Glossary of
Stability and Trim Terms
A
B C
D E
F G H
I J
K L
M N
O P
Q R
S T
U V
W X
Y Z
A
Added Weight
Method: A method of solving for damage stability where the water
which enters the vessel is considered as an added weight.
Anti-rolling
Devices: These include the bilge keel, or rolling chocks,
anti-rolling tanks, gyro stabilizer, and stabilizing fins.
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B
B - Symbol for
center of buoyancy.
Beam-draft
Ratio: Ratio of beam to draft. This ratio has an important bearing
on the height and movement of M.
Block
Coefficient: A coefficient of fineness which expresses the
relationship between the volume of displacement and a block having
the length, breadth, and draft of the vessel.
BM: Symbols for
metacentric radius; distance between B and M.
Buoyancy: The
upward vertical support supplied by the liquid in which a vessel is
floating.
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C
Capacity
Calculator: A device which calculates quickly the capacity of a
hold at any point in the loading of the hold.
Capsize: To
"turn turtle" due to loss of transverse stability.
Center of
Buoyancy: 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
Flotation: The center of gravity of the waterplane; the point
around which a vessel trims.
Center of
Gravity: That point at which all the vertically downward forces of
weight are considered to be concentrated; the center of the mass of
the vessel.
Change of Trim:
The algebraic sum of the initial trim and the trim after weight has
been shifted, loaded, or discharged.
Compartment
Standard: The number of compartments in any location which can be
flooded up to the margin line without causing the vessel to sink.
Based on certain permeability, usually 63 percent for cargo spaces
and 80 percent for machinery space.
Couple Moment:
Created by two equal forces exerted in opposite directions and along
parallel lines. In stability, the forces through G and B.
Crank Ship: A
vessel with small metacentric height; top-heavy.
Criterion of
Service Numeral: A number (usually between 23 and 123) based on the
dimensions and service in which the vessel is engaged, which is used
to obtain the subdivision requirements for a vessel.
Cross Curves of
Stability: Curves for various angles of inclination up to 90
degrees. The ordinates are displacements. Intersections of ordinates
with curves produce the abscissas (righting arms).
Curves of Form:
See Hydrostatic Curves.
Curves of
Statical Stability: See Statical Stability Curves.
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D
Damage
Stability: Stability of a vessel after flooding.
Dead Rise: The
distance the bottom rises in one half the beam.
Deadweight
Scale: A scale of values of TPI, MT1, displacement, and deadweight,
for all drafts.
Deep Tank: A
tank usually used for carrying sea-water ballast, but also used for
carriage of fuel oil and cargo.
Deflection:
Distance along the batten that the inclining experiment pendulum
moves in a horizontal direction after inclination.
Degree of
Subdivision: A relative term expressing the relation of actual
subdivision to required compartment standard.
Density:
The density of any
given material is the weight of that material that occupies a given
volume (i.e., weight per unit volume). Therefore, density may be
expressed as the weight per gallon or weight per cubic foot or
weight per other unit of volume of any liquid or bulk (solid)
material.
Using a common example: fresh water has a density of 62.4 lbs. per
cubic foot This means that a cube of fresh water, one foot on each
side, weighs 62.4 lbs. Salt water is more dense than fresh water
with a density of 64.0 lbs. per cubic foot.
Displacement:
The weight of water displaced by a floating object. Equal to the
weight of the object.
Draft: The
linear distance from the bottom of the keel to the waterline.
Dynamical
Stability: The energy which a vessel possesses to right herself due
to the work performed in inclining her.
Dynamic:
Referring to movement.
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E
Equilibrium:
Vessel is in a state where there is no movement; G must be in the
same vertical line with B.
Even Keel: A
condition when the draft forward is the same as the draft aft.
Expansion
Trunk: Narrow upper part of tank on an oil tanker, used as an
allowance for expansion of oil at high temperatures.
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F
Factor of
Subdivision: A number less than 1 obtained from curves of factor of
subdivision which, when multiplied by floodable length produces
permissible length of compartment. It is the reciprocal of the
compartment standard.
Flare: The
outward curvature of a ship's side.
Floodable
Length: At any point of a ship, the length of the space having its
center at that point, which can be flooded without causing the ship
to sink.
Founder: To
sink due to loss of reserve buoyancy.
Freeboard: The
distance from the waterline to the upper deck.
