Stability & Buoyancy Flashcards
What is the tendency of a vessel to remain upright?
Stability
What are the two primary forces acting on a floating vessel that affect stability
Weight and buoyancy. The interaction between these two determines the vessels stability
The point at which the weight of the vessel acts vertically downwards
the center of gravity
What shifts the center of gravity
adding or subtracting weight
shifting weight
When weight is changes the center of gravity
moves towards the added the weight
If a vessel has been damaged so that water is flowing in and out of a hole below the waterline, this is known as
free communication with the sea
The upward force of water displace by the hull
is the buoyancy of a shp
The center of gravity of the displace water of a boat is known as
the center of buoyancy
A vessel is said to be in equilibrium when the vessel is at rest and
the center of buoyancy is pushing upwards and is below the center of gravity pushing downwards.
A ship’s equilibrium is affected by
the movement of the center of gravity or center of buoyancy or by some outside forces, such as wind and waves.
When a vessel rolls, the forces of the center of gravity will
move in the same direction as the tool. The downward force of gravity is offset by the upward force of buoyancy and causes the vessel to heel
When the underwater volume of a vessel changes shape, the center of buoyancies moves. It will move towards the part of the hull that is more deeply immersed causing the center of buoyancy to lose alignment with the center of gravity. This is called
heeling
On a ship, the intersection of the vertical line though the center of buoyancy and the vertical centerline is called the
metacenter
The distance between the center of gravity and the metacenter is called
the metacentric height
When the metacentric height (the distance between the center of gravity and the metacenter) is positive, meaning that the metacenter if above the center of graffiti, the center go buoyancy shifts so that is outboard of the center of gravity and the vessel is considered to be
stability and the forces of buoyancy and gravity will act to bring the vessel back to an uprightpoistion.
If the center of buoyancy is inboard of the center of gravity, the metacentric height is negative, the forces of buoyancy and gravity will tend to
roll the vessel further towards capsize.
If the center of gravity is not on the centerline of the vessel, the vessel will heel until equilibrium is reach with the cuter of buoyancy and the center of gravity in alignment. This condition is referred to as
list.
Helling is a temoirary leaning, listing is a ___ leaning
permanent
A vessel has tow principle types of stability:
Longitudinal and TRansverse
A vessel is usually much longer than it is wide. Therefore, the ______ plane (___ and ___) is more ____ than its ____ plane (____)
longitudinal (fore and aft) stable transverse plan (beam)
pitchpoling
when a vessel pitches end-over-end
Which stability tends to keep the vessel from rolling over (capsizing)
Transverse (athwartships) stability
A shipping rolling over is
capsizing
Additional weight above the center of gravity increases the distance from the center of gravity up to the center of buoyancy, which _____ stability
decrseases
Removal of weight from below the center of gravity
decreases stability
The force that causes a vessel to return to an even keel, or upright position, is called the vessel’s
moment
The force causing a vessel to react agains a tool and return to an even keel is the
=righting moment.
The broader a vessel’s beam,
the more stable that vessel will be and the less likely it is to capsize
As a vessel heels, the center of buoyancy moves to the lower side of the vessel forming an
angle of inclination
Larger changes int he movement of the center of buoyancy will result with any given
angle of heel
A vessel may also capsize when aground as the
volume of water beneath the vessel decreases and the vessel loses blance
The two principle forces that affect stability are
static and dynamic forces
Static forces are caused by
the placement of weight within the hull. Adding weight on one side of a vessel’s centerline or above it’s center of gravity usually reduced stability. Flooding or grounding a vessel makes it susceptible to statice forces with may adverse affect stability.
Dynamic forces are caused by
actions outside the hell such as wind and wave. Strong gusts of wind or heavy seas, especially in shallow water, may build up a dangerous sea tending to capsize a vessel.
A liquid that only partly fills a compartment is said to have
free surface
Water in a partly filled compartment is called
loose water
When loose water shifts from side to side or forward and aft due to turning, speed changes or wave actions the vessel
looses stability and does not want to right itself.
If fish are free to move about inside a compartment (a commonly seen condition) this will
change a vessel’s stability
To correct free surface effects
Minimize the number of partially filled tank
Fish wells should be completely empty or filled
Cargo such as fish should be prevented from rolling back and forth on the deck
To correct free communication with the sea, you should
Patch the hull opening
Place weight on the high side to decrease the list towards the damaged side
Remove weight above the center of gravity on the damaged side
Icing can
increase the displacement of a vessel by adding weight above the center of gravity and causing it to rise
To correct for icing
Change course, speed or both to reduce the freezing spray and rolling
Physically remove the ice
The entry of water into the hull resulting in progressive flooding and loss of stability is
downflooding
To correct downing flooring you need to
keep all watertight fittings and openings secured when a vessel is underway way
pump out the water
