principles of stability lrg

Grunt Productions 2004

GENERAL PRINCIPLES OF STABILITY

A Brief By Lance Grindley


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


Archimedes Principle

It’s why ships float


WATERLINE

DEPTH

OF

HU

LL

DR

AFT

FREEB

OA

RD

RESERVE BUOYANCY


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”?


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”


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)


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


EFFECT OF DRAFT ON RIGHTING ARM


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


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


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?


QUESTIONS ????

principles of stability lrg

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