TRIM, STABILITY, AND STRESS (PRELIM) Flashcards
K
KEEL
M
Meta Center
KM
HEIGHT OF META CENTER
G
CENTER OF GRAVITY
KG
HT OF CENTER OF GRAVITY
GM
METACENTRIC HT
B
CENTER OF BUOYANCY
KB
HT OF CENTER BUOYANCY
G = B
V/L FLOATS
G > B
V/L SINKS
That point at which all the vertically downward forces of weight are considered to be concentrated the center of the mass of the vessel
Center of Gravity.
That point at which all the vertically upward forces of buoyancy are considered to be concentrated; the center of volume of the immersed portion of the vessel
Center of Buoyancy.
Metacentric height distance from the center of gravity to the transverse metacenter
GM
Linear distance from the keel to the center of buoyancy when vessel is upright.)
KB
Height of center of gravity above keel
KG
Height of metacenter above keel
KM
The highest point to which G may rise and still permit the vessel to have positive stability. Found at the intersection of the line of action of B when the ship is erect with the line of action of B when the ship is given a small inclination
Metacenter.
Distance between B and M
Metacentric Radius.
Vessel with low center of gravity and large metacentric height
Stiff Ship
A vessel with small metacentric height; top-heavy
Crank Ship or Tender Ship
is the vessel’s ability to return to an upright position after being heeled by an external forces
stability
A ship is lying at an _ when the weights onboard are unevenly distributed and the static condition of the vessel is at an angle of inclination away from the vertical
angle of List
A ship is lying at an _ when external forces, such as waves or wind, shift the vessel over to an angle of inclination away from the static condition, perhaps only for a short time
angle of Heel
this results in a moment that brings the ship back to its original upright position
stable equilibrium
for a ship to be in stable equilibrium
the center of gravity (G) must be below the metacenter (M)
Exists when G coincides with
M. The vessel does not tend to return to an upright
position if inclined, nor to continue its inclination if
the inclining force is removed
Negative Stability
this causes the ship to heel over to one side and will at that angle of heel
neutral buoyancy
for a ship to be in a neutral buoyancy
the center of gravity (G) and the metacenter (M) coincide or nearly coincide
this is a dangerous state and too much heel would capsize the ship
unstable equilibrium
unstable equilibrium state
the centre of gravity is above the metacenter
- Archimede’s
Principle
Principle of Flotation
states that when a
body is wholly or partially immersed in a
fluid it appears to suffer a loss in mass
equal to the mass of the fluid it displaces
“Archimede’s” Principle
“A ship displaces
a weight of water
that is equal to its
own weight.”
“Archimede’s” Principle
, states that when a body floats in a fluid, the
weight of the body is exactly equal to the weight of
the fluid it displaces
Archimedes Principle, when applied to a floating
body
“Archimede’s” Principle vessel will
experience
an
upthrust that is
equal to the weight
of the displaced
water.
the vessel will float when
When Buoyancy (B)
is equal to Gravity (G)
THE WEIGHT OF ANY SHAPE IS ACTING ONLY
AT A CERTAIN POINT WHICH IS CALLED
CENTRE OF GRAVITY
IS DEFINED AS A POINT WHERE THE SHIPS WEIGHT
IS CONCENTRATED
CENTRE OF GRAVITY
is the point at which
all the mass of the body may be assumed to be
concentrated and is the point through which the
force of gravity is considered to act vertically
downwards, with a force equal to the weight of the
body. It is also the point about which the body
would balance
center of gravity of a body
The center of gravity of the body will always move
_ of any weight
moved within the body
parallel to the shift of the center of gravity
is the point
through which the force of gravity may be
considered to act vertically downwards
center of gravity of a body
When a _ is center of gravity is
considered to be at the point of suspension.
weight is suspended
The center of gravity of a body will _
from the center of gravity of any weight removed.
move directly away
he center of gravity of a body will _ the center of gravity of any weight added
move directly
towards
is the mutual actual between the parts
of a material to preserve their relative positions
when external loads are applied to the material
stress
- is defined as the load put on a piece of
material or a structure.
