STABILITY AND TRIM

DDEFINITION OF TERMS

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Added Weight Method. A method of solving for damage stability where the water which enters the vessel is considered an added weight.

Block Coefficient. A coefficient of fineness which expresses the relationship between the volume of displacement and a block having the length, breadth, and draft the vessel.

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 . The 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 a certain permeability, usually 63% for cargo spaces and 80% 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 or Tender Ship. A vessel with small metacentric height; top-heavy.

Density. The weight per unit volume of a substance.

Displacement. The weight of water displaced by a floating object. Equal to the weight of the object.Downflooding Angle. The angle of heel where water first enter the hull of a vessel through an opening. This angle is the maximum angle at which the intact stability curves are valid.

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.

Equilibrium. Vessel in a state where there

is no movement; G must be in the same

vertical line with B.

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.

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.

GM. Metacentric height; distance from the center of gravity to the transverse metacenter.

Heel. The transverse angle of inclination of a vessel.

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Heeling Moment. The moment tending to heel the vessel. Opposed by the righting moment.

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, MTI,

height of B and M above the keel, increase of displacement for one foot trim by the stern.

Initial Stability. Stability of a vessel for small angles of inclination (up to 15 deg.)

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.

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 a light condition.

List. Transverse angle of inclination of a vessel.

Load Displacement. Weight of vessel in long tons when fully loaded.

Longitudinal Stability. The tendency of a vessel to return to its original longitudinal position.

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 Radius. Distance between B and M.

Moment. Created by a force or weight moved through a distance.

Negative Stability. Exist 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.

Permissible length. The maximum length permitted between main transverse bulkheads. Found by multiplying factor of subdivision by floodable length.

Range of Stability. The end of stability is reached at an angle of inclination when 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.

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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 complete roll, that is, from port to starboard and back to port again.

Stability. The tendency of a vessel return to an erect position after being inclined by an exterior force.

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 abscissae (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.

Synchronous Rolling. Occurs when the rolling period of the vessel is the same as the wave period; a condition to be avoided.

TCG. Transverse position of center of gravity.

TPI. Number of tons necessary to change the mean draft of a vessel by one inch; varies with draft.

Trim. The difference between the drafts forward and aft.

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.

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.

Volumetric Heeling Moment. A moment produced by a volume acting through a distance. Units of volumetric heeling moment are in feet, or meters. If a volumetric heeling moment is divided by a stowage factor, a weight moment will be produced, as done in the practice of stability calculation for bulk grain.

Water Plane. The plane defined by the intersection of the water in which a vessel is floating with the vessel sides.

Water Plane 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 water plane.

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