Free Surface:
Condition existing when a liquid is free to move in the tank or
compartment of a vessel. Causes a virtual rise of the ship's center
of gravity.
FSC (free surface correction)
FSM (free surface moment)
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G
G: Symbol for
center of gravity.
GG: Distance
that the center of gravity moves due to weight movement or free
surface of liquid.
GM: Metacentric
height; distance from the center of gravity to the metacenter.
GZ: Righting
arm or lever. Distance between lines of force through G and B.
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H
Heel: The
transverse angle of inclination of a vessel.
Heeling Moment:
The moment tending to heel the vessel. Opposed by the righting
moment.
Heterogeneous
Cargo: Cargo of a varied nature; general cargo.
Homogeneous
Cargo: Cargo of the same density throughout.
Hydrostatic
Curves: Curves based on the form of the immersed portions of a
vessel. They include: Coefficients of fineness, TPI,
displacement in salt and fresh water, MT1, Height of B and M above the keel, Increase of displacement for one foot
trim by the stern.
Hold: The
large, bottom space of a cargo compartment of a vessel.
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I
I: Symbol for
moment of inertia.
Initial
Stability: Stability of a vessel for small angles of inclination
(up to 15 degrees).
Inner Bottom:
Tank tops over double bottom tanks, forming an inner skin for the
vessel.
Intact
Buoyancy: Intact space below the surface of a flooded area.
Inclining
Experiment: Experiment, which by inclining a vessel a few degrees,
produces with the aid of a formula the metacentric height (GM) and
the position of the center of gravity of a vessel.
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J
K
K - Symbol for
keel.
KB - Linear
distance from the keel to the center of buoyancy. (When vessel is
upright.)
KG: Height of
center of gravity above keel.
KM: Height of
metacenter above keel.
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L
Light
Displacement: Weight in long tons of vessel in a light condition.
List:
Transverse angle of inclination of a vessel.
Load
Displacement: Weight of vessel in long tons when fully loaded.
Longitudinal Center of Gravity
(abbreviated LCG) is a term that may be applied to your vessel as a
whole or to any individual tank or compartment within your vessel.
When applied to the vessel as a whole, LCG refers to the
longitudinal position of the center of gravity or the location of
the vessel's center of gravity in relation to the bow and stern.
When applied to a tank or compartment, LCG refers to the
longitudinal distance of a point within that tank or compartment
from the vessel's tipping center.
Longitudinal
Stability: Tendency of a vessel to return to its original
longitudinal position. Longitudinal stability terms: Longitudinal metacenter, GML, BML, Center of buoyancy, Center of gravity.
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M
M: Symbol for
metacenter.
Mean Draft:
That draft midway between the draft forward and draft aft.
Metacenter: 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.
Metacentric
Height: Distance between G and M. Used as a measure of initial
stability.
Metacentric
Radius: Distance between B and M.
Moment: Created
by a force or weight moved through a distance.
MT1: The moment
necessary to change the trim of the vessel one inch.
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N
Negative
Stability: Exists when G is above M, that is, when there exists a
negative GM or GZ.
Neutral
Equilibrium: 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.
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O
P
Parallel Sinkage:
Vessel increases her draft so that the drafts forward and aft are
increased by the same amount; increase of draft without change of
trim.
Permeability:
The percentage of the volume of a compartment which can be occupied
by water if flooded.
Permeability of
Surface: The percentage of the surface of a flooded compartment
which is occupied by water.
Peak: Found at
the ends of the vessel are the fore peak and after peak spaces.
Permissible
Length: Maximum length permitted between main, transverse
bulkheads. Found by multiplying factor of subdivision by floodable
length.
Plimsoll Mark:
The load line which is stenciled in and painted amidships on the
side plating.
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Q
R
Ralston
Stability and Trim Indicator: A device for calculating the
stability and trim of a vessel by adding or removing representative
weights on a metal profile of the vessel.
Range of
Stability: The end of the range of stability is reached at an angle
of inclination when the righting arm is equal to zero. Practically,
the range of stability is ended shortly after deck edge immersion in
most vessels.
Reserve
Buoyancy: The volume of all intact space above the waterline.
Righting Arm:
The distance between the line of force through B and the line of
force through G, when there is positive stability.
Righting
Moment: The product of the weight of the vessel (displacement) and
the righting arm (GZ).
