# Basic-Ship-Stability

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[MUSIC PLAYING]
January of 2006,
the tugboat, Valor,
was pulling a barge off the
coast of North Carolina when
the weather turned bad.
30 knot winds and 10 foot
waves buffeted the Valor,
and while she should have
been up to the challenge,
she instead developed a bad
list to port and capsized.
The Valor, along with three
of her crew, was lost.
A ship does more than simply
sit on top of the water.
It balances in it.
Understanding this balance is
critical for ships' officers.
They need to know how
to keep their vessel
upright in the water.
A ship's ability to keep itself
up right is called stability.
What keeps a ship afloat is the
force of buoyancy acting upward
against the hull.
A force equal to the weight
of the volume of water
it displaces.
But there's a twist.
The upward force
of buoyancy is not
in the same place as the
downward force of gravity.
The relative position of the
two is the essential balance
that all vessels must find and
is the root of their stability.
This video will
explore this balance
and explain how a vessel
rests in the water.
By taking the time to
understand these principles,
officers and sailors
alike will better
understand the stability
of their vessel
and how to best keep
it safely upright.
Working up from the
basics, this video
will build an understanding
of where a ship's stability
and trim come from.
[MUSIC PLAYING]
The following are
the standard terms
used to describe
the hull of a ship
even before it is
put into the water.
These first few terms
are fairly basic.
Depth is the height of
a hull from the highest
point of its main deck
to its lowest point.
In the other direction,
a ship's beam or breadth
is its width at
its widest point.
The centerline is
a vertical plane
that runs the length of the ship
at the midpoint of its beam,
and the baseline is the
horizontal plane perpendicular
to the centerline located at
the lowest point of the hull.
The keel is the principal
structural member
of a ship running lengthwise
along the centerline from bow
to stern to which the
ship's frames are attached.
The lowest point
of the keel, or K,
is the point from which
vertical distances
are measured on a ship.
K is located at the
intersection of the centerline
and the baseline.
The waterline is
the intersection
of the surface of the water
a ship is floating in,
with the sides of
the ship's hull.
When a ship is designed,
the naval architect
determines the design
waterline or DWL.
It represents the
waterline of a ship
under full load or maximum draft
conditions on an even keel.
The forward perpendicular
or FP is a vertical line
drawn at the intersection
of the design waterline
and the fore side of
the stem of the hull.
The after perpendicular
or AP is a vertical line
drawn at the intersection
of the design waterline
and the aft most point
of a ship's hull.
For most commercial
vessels, this
is generally where the
rudder post is located.
Midships is the
horizontal point halfway
between the forward
and aft perpendiculars.
And the length between
perpendiculars or LBP
is the total horizontal distance
between the forward and aft
perpendiculars.
Length overall or LOA
is the total length
of a ship at its longest point.
Note that this may be a
little longer than the LBP
because a ship can
extend slightly
past the perpendiculars.
Distances onboard
ships are measured
in one of three directions,
longitudinally, transversely,
and vertically.
Longitudinal is the
horizontal direction
along the length of a ship.
Longitudinal
distances are measured
from one of three places,
the forward perpendicular,
the aft perpendicular,
or midships.
Where longitudinal
measurements are taken from
will vary from ship to ship.
Transverse is the
horizontal direction
across the beam of a ship.
Transverse distances
are measured port
or starboard from
the centerline,
with one written as a positive
distance and the other as
negative.
It is not standard
which is which however,
and this varies from
ship to ship as well.
Vertical distances on a
ship are measured upward
from the baseline or
lowest point of the keel.
With these terms
in mind, we can now
look at how a ship
interacts with the water.
These concepts are the
foundation of stability
and trim.
What holds a ship above the
water is the force of buoyancy.
A force that equals the
weight of the water the ship
displaces.
Displacement is the amount of
water pushed aside or displaced
by a ship when it is floating.
A ship's displacement is always
equal to the total weight
and is measured in tons.
Depending on your
vessel, ship stability
can be calculated in either
the metric or imperial system.
So you must be
familiar with both.
Displacement was first
understood by the Greek thinker
Archimedes over 2000 years ago.
The king of Syracuse
had asked Archimedes
to determine if a crown he
had commissioned was pure gold
or if the jeweler
had cheated him
by mixing in some lesser metal.
While he was still
thinking about it,
Archimedes noticed the
water in his bathtub
rised as he stepped into it.
Eureka, he shouted and
ran naked into the streets
in his excitement.
By using displacement,
he understood
that he could measure the
exact volume of the crown.
This allowed him to
find its density,
telling him if the
metal was pure or not.
It wasn't.
Draft is the vertical
distance between the waterline
and the lowest
point of the hull.
A ship's draft can
be found or taken
by reading the draft marks that
are welded onto a ship's hull,
forward and aft.
Tons per inch of
immersion or TPI
is the number of tons necessary
to change the draft of a vessel
by one inch.
