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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 |
