Ships types and
terms
Tender
Ship: A ship
with a small metacentric height will have a small righting level at any angle
and will roll easily then ship is said to be tender.
Stiff
Ship:
A ship with a
large metacentric height will have a large righting level at any angle and will
have a considerable resistance for rolling then ship is said to be stiff.
TPC
(Tonne Per Centimeter): TPC is a measure of the amount of mass in tonnes which is
required to change a vessel’s draught by one centimeter.
Angle
of loll: the
angle at which the initially constable ship gates natural equilibrium is called
angle of loll.
Righting
lever: Perpendicular distance between the
vertical lines through the center of gravity and new center of buoyancy in
incline position.
Displacement: light weight + dead weight
Light
weight: the
mass of empty ship without cargo, fuel, water, crew and their effects.
Dead
weight: the
mass of cargo, fuel, stores etc. a ship carries is known as dead weight. The
dead weight is difference between Displacement and Light weight.
Camber/
Round/ Beam:
The transverse curvature of the deck from the centerline down to the sides.
Raise
of floor:
The light of bottom shell plating above the base line is called as raise of
floor.
Stress of ship:
Hogging: it is the longitudinal bending
stress which may occur when a ship in a sea way or due to uneven loading when
too match weight in the ends.
Sagging: it is the longitudinal bending stress
which may occur when too match weight in the middle.
Panting: Panting is the in and out motion of
the plating in the bows of a ship and it is caused by unequal water pressure
through successive waves.
Racking: When a ship rolls there is a
tendency to deform transversely, it is known as Racking.
Pounding/
Slamming:
when a ship is pitching her bows often lift clear from the water then comes
down heavily & get heavy thrust at bottom part, it is known as pounding.
Ships motion
Rolling: The rotational motion of a ship
about a longitudinal axis is known as rolling.
Pitching: The rotational motion of a ship
about a transverse axis is known as Pitching.
Surging: The foreword and aft liner motion of
a ship is known as surging.
Swaying: The side to side liner motion of a
ship is known as swaying.
Heaving: The up and down liner motion of a
ship is known as heaving.
Yawing: The rotational motion of a ship
about a vertical axis is known as yawing.
Bilge
keel: It is
on kind of flat plate fitted outside mounted portion of ship both port and
starboard.
Purpose:
1.
Prevent rolling.
2.
Give longitudinal strength.
3.
Protect the bilge in ever of grounding.
Six
motion:
Rolling, pitching, Towing, Surging, Swaying, Yawing.
Metacentric
height GM: It is a measurement of the initial static stability of a floating
body. It is calculated as the distance between the center of gravity of a
ship and it’s meta center.
Cross curve
of stability: These are a set of curves from which the righting lever about an
assumed center of gravity for any angle of heel at any particular displacement.
Static
stability:
1.
It is defined as the ability of a ship to regain its upright equilibrium
position, after the removal of external factor which caused the vessel to heel
at an angle.
2.
It gives the stability information of a vessel under the condition that the
outside water is static.
3.
It’s unit is meter.
4.
Static stability at two different angle of heel can be the same.
Dynamic
stability:
1.
It is defined as the energy required heeling the ship from upright equilibrium
till the angle of heel in question.
2.
It gives the stability information of a vessel considering dynamic behavior of
sea.
3.
It’s unit is ton-meter-radian.
4.
The dynamic stability at two different angle of heel cannot be the same.
Importance
of metric light (GM)
For a vessel
to be stable the numerical value of GM must must be positive. This means that G
must always be located below M.
If
GM<0.2m, the ship will be tender. As the value of GM is very small there is
more possibility to sink.
If GM>1m,
the ship will be stiff. As the value of GM is large there is less possibility
to sink.
Tender
ship:
Advantage:
1.
It will roll easily and comfortably.
2.
It have a large rolling period and will not roll quickly from side to side.
Disadvantage: As GM small there is a possibility
to sink.
Stiff
ship:
Advantage:
1.
It has a considerable resistance to rolling.
2.
There is less possibility to sink for a stiff ship.
Disadvantage:
1.
It will be very uncomfortable as rolling period is less.
2.
It may result in structural damage.
3.
It has a very small rolling period.
4.
It may roll violently from side to side.
Advantage
of a balanced rudder:
1.
It offers good maneuverability.
2.
Not much strength is applied to the rudderstock.
3.
The steering gear is quite concept.
Advantage of
semi-balanced rudder:
1.
It offers good maneuverability.
Balanced
rudder: A
rudder which have (25-30)% of area of foreword furring axis is known as
balanced rudder.
Semi-balanced
rudder: A
rudder which have less than 20% of area of foreword furring axis is known as
semi-balanced rudder.
Ship
resistance:
This is the resistance to friction of the water along the hull.
1.
Pressure resistance.
2.
Frictional resistance.
Total resistance RT =Rf + Rr
Formula for calculating GRT = 0.2 + 0.02 log10 V
Collision
bulkhead:
the foremost major watertight bulkhead which extends from bottom to the main
deck.
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