In the metric system, this
is tons per centimeter
of immersion or TPC.
Load lines are marks welded on
the side of the vessel's hull
at midships, showing the draft
under maximum safe loading
conditions.
This mark is called
the plimsoll mark.
If a vessel is overloaded
and its draft is deeper
than its load line,
it is unsafe and is
in violation of the
international load line
convention.
It will be subject to fines
or other legal actions,
and its insurance is void.
Trim is the difference in
draft, forward, and aft.
The trim of a vessel can be
found by reading the draft
marks on its hull.
These draft marks are placed
as close to the perpendiculars
as the shape of the hull allows.
A vessel's freeboard is
the vertical distance
between the waterline and
the highest watertight
deck of the hull,
usually the main deck.
Freeboard is
important because it
is part of what determines
the volume of the space
above the water line.
The waterplane area
or the AWP of a vessel
is the horizontal
intersection of the waterplane
and the vessel's sides.
The bigger a vessel's
waterplane area
is, the greater the
surface of the hull
is that buoyancy will
be acting against.
For this reason, in
the case of two vessels
with the same weight
added, the one
with the greater
waterplane area will
have a smaller change in draft.
As a vessel rolls in the
water, its waterplane area
increases as long as
there is freeboard
available to add to it.
The larger a vessel's
waterplane area
becomes when it rolls, the
more buoyant force it develops.
This is why freeboard is
indicative of a ship's reserve
buoyancy or additional buoyancy
at larger angles of roll.
Longitudinal center
of flotation or LCF
is the geometric center
of the waterplane area.
The LCF is the point about
which the vessel trims.
Note that the LCF is
not necessarily located
at midships.
Its location is
determined by the shape
of the waterplane area and
the trim of the vessel.
Stability is the tendency
of a vessel to right itself.
When a vessel is tilted by
an outside force such as wind
or waves and it returns
to its original position,
it has positive stability.
If it does not, it has
neutral or negative stability.
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 are 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.
The vertical center of
gravity or VCG of KG
is measured upward
from the keel.
The transverse
center of gravity,
TCG, is measured
from the centerline.
The longitudinal
center of gravity,
LCG, is usually measured from
the forward perpendicular
or from midships.
Understanding where a vessel's
center of gravity is located
and how it moves is
vital for ship's officer
because it is the one
property of a ship's 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 principle
will keep his vessel
afloat in heavy weather.
Center of buoyancy, or
B, is the single point
where the upward force
of buoyancy acts.
It is located at the geometric
center of the displaced volume
or the area of hull
beneath the waterline.
When a ship rolls in
the water, the shape
of the area beneath
the waterline 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 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.
A moment is nothing more than
a force or weight multiplied
by its distance from
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.
The effect of the longitudinal
difference between G and B
is trim.
Trim is defined as the
difference between the forward
and after drafts.
The opposing forces of G
and B across the trim arm
create a trimming moment
that causes the vessel
to trim around the LCF.
Keep in mind that the deeper
draft of a ship's 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 LCD is aft of LCB, then
the after draft increases.
Moment to trim one inch,
or MT1, is the moment
to change the trim 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 it should be quickly
and easily corrected.
As we've seen, when
a vessel rolls,
B moves in the direction of
the roll and out from under G.
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 arc
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 centerline
at small angles of yield.
The location of
the metacenter is
provided for the ship's
officers by the naval architect
and is a key value in
calculating stability.
The vertical distance
between the center
of buoyancy and the
metacenter is called
the metacentric radius or BM.
Once there is a
horizontal distance
between the center of gravity
and the metacentric radius,
the opposing forces of
gravity and buoyancy
create a righting moment
that returns the vessel
to it's upright position.
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
vessel's total displacement.
KM is the vertical position of
M measured upward from the keel.
Metacentric height or GM
is the vertical distance
between G and M. When we
put these things together,
we see this ship's
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 ship's 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 vessel's stability
however, because if a vessel has
enough initial
stability, it is believed
to have sufficient stability
for larger angles of roll.
Knowing the fundamentals
and terminology of stability
is just the beginning.
Once you're comfortable
with the terms of stability,
you could begin to understand
the stability calculations
required of every officer.
As you continue to
study stability,
you will become familiar with
the hydrostatic tables provided
for your ship, and
use them to help you
find it's required stability.
You will use initial
stability to determine
your ship's overall
stability as well
as range of stability, maximum
righting arm, and danger angle.
Understanding the
stability of your ship
is the key to making
every voyage a safe one.
A vessel's stability should
never be taken for granted.
Many assume the naval
architect took care of it
in the ship's design.
Most crew members give
it a little thought.
A responsible officer
does not have this luxury
and must always be aware
of their ships motion
and understand what it means.
Yes, firefighting,
housekeeping, and navigation
are very important,
but none of it
matters on a ship that can't
stay on top of the water.
Only when your ship
is upright and stable,
are you in a position
to deal with anything
else the sea throws at 'you.