STABILITY DEFINITIONS

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Stability Definitions Centre of Gravity A point on the vessel through which all forces of gravity act vertical downwards Forces of Graphic All forces of gravity acting vertically downwards Centre of Buoyancy A point on the vessel through which all forces of buoyancy act vertically upwards equal to the water displaced Forces of Buoyancy A floating body experiences an upward force equal to the water it displaces Metacentre A point on the centre-line of a vessel through which all the forces of buoyancy pass when the vessel is heeled Righting Lever When the vessel is heeled by an external force, the centre of buoyancy/centre of gravity are not in the same line, now a horizontal distance exists, the buoyancy pushing the vessel upright (the righting lever Gz) Metacentric Height The distance from the Centre of Gravity to the Metacentre (G.M.) Height of the Metacentre The distance from the Keel to the Metacentre (K.M.) Displacement Is the total weight of the vessel equal to the water it displaces (Displacement = Lightship + deadweight Draught The vertical distance from the Keel to the waterline Freeboard The vertical distance from the waterline to the lowest deck-edge Under keel allowance The distance from the keel to the seabed Trim This is the difference between the fore and aft draughts Mean Draft This is the forward and aft draft added together and divided by the number 2 Stable Equilibrium This is when a vessel has a positive righting lever (G below M) Neutral Equilibrium This is when the vessel has no righting lever (G & M together) (Danger of Capsize) Unstable Equilibrium This is when the vessel has a negative righting lever (G above M) (Capsizing lever) Stiff Vessel This is a vessel with a very large righting lever (G near the Keel) Tender Vessel This is a vessel with a vessel small righting lever (G very near M) Angle of Loll This is a vessel that is initial unstable but when heeled has a vessel small righting lever (Very dangerous condition, get rid of any weights on deck either by putting it overboard or down into the hold) (Caution watch an angle of loll through ice accretion, always take the ice off all rigging first the from the high side and push it towards the low side giving you a bigger list but your forces of buoyancy work harder to keep your vessel upright) List A list is caused by you moving anything on the vessel to one side Curve of Statical Stability this is a curve that shows the following : (1.) angle of maximum stability (2.) maximum g.z. (3.) the righting lever at any angle (4.) angle of vanishing stability (5.) the range of stability (6.) angle where deck-edge immersion begins (7.) the amount of dynamic stability a vessel has (8.) the point of contra flexure (9.) the angle of inclination (10.) the initial g.m. (11.) the radians for that vessel Stability This is an act of keeping the vessel stable Transverse or Statical Stability The vessels ability to return to the upright position Reserve Buoyancy This is the volume of air trapped in a watertight space above the waterline Centre of Floatation This is the centre of the water-plane area of a vessel at any draught Deadweight This is the cargo, stores water, fuel that you've taken aboard Light Displacement The total weight of the vessel, machinery etc that stays on the vessel and cannot be moved, (stores, fuel water etc not included) Lightship The total weight of the vessel, machinery etc that stays on the vessel and cannot be moved, (stores, fuel water etc not included) A righting moment or a moment of statical stability The total weight X the righting lever (Gz) A moment A moment = weight x distance Loaded weight regarding the centre of gravity When a weight is loaded onto a vessel the centre of gravity moves towards it Discharged weight regarding the centre of gravity When a weight is discharged from a vessel the centre of gravity goes back to where it was before the weight came on board (Opposite direction from where the weight was placed at on the vessel) Shifted weight regarding the centre of gravity When a weight is shifted on a vessel the centre of gravity moves from where the weight was to the weights new position Dynamic stability The amount of work taken to bring a vessel back to its upright position Range of positive stability This is on a curve of statical stability , where the curve starts on the angle of inclination to where the curve stops at the point of vanishing stability Angle of vanishing stability This is on the curve of statical stability and where the curve comes down and has no (g.z.) ( + or - ) then this is where stability vanishes Initial GM This is on the curve of statical stability, on the angle of inclination at 57.3 degrees there is a radian line , and a tangent line which starts from 0 degrees and leaves the first arc of the curve of statical stability and where the tangent line and the radian line at 57.3 degrees meet then this is the initial g.m. Angle of Maximum stability This is on the curve of statical stability, on the curve itself at the top of the curve down to the angle of inclination and this is the angle of maximum stability Maximum GZ (on curve of static stability) This is on the curve of statical stability, at the top of the curve look at the distance on the scale (metres) and this is the maximum g.z. Importance of adequate freeboard With freeboard raised then this will give you (1.) a greater range of stability (2.) a greater range of vanishing stability (3.) a greater maximum g.z. (4.) the maximum g.z. occurs at a greater angle (5.) greater dynamic stability Density The mass of any object expressed in cubic metres (i.e.) a dice is length x breadth x width = Volume of displacement This is where the vessel is equal to the water displaced and expressed in cubic metres