Short Definitions Flashcards
Baseline
Baseline
All measurements are made using a baseline, which is a horizontal line drawn through the lowest point of the hull.
Free surface moment
The force of Free surface water on the vertically downward righting moment force
Free surface effect
Free movement of liquids, which may or may not be
confined in tanks.
Free surface correction
The virtual rise in the ship’s centre of gravity creates a
new virtual gravity. This new value is used when making
stability calculations. The difference between GM & GvM
is called the free surface correction.
Assigned Freeboard
Assigned Freeboard The vertical distance between the freeboard deck and
the SLL at amidships.
Freeboard
Freeboard The vertical distance between the uppermost continuous
watertight deck (freeboard deck) and the waterline at
amidships.
Reserve buoyancy
Reserve of Buoyancy This will allow the freeboard deck to remain
unsubmerged in the event of us being holed below the
waterline. It is the volume of enclosed spaces between
the waterline and the freeboard deck. Expressed as a %
of the total enclosed volume of the vessel.
Transverse cross section
Transverse cross section A drawing showing standard cross sections of the hull at,
or near amidships, and presenting the scantlings of the
principal structural members.
Freeboard Deck
Freeboard Deck
The uppermost continuous watertight deck having permanent means for closing all
openings to maintain watertight integrity. Below which all openings in the vessel’s side
have permanent means for watertight closure.
Pounding stress
Pounding Stress
Occurs in the bottom shell and framing forward when a vessel drives into head seas. It
creates a loud bang followed by vibration as the stresses transfer through the hull. Most
severe if a lightly ballasted ship slams into the trough of a wave. If the area of impact with
the water is far enough back, then it will not have the fine shape of the bow. All ships are
prone to pounding and the most vulnerable area is the forward third of the hull.
Panting stress
Panting Stress
The hull squeezes in and out like a bellows due to the fluctuating pressures on the hull as
the ship moves through waves. The effect is most pronounced at the ends when the ship is
pushing through waves and is pitching heavily. The large pressure changes at the bow and
stern over a short time create considerable stresses in these areas.
Law of floatation
Law of Flotation: Every floating body displaces its own Mass of the liquid
in which it floats.
Archimedes’ Principle
Archimedes’ Principle: When a body is wholly or partially immersed in a liquid,
an upthrust (apparent loss of Weight) equal to the Mass
of liquid Displaced. This is called Buoyancy Force (Bf).
Buoyancy
Buoyancy
An upward force exerted by a fluid that opposes the
weight of a partially or fully immersed object.
Centre of Buoyancy (B)
Centre of Buoyancy (B)
Point where the total buoyancy force (Bf) of the ship is
considered to act vertically upwards. This will be at the
centroid of the ship’s underwater volume.