Rolling Period:
The time it takes a vessel to make a complete roll, that is, from
port to starboard and back to port again.
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S
Settlers: Tanks
used for "settling" fuel oil before using.
Slack Tank:
Tank which is not completely filled or empty.
Specific Gravity of a body (which may be a
solid, liquid or gas) is the ratio
(i.e., a comparison) of its density to the density of pure fresh
water at a temperature of 4° Celsius. The temperature of 4° Celsius
is specified because water is at its most dense state at that
particular temperature. You can determine specific gravity by
dividing the weight of salt water or any other liquid or solid by
the weight of an equal volume (i.e., the same number of gallons) of
fresh water.
Sponsons:
Bulges on the upper sides of canoes and other small boats which add
breadth when the boat inclines.
Stability: The
tendency of a vessel to return to an erect position after being
inclined by an exterior force.
Stability
Tables: Tables which show the proper and improper distribution of
weights and their effect on the GM and rolling period of a vessel.
Stabilogauge: A
device which automatically calculates GM when actuators indicating
weights loaded or discharged are turned.
Statical
Stability Curves: Curves for various displacements up to and past
load displacement. The ordinates are angles of inclination.
Intersection of ordinates with curves produces -the abscissas
(righting arms).
Stable
Equilibrium: Exists when M is above G. A vessel will tend to return
to an erect position if inclined to a small angle.
Stiff Ship:
Vessel with low center of gravity and large metacentric height.
Swash Bulkhead:
Longitudinal bulkhead, with or without lightening holes, installed
for the purpose of reducing free surface effects.
Synchronous
Rolling: Occurs when the rolling period of the vessel is the same
as the wave period; a condition to be avoided.
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T
Table of
Stability: See Stability Tables.
Tender Ship:
See Crank Ship.
TIPPING CENTER
OR
LONGITUDINAL
CENTER OF FLOTATION (LCF)
These two terms are
essentially the same and refer to the point around which a vessel
"trims". Trim is
defined as the
difference between the draft forward and the draft aft. One
practical and well-known method of
"trimming" an
offshore supply vessel, for example, is to pump seawater into either
her forepeak or afterpeak (i.e., "wraparound") ballast tanks.
TPI: Number of
tons necessary to change the mean draft of a vessel one inch; varies
with draft.
Transverse
Metacenter: See Metacenter.
Transverse Center of Gravity is
referenced in the transverse (athwartships) direction from the
centerline of the ship and is labeled TCG.
Trim:
Difference between the drafts forward and aft.
Trim
Calculator: A device which calculates quickly the trim of a vessel
after loading or discharging.
Trimming
Tables: Tables which calculate change of mean draft and change of
trim after loading, discharging, or shifting of weights.
Trimming Tanks:
The forward and after peak tanks.
Tumble Home:
The inward curvature of a vessel's sides.
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U
Unstable
Equilibrium: Exists when G is above M. Vessel does not tend to
return to an erect position after being inclined but, for small
angles, tends to continue inclination.
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V
Vertical Center
of Gravity (VCG): The vertical height of the center of gravity of a
compartment above its bottom, or of the center of gravity of a
vessel above its keel. Vertical Center of Gravity is a term that may
be applied to your entire vessel. The term may also be applied to
any individual tank, compartment or weight on or within your
vessel.Consequently, VCG may be given a dual definition as the 1)
vertical height of the center of gravity of the contents of any tank
or compartment above its bottom, or 2) the vertical height of the
center of gravity of the entire vessel above its keel.
Virtual Rise of
G: Caused by the "swinging" motion of water in a slack tank.
Volume of
Displacement: The volume of water displaced by a floating object;
weight of this volume of water is equal to the weight of the object.
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W
Waterplane: The
plane defined by the intersection of the water in which a vessel is
floating with the vessel sides.
Waterplane
Coefficient: A coefficient of fineness which expresses the
relationship between the area of the water plane and a rectangle
having the length and breadth of the vessel at that waterplane.
Wing Ballast
Tank: Tank, usually located in the upper 'tween deck on either side
of the engine room casing, which is especially valuable in raising
the center of gravity of a light ship. These tanks also serve to
dampen the period of roll of a vessel. Any weights" winged out"
increase the" mass moment of inertia" of a vessel, thus
dampening
rolling.
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X
Y
Z