stress
is defined as the permanent deformity or
weakness caused by excessive stress
STRAIN-
3 MAIN TYPES OF STRESS
- Tensile/ Tensioning
- Compressive/ Compression
- Shear
When an external load is applied to a material in
such a way as to cause an extension of the
material it is called a
‘tensile’ load
an
external load tending to cause compression of
the material is a
‘compressive’ load
tendency to pull the material apart
tensioning
tendency to crush the material or to buckle
compression
is the effect of two forces acting in opposite directions and along parallel lines
shear
is a stress within a material
which tends to break or shear the material
across
shearing stress
If the wave crest is considered at mid-ships then
the buoyancy in this region will be increased. With
the wave trough positioned at the ends of the ship,
the buoyancy here will be reduced. This loading
condition will result in a significantly increased
bending moment, which will cause the ship to
hog
This will be an extreme condition giving the
maximum bending moment that can occur in the
ship‟s structure
Hogging due to waves
Consider a ship loaded with the weights concentrated at
the bow and the stern, which tends to droop. This leads
to
hogging of the ship hull
In a heavy seaway, a ship may be supported at the ends
by the crests of waves while the middle remains
unsupported. If the wave trough is now considered at
midships then the buoyancy in this region will be
reduced. With the wave crest positioned at the ends of
the ship, the buoyancy here will be increased. This
loading condition will result in a bending moment which
will cause the ship to
sag.
Consider heavy weights concentrated at the midships of
a ship. The middle hull part tends to droop more than the
ends. This causes
sagging of ship hull
The forces acting on a ship may be
static or
dynamic.
the ship at sea or lying in still water is constantly being subjected to a wide variety of stresses and strains, which results from the actions of forces from outside and within the ship. these forces may initially be classified into
static or
dynamic. forces
are due to the
difference in the weight and buoyancy,
which occur through out the ship
static forces
are cause by the
motion of the ship at sea and the action of
the wind and wave
dynamic forces
They result from
* The ship’s motion at sea.
* The action of wind and waves.
* The effects of operating machinery.
dynamic forces
These are due to
* Internal forces resulting from structural weight, cargo and machinery weight.
* External static forces including the hydrostatic pressure of the water on the hull.
STATIC FORCES
ship movement of dynamic forces
six degrees of freedom ( three linear and three rotational)
is the motion of the ship when the ship have being up by a wave or sea.
Heave
is the swing of a mast or bow of a ship from side to side as the vessel progresses in a heavy sea.
Sway
is the movement forward as the bow of a ship rises and dips when it encounter waves which are strong enough to life it.
Surge
is the motion of a ship in rising the crest of a wave then descending into the following trough.
Pitch
is the motion of a ship from side to side as she moves through the water.
Roll
is where the bow of a ship falls away or sways erratically from side to side as the vessel moves through the water.
Yaw
Forces produce stresses in the ship’s structure which may be divided into two categories:
*Global stress, Local stress
- affects the whole ship
Global stress
affects a particular part of a ship
Local stress
When a ship rolls in a seaway, it results in forces in the
structure tending to distort it transversely and may cause
deformation at the corners. The deck tends to move
laterally relative to the bottom structure, and the shell on
one side to move vertically relative to the other side. This
type of deformation is referred to as
“racking”
When a ship is _, the accelerations on the ship’s structure are liable to cause distortion in the transverse section.
rolling
Greatest effect is under light ship condition
“racking”
When any body is subjected to a twisting moment, which
is commonly referred to as torque, that body is said to be
in „
torsion‟
. A ship heading obliquely to a wave will be
subjected to righting moments of opposite direction at its
ends twisting the hull and putting it in
torsion‟
In most
ships, _ are negligible but
in ships with extremely wide and long deck openings
they are significant.
torsional moments and stresses
A ship traversing a wave train at angle will be subject to righting moments of opposite directions at its ends.
torsion.
The hull is subject to a twisting moment and the structure is in
torsion.
increases with depth and tends to set in
the ship‟s plating below the water line
Water pressure
_ is subjected to a static pressure from
the surrounding water in addition to the loading resulting
from the weight of the structure, cargo, etc
transverse
section of a ship
are of lesser magnitude than
longitudinal stresses, considerable distortion of the
structure could occur, in absence of adequate stiffening.
transverse stresses
tends to set the keel upwards because of
the up-thrust of the keel blocks.
Dry- docking
There is a tendency for
the ship‟s sides to bulge outwards and for the bilges to
sag.
Dry- docking
- Tends to set the keel upwards.
- Due to the up-thrust of the keel blocks.
- Tendency for the ship’s sides to bulge outwards.
- Bilges tend to sag.
stresses due to dry docking
This is a stress which occurs at the ends of a vessel due to variations in water pressure on the shell plating as the vessel pitches in a seaway.
panting
The effect is accentuated at the bow when making headway
panting
can give
rise to stresses due to localized distortion of the
transverse section
Heavy weights, such as equipment in the machinery
spaces are particular items of general cargo,
The fitting of transverse bulkheads,
deep plate floors and web frames
reduce such stresses
tends to distort the ship’s structure
localized loads
may give rise to localized distortion of the transverse section.
localized heavy loads
creates areas of high local stress due to lack of continuity of structure
deck opening
is a localized area
in a structure at which the stress is
significantly higher than in the
surrounding material.
stress concentration
Two types of discontinuity in ships are
* built into ship unintentionally by the
methods of construction e.g.
rolling,
welding, casting.
