# ShipStability_Basic Concepts

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[MARITIME TRAINING SERVICES INC.]
[In case of any conflict between the requirements shown in the movie and the company's safety management system (SMS), please follow the company's SMS requirements]
[SHIP STABILITY BASIC CONCEPTS]
[G]
The center of gravity or G,
is the single point where the downward force of gravity acts.
The center of gravity is the combined effect
of the position and weight of everything on board a vessel.
The center of gravity moves toward any added weight,
away from any removed weight,
and in the same direction as any shift in weight.
On a properly managed vessel,
the center of gravity should be maintained on the center line for maximum stability.
Because it is a point in three dimensional space,
there are three coordinates used to describe its position.
[VERTICAL CENTER GRAVITY]
The vertical center of gravity
[VCG, KG]
or VCG or KG is measured upward from the keel.
[TRANSVERSE CENTER OF GRAVITY]
The transverse center of gravity,
[TCG]
TCG, is measured from the center line.
[LONGITUDINAL CENTER OF GRAVITY]
The longitudinal center of gravity, LCG,
[LCG]
is usually measured from the forward perpendicular or from midships.
Understanding where a vessels center of gravity is located
and how it moves is vital for a ships officer
because it is one property of a ships' stability that they have the most control over.
As cargo is loaded or ballast tanks are filled,
the center of gravity moves accordingly.
How well an officer understands this principal,
will keep his vessel afloat in heavy weather.
[B]
Center of buoyancy or B, is the single point
where the upward force of buoyancy acts.
It is located in the geometric center of the displaced volume
or the area of hull beneath the water line.
When a ship rolls in the water, the shape of the area beneath the water line changes.
The center of buoyancy is constantly moving
to stay in the center of that area.
The key to understanding stability
is understanding how the center of buoyancy moves.
[G, B]
G and B have equal force acting in opposite directions.
As a vessel rolls, the distance between the downward force of G
and the upward force of B
creates a righting moment that returns the vessel to the upright position.
[TRIM]
[G]
A moment is nothing more than a force or weight,
multiplied by its distance to a particular point.
Think of it as a lever.
A moment can have a greater effect because the distance increases
or because the force increases.
In the imperial system, moments are measured in foot tons.
[G, B]
The effect of the longitudinal difference between G and B is Trim.
Trim is defined as the difference between the forward and after drafts.
[LCG]
The opposing forces of G and B across the trim arm
[LCB, LCF]
create a trimming moment that causes the vessel
to trim around the LCF.
Keep in mind that the deeper draft of a ships trim
will always be on the same side of the trim arm as the LCG.
If the LCG is forward of the LCB,
then the forward draft increases.
If LCG is aft of LCB, then the after draft increases.
[MT1]
Moment to trim one inch, or MT1,
is the moment that change the trim of a vessel by one inch.
In the metric system, this is the moment to trim one centimeter
or MTC.
These final concepts are the core of ship stability and trim.
Every officer needs a working knowledge of these terms
in order to understand the stability of their vessel.
Heel is the angle a ship assumes as a result of an outside force
such as wind or waves.
Unlike heel, a vessel is said to list
if it is resting in the water at an angle without an outside influence.
Normally, this is due to what is called an asymmetrical load.
An uneven load that causes a vessel's center of gravity to move off of the center line.
A list should be quickly and easily corrected.
[G, B]
As we've seen, when a vessel rolls, B moves in the direction of the roll
and out from under G.
[METACENTER, M]
The metacenter or M,
is the intersection of the upward force of B when a vessel is upright
and when it is at an angle.
Think of it as the center of the ark that B moves through
as a vessel rolls in the water, like the anchoring point of a pendulum.
It is considered to be on the center line at small angles of heel.
The location of the metacenter
is provided for the ships officers by the Naval Architect
and is a key value in calculating stability.
The vertical distance between the center of buoyancy and the metacenter
[METACENTERIC RADIUS, BM]
is called the metacenteric radius or BM.
Once there is a horizontal distance between the center of gravity and the metacenteric radius,
the opposing forces of gravity and buoyancy
create a righting moment that returns the vessel to its upright position.
[GZ]
This horizontal distance is called the righting arm or GZ.
The righting moment, the rotational force that rights the ship
is the length of the righting arm
multiplied by the vessels total displacement.
[KM]
KM is the vertical position of M measured upward from the keel.
[METACENTRIC HEIGHT, GM]
Metacentric height or GM,
is the vertical distance between G and M.
When we put these things together, we see the ships' stability triangle.
This shows us that the larger the GM is,
the longer the righting arm, or GZ, will be.
The longer the righting arm is, the greater the force of the righting moment will be
and the ship will have a greater ability to right itself.
For this reason, GM is used as the standard measure
of a ships initial stability.
This is the stability for small angles of heel,
usually less than 7 or 10 degrees.
When a ship heels any more than that,
it's metacenter moves off of the center line and stability becomes a more complex problem.
Initial stability is still used as the standard measure of a vessels' stability however,
because if a vessel has enough initial stability,
it is believed to have sufficient stability for larger angles of roll.