STABILITY
The static stability of a ship is the measure of her tendency to return to the upright after being inclined by external forces such as wind or waves. A ship's stability is influenced by her underwater form, or shape, and by the amount and position of the weights or loading placed aboard the ship. Determining the dimensions, proportions, and shape of the hull for stability in a properly loaded ship is the business of the ship designer. It is the ship's Master and Chief Mate, who have control over her loading. You must understand the basic principles of stability to avoid loading conditions that will produce too little or too much of it. There are several programs out that help in working with stability, this is just some basic things that are good to know.
WEIGHT AND BUOYANCY - A ship when afloat is acted on by two principal forces, its weight and its buoyancy, that are equal to each other but act in opposite directions. The weight acts downward through a point called the center of gravity. The buoyancy acts upward through the center of buoyancy.
To find the height of the center of gravity of a ship above the keel a very simple principle is used, but it can be complex and time consuming. The weight of each sizeable piece of hull or machinery is multiplied by its distance above the keel. This gives a number of things when added together equal the moment sum. The sum of the weights of all these parts is also made and is the total weight of the ship, This is called ships displacement. Height of the ship's center of gravity above the keel is the moment sum divided by the ship's displacement.
DISPLACEMENT - Should a graving dock be filled with water to the top of the gates and a launch lowered by crane into the water some water will then flow over the top of the gates. If this water is caught in tanks and weighed the weight will be found to be the same as the launch. And the volume is the same as the underwater part of the launch. In other words the vessel removes or displaces a weight of water equal to its own weight. I should mention a displacement ton is 2,240 pounds. The relation of the volume of the underwater part of the vessel to a rectangular block of the same length, beam, and depth is called the fineness coefficient. The displacement of a vessel is then the product of the length, beam, draft, and fineness coefficient all divided by thirty-five. In ship stability calculations the center of gravity of the volume of the underwater part of the ship is very important.
The position of this center of buoyancy fore and aft is called the longitudinal center of buoyancy. The position of the center of buoyancy measured in a vertical direction above the keel, or in some cases from the plane of water line is called the vertical center of buoyancy. The same is true of all the usual surface ships.
WORKING BUOYANCY - is the amount of buoyancy available for carrying of cargo, that is the difference in displacement in tons between the ship when light and when loaded down to the Plimsoll mark.
The center of buoyancy curve is a curve showing the height of the vertical center of buoyancy for various drafts of the ship. The freeboard of a ship is the distance from the top of the freeboard deck to the water line. The freeboard deck is the uppermost complete deck having permanent means of closing all openings in weather portions of the deck. Freeboard is measured at the center of the ship and if necessary the top of the freeboard deck plank is continued through the water way.
SPECIFIC GRAVITY - is the weight of a given number of cubic inches of a given material divided by the weight of the same number of cubic inches of water. Inertia is the tendency of a body in motion to continue in motion, and if at rest to continue at rest. The moment of inertia of a plane about an axis is the sum of the products of each small part of the surface multiplied by the square of the distance of each part from the axis.
The polar moment of inertia which is used for period of roll is the sum of the weights of all parts of the ship multiplied by the squares of each individual distance from a horizontal line passing fore and aft and through the ship's center of gravity.
The longitudinal metacenter is similar to the transverse metacenter except in a fore-and-aft direction but is not of much importance because the period of pitching is so very small compared with the periods of the waves that there is no synchronism set up. The period of seconds of a complete roll of a ship, that is a roll from port to starboard and back to port again
SYNCHRONISM - Is the condition resulting from waves reaching the ship in such succession that each catches the ship at the same period of roll and results in a rapidly increasing amplitude. This can become so dangerous that to avoid it the ship's course must be changed.
METACENTRIC CURVES - Are curves showing the heights above the keel of metacenters and centers of buoyancy for varying displacements of the ship.
METACENTRIC HEIGHT - Transverse metacentric height is a measure that determines a ship's initial stability. The larger it is in positive value the "stiffer" the ship will be. Should it be too great the ship will be very uncomfortable and if it is too small the ship will be "tender" and her safety may not be what it should be.The distance between the two lines is called righting lever. Which is a kind of force tending to right the ship at any time, is the displacement of the ship multiplied by its righting lever.
WEDGE OF IMMERSION - is the wedge of the ship which will be immersed when the ship is inclined and the wedge of emersion is the corresponding wedge of the ship on the opposite side which will emerge from the water during the operation.
The distance between the center of buoyancy and the transverse metacenter equals the moment of inertia of the water plane about its center line.
TONS PER INCH OF IMMERSION - is given in curves showing the displacement change for every inch of increased draft.
Stability studies relate to the tendency of a ship when inclined to resume its upright position. Statical stability is the product of the righting arm multiplied by the displacement.
INITIAL STABILITY - is the stability of a ship at small angles of inclination as measured by metacentric height and displacement. The curve of stability is a curve giving the righting lever in feet for various large angles of inclination.
RANGE OF STABILITY - is the angle of ship's heel in degrees at which the righting arm disappears and a ship is just as likely to capsize as to right itself. This is only of meaning if all openings by which flooding can take place are closed, and is in general greatest in ships of large freeboard.
DYNAMICAL STABILITY - of a ship is the work done in inclining the ship to any angle and equals the displacement multiplied by the increased vertical separation of the centers of gravity and buoyancy of the ship from upright to an inclined position.
For every ship stability curves are worked out for various drafts and loadings.
The longitudinal metacentric height is computed similarly to the transverse metacentric height except that in computing the moment of inertia of the water plane is taken about an athwartshlp axis passing through or directly above the center of gravity. The longitudinal metacentric height is used in calculating changes of trim. A change of trim is the sum of the change of drafts forward and aft and changes of trim can be caused by a shift of weights on board in a fore-and-aft direction.
