principles of stability lrg
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GENERAL PRINCIPLES OF STABILITY
A Brief By Lance Grindley
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Objectives
Understand Archimedes’ principle Understand the following terms and how they relate to ship
stability– Center of Gravity– Center of Buoyancy– Reserve Buoyancy– Couple– Righting Arm– Righting Moment
Be able to define the terms related to how the ship moves Be able to explain the free surface effect and how it is
related to ship stability
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Archimedes Principle
It’s why ships float
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WATERLINE
DEPTH
OF
HU
LL
DR
AFT
FREEB
OA
RD
RESERVE BUOYANCY
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Terms
Center of Gravity – “G”– An imaginary point within the ship through which all gravity
can be seen to act– A spatially weighted average of smaller, individual weights
around the ship– Is this fixed or can it move?
Center of Buoyancy – “B”– An imaginary point within the ship through which the
buoyant force can be seen to act– Is this fixed or can it move?– How would you want your “B” to be positioned relative to
“G”?
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Terms (cont)
Metacenter – “M”– Intersection of two successive lines of action of the
buoyant force as the ship heels through a very small angle
– What does this tell us?• It’s position with respect to “G” (“GM” or metacentric height)
gives us an indication of stability
“Z”– A point along the line forming the metacenter that
creates a right triangle with “M” & “G”
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More Terms
Couple– Two forces of equal magnitude acting in opposite and
parallel directions– When inclining a ship, this couple produces either a
righting moment or an upsetting moment depending on the relative location of B and G
Righting Arm– GZ = GM (sin (angle)) (in feet)
Righting Moment– RM = Weight x GZ (W) (ft-tons)
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STABILITY CURVES
A plotted series of GZ values at successive angles
Factors involved– G does not change position as heeling
angle changes– B is always at the geometric center of
the volume of the underwater hull– The shape of the underwater hull
changes as heeling angle changes
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EFFECT OF DRAFT ON RIGHTING ARM
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How the ship moves
Roll– Back/forth motion transversely
Pitch– Back/forth motion longitudinally
Yaw– Side to Side
List– Diff. between Port and Stbd draft
Trim– Diff. between fwd/aft draft
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FREE SURFACE EFFECT
Occurs when a compartment or tank is only partially flooded/filled– As the ship rocks, water wants to maintain a
level condition– Causes “sloshing” of water– Momentum of water will bang against the
tank’s sides, further causing instability of the vessel in that direction
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Review/Summary
Define the following:– Trim– List– Pitch
What is the center of gravity? What is the metacenter and what does it tell us
about ship stability? State Archimedes’ principle. Why is it important? What is a righting arm?
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QUESTIONS ????