conditions
of the ship. Other than this, there are also wave induced stresses that the ship
also has to face during transit. Stresses such as parametric rolling sagging or
hogging are such wave induced stresses. Parametric rolling is a type of movement
that is experienced only on the container ships/. Parametric rolling is an
unstable phenomenon which can quickly generate large roll angles that are
coupled with significant pitch motions leading to breaking of container lashing
loss of cargo and in the worst case scenario capsizing of the ship. If the
vessel is heading against the wind and with the increase of the wind the wave
height will increase which results in rolling of the vessel. Now the wavelength
reduces and the encounter period of waves by the ship also reduces, this
result in decrease in the ship's speed and increase in the pitching angle along
with rolling of the ship where pitching period is equal to or half of that of
the time taken to complete one roll. This leads to heavy rolling of the
vessel which can be up to 30 to 40 degrees on both sides. When the buoyancy
amidships exceeds the weight due to loading or when the wave crest is at the
midship the ship will hog as a beam is supported at mid length and loaded at
the end. In this condition the overall weight is greatest near the bow and
stern with buoyancy being larger near midship. This has the effect of bending
the structure in the other direction placing the keel in compression and the
deck in tension
if the weight amidships exceed the buoyancy or when the wave trough is at midships
the vessel will sag as a beam supported at an end and loaded at mid-length. It is
fairly evident that this longitudinal bending condition is creating
significant stresses in the structure termed
bending stressed