The high stresses at the corner of the
hatch may result in
cracking
If the ends of the superstructures
are ended abruptly, there is a major
discontinuity of the ships structure, which
may give rise to localized stresses
resulting in
cracking of the plating
is the mutual actual between the parts of a
material to preserve their relative positions when
external loads are applied to the material
stress
are used to calculate the stress in a
material under different loads.
Stress tables
They provide information on the
maximum allowable stress levels for different
parts of the hull, which helps ensure the ship’s
structural integrity and safety. To interpret a ship’s
stability conditions, the _ are used to
determine the stress on the hull due to the weight
of the ship and the forces acting on it.
stress tables
The relationship between weight and volume is
called _ It is defined as ‘mass per unit
volume’.
density
One metric tonne of fresh water has a volume
of
one cubic metre
The is _
defined as the ratio of the weight of the substance to the
weight of an equal volume of fresh water. In other words,
it is simply a comparison of the density of a substance
with the density of fresh water
relative density (or specific gravity) of a substance
DEFINED AS THE RATIO BETWEEN THE DENSITY OF ANY LIQUID
TO THE DENSITY OF FRESH WATER.
RELATIVE DENSITY:
THE MASS PER UNIT VOLUME MEASURED IN
Kg/m3 OR TON/m3
DENSITY :
There are two main types
of density used in stability
calculations
The density of Fresh Water, The density of Salt Water
To measure the weight
of a ship the density of
the water in which it is
floating is required. This
can be found by the use
of a
hydrometer
A RELATION BETWEEN THE DENSITY & VOLUME WOULD BE
;
INV. PROPORTION
A RELATION BETWEEN THE DENSITY & MASS WOULD BE ;
DIRECT PROPORTION
DENSITY
must be tested at least
once every three months by
simulated loading condition
excerpted from the Loading Manual
and results compared.
The loading computer - (approved
or otherwise)
– A
steady angle of heel
created by an
external force, such
as wind or waves.
Angle of heel
– A steady angle of
heel created by forces within the ship. For
example, when the ship is inclined due to
her asymmetric construction, or by shifting
a weight transversely within the ship. The
list reduces of ship’s stability. Therefore it
is essential to keep the ship upright by
asymmetrical distribution of masses
Angle of the list, list
The angle at which a ship
with a negative initial
metacentric height will lie at
rest in still water. In a seaway,
such a ship will oscillate
between the angle of loll on
SB and the one on PS.
Angle of loll –
can be corrected only by lowering the
centre of gravity, not by moving loads
transversely. It can be done by moving
weight downwards, adding water
ballast in double bottom tanks, or
removing weight above the ship’s
vertical centre of gravity
An angle of loll
- is a
process by which sea water is
taken in and out of the ship when
the ship is at the port or at the
sea. The sea water carried by the
ship is known as ballast water
Ballasting or de-ballasting
- is sea
water carried by a vessel in its
ballast tanks to ensure its trim,
stability and structural integrity.
Ballast tanks are constructed in
ships with piping system and high
capacity ballast pumps to carry
out the operation.
Ballast or ballast water
- In ancient times, ships
used to carry solid ballast for
stability as the cargo was minimal
or there was no cargo to be
carried. However, as time passed
difficulties were faced during
loading and discharging of solid
cargo.
Ballasting
The process of transferring of solid
cargo was also time-consuming and
for this reason, solid ballast was
replaced by water ballast. As sea
water was readily available and in
huge amount, it was used for the
ballasting and de-ballasting process.
Ballasting –
is required
when the ship is to enter a channel,
cross any canal-like Panama canal
and Suez Canal, during loading or
unloading of cargo, and when ship is
going for berthing.
Ballasting or de-ballasting
IN ORDER TO UNDER STAND THE EFFECT WE
SHOULD VERY WELL UNDERSTAND THE
PLIMSOL MARK ( DRAFT MEASURES)
is a
marking indicating the extent to which the
weight of a load may safely submerge a
ship, by way of a waterline limit. It is
positioned amidships on both sides of a
vessel’s hull and indicates the draft of the
ship and the legal limit to which a ship may
be loaded for specific geographical areas
and seasons of the year.
load line, also called Plimsoll mark,
is a reference mark located on a
ship’s hull that indicates the maximum depth to
which the vessel may be safely immersed when
loaded with cargo. This depth varies with a ship’s
dimensions, type of cargo, time of year, and the
water densities encountered in port and at sea.
Plimsoll line
is to ensure that a ship
has sufficient freeboard (the height from the
waterline to the main deck) and thus sufficient
reserve buoyancy (volume of ship above the
waterline).
