Glossary
of Stability and Trim Terms
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.
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.
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.
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.
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.
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)
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.