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.

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