It should also ensure adequate stability and avoid
excessive stress on the ship’s hull as a result of
overloading.
load line
is a special marking positioned
amidships. All vessels of _ are required to have this Load line
marking at the centre position of the length
of summer load water line
Load Line, 24 meters and
more
There are two types of Load line markings:
Standard Load Line marking, Timber Load Line Markings
– This is
applicable to all types of vessels
Standard Load Line marking
– This is
applicable to vessels carrying timber
cargo
Timber Load Line Markings
These marks shall be punched on the surface of the hull
making it visible even if the ship side paint fades out.
The marks shall again be painted with_ on a dark background / black on a light
background.
white or yellow
colour
– It is a horizontal line measuring 300mm by
25mm. It passes through the upper surface of the
freeboard
Deck Line
– It is 300mm diameter and 25mm thick
round shaped disc. It is intersected by a horizontal line.
The upper edge of the horizontal line marks the ‘Summer
salt water line’ also known as
Load Line Disc, ‘Plimsol Line
are horizontal lines extending
forward and aft from a vertical line placed at a distance of
540mm from the centre of the disc. They measure 230mm
by 25mm. The upper surfaces of the load lines indicate
the maximum depths to which the ships maybe
submerged in different seasons and circumstances
– Load lines
indicated by the
upper edge of the line which passes
through the centre of the ring and also by a
line marked S
Summer Load Line
indicated by the
upper edge of a line marked W.
The Winter Load Line
indicated by the upper edge of a line marked WNA. It is
marked 50mm below the Winter load line. It applies to
voyages in North Atlantic ( above 36 degrees of latitude)
during winter months.
Winter North Atlantic Load Line
indicated by the upper
edge of a line marked T.
Tropical Load Line
indicated by the upper edge of a line
marked F
Fresh Water Load Line in summer
indicated by the upper edge of a line
marked TF, and marked abaft the vertical line
Tropical Fresh Water Load Line
:- Its upper edge marks the
summer salt water timber loadline. It is situated
at a specified level above the Plimsol line.
LS – Lumber Summer
:- It is 1/36th of the lumber
summer draft below LS
LW – Lumber Winter
:- It is 1/48th of the lumber
summer draft above LS.
LT – Lumber Tropical
:- It is at
the same level as WNA.
LWNA – Lumber Winter North Atlantic
:- It is situated above
the LS by an amount equal FWA
LF – Lumber Fresh water
- It is
positioned above LT by an amount equal to FWA.
LTF – Lumber Tropical Fresh Water :
Every ship that has been surveyed and marked in
accordance with the present Load line convention are
issued by the authorized administration, an
International Load Line Certificate.
The certificate will
have a validity of _ and will
contain all vital information that includes the assigned
freeboard and fresh water allowance
not more than 5 years
is the difference between fwd and aft draft
TRIM
is when the aft draught is larger than the fwd
draught
Aft Trim
is when the fwd draught is larger than the aft draught
FWD Trim
The three basic objectives of the damage control are:
*PREVENTION,
*MINIMIZATION,
*RESTORATION
means to take all practical preliminary measures, such
as maintaining watertight integrity, providing reserve buoyancy and
stability before damage occurs
Prevention
is to minimize and localize damage by taking
measures to control flooding, preserve stability and buoyancy
Minimization
is to accomplish as quickly as possible, emergency
repair or restoration after occurrence of damage. Restoration
requires regaining a safe margin of stability and buoyancy
Restoration
One of the most important damage control measures is
to
control flooding
The primary duty of the damage control organization is
to
control damage
There are two major types of flooding:
*SOLID
*PARTIAL
If the ship has received severe underwater
damage, compartments will be badly ruptured and
completely flooded. Little or nothing can be done to
correct this damage. Isolate the compartments to
permit concentration on compartments that can be
repaired to prevent progressive flooding.
SOLID:
refers to a compartment that is
completely filled from deck to overhead.)
Solid flooding
refers to a condition in which an
intact compartment is not completely flooded.An
“intact compartment” means that the deck on which
the water rests and the bulkheads that surround it
remain watertight. If the boundaries remain intact,
water will neither run into nor out of the flooded
compartment as the ship rolls.
Partial flooding
The final result of _ is usually a decided loss in overall stability
partial
flooding
has
no other effect than to add weight at the center of
gravity of the ship
Solid flooding
are the bulkheads and decks
restricting the partially flooded area from the flooding
boundary
Flooding boundaries
usually results in
the entrance of a great mass of water with extensive
free surface, the combined result of which is a
reduction of stability
substantial underwater explosion
List, or capsizing in the ultimate case, is due to
negative GM, or unsymmetrical flooding, or a
combination or both.
Flooding in the middle length increases
sagging
stresses
flooding at the ends increase
hogging
stresses.
