FORCES?
ANS)Ship stability can be defined in simple terms as its characteristics or tendency to return to its original state or
upright state, when an external force is applied on or removed from the ship.
A ship is at equilibrium when the weight of the ship acting down through centre of gravity is equal to the up thrust force of water acting through centre of buoyancy and when both of these forces are in same vertical line.
B is center of buoyancy and G is center of gravity
A ship will come to its upright position or will become stable, when an external force is applied and removed, if the centre of gravity remains in the same position well below metacentric height of the ship. When ship is inclined, centre of buoyancy shifts from B to B1, which creates a movement and the righting lever returns the ship to its original position and makes it stable.
M is metacenter and GZ is righting lever
A ship is seaworthy if it fulfills two important stability criteria- Intact and Damage stability.
Intact and damage stability are very important factors that govern the overall stability of the ship.
Q75) WHAT IS METACENTER & METACENTRIC HEIGHT? ANS) Metacenter: -
Top: upward thrust of buoyancy (B) and downward thrust of gravity (G) allow a stable ship to right itself when heeled
Bottom: with a metacenter (M) below gravity, forces of gravity and buoyancy are further apart and will cause an unstable ship to capsize when heeled
The metacenter had to be determined which is a point where an imaginary vertical line (through the center of buoyancy) intersects another imaginary vertical line (through a new centre of buoyancy) created after the ship is displaced, or tilted, in the water. The center of buoyancy in a floating ship is the point in which all the body parts exactly balance each other and make each other float. In other words, the metacenter remains directly above the center of buoyancy regardless of the tilt of the floating ship. When a ship tilts, one side displaces more water than the other side, and the center of buoyancy moves and is no longer directly under the center of gravity; but regardless of the amount of the tilt, the center of buoyancy remains directly below the metacenter. If the metacenter is above the center of gravity, buoyancy restores stability when the ship tilts. If the metacenter is below the center of gravity, the boat is unstable and capsizes.
METACENTRIC HEIGHT: - The distance from the centre of gravity of a ship to the metacentre; it is considered positive if the metacentre lies above centre of gravity
Ship Stability diagram showing centre of gravity (G), centre of buoyancy (B), and metacentre (M) with ship upright and heeled over to one side. Note that for small angles, G and M are fixed, while B moves as the ship heels, while for big angles both B and M are moving.
The metacentric height is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre (GM). A larger metacentric height implies greater initial stability against overturning. Metacentric height also has implication on the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll, which are uncomfortable for passengers. Hence, a sufficiently high but not excessively high metacentric height is considered ideal for passenger ships.
Q76) what is tender and stiff ship?
ANS) Tender Ship: - The ship with a small Metacentric height has a small righting lever at any angle & will roll easily is said to be tender ship. In tender ship, in this centre of gravity lies below the transverse metacentre. The GM is more than GZ. &
these kinds of ship are more stable.
Stiff Ship: - The ship with a large Metacentric height has a large righting lever at any angle & has considerable resistance to rolling. A stiff ship is very uncomfortable. In it the Centre of Gravity lies above the transverse metacentre.
Q77) WHAT IS FREE SURFACE EFFECT & HOW IT IS REDUCED CONSTRUCTIONALLY?
ANS)Free Surface Effect: - It has a lot to do with the stability of a ship. A ship that has taken in a lot of water will also experience this kind of phenomenon that will make it unstable. Ships carrying liquid cargo, or Tankers, have to be designed so as to minimize the effects of free liquid surface. Water ballast, fuel oil, fresh water, lubrication oil, and other liquid carried in the ship can also contribute to the free surface effect.
The drawing shows a cross section through the midship of a tanker ship. If there is some dynamic force that makes a ship tilt to one side, notice how the oil in the tank finds its own level and tends to shift more towards the tilting side.
The center of gravity of the oil in the tank will also shift. If the ship has enough buoyancy, it is able to right itself.
However, if the tilt is too big, the shift in the center of gravity of the oil may become too big. Instead of righting the ship, the buoyancy force on the ship may even turn the ship in the same direction of tilt, and the ship rotates and overturns.
What can be done to minimize the free surface effect?
The ship is fitted with compartments so that there are several tanks instead of one big tank. Even though the same quantity of oil is carried, notice how the oil behaves. The center of gravity of individual oil tanks will also shift, but the summation of all the centers of gravities does not shift the center of gravity of the ship that significantly as before.
Another way to minimize the free surface effect is to fill the tanks nearly full. In this case there is less room for the liquid to move about freely. This method may be a bit difficult to control for tanks carrying consumables like fuel oil, domestic water, and potable water.
The shape of the tanks can also be built to ensure stability, but in most cases, ships are built for maximum storage capacity and the rectangular cross sectional shape is most feasible.
The tanks in a Tanker are built in compartments for this purpose. The sides of the tanks also serve to protect the ship from complete flooding should some damage to its hull occur. REFER:-http://www.freemarine.com/i8freesurface.htm
Q78) EXPLAIN THE PURPOSE & LOCATION OF COLLISION BULKHEAD?
ANS)Purpose: -
Avoids flooding of ship in case of damage to bows. Location: -
Location is such that it is not so much forward as to get damaged on impact, Neither it should be too far aft so that compartment flooded forward causes extensive trim by head. As a rule located at minimum distance to get maximum space for cargo.
Minimum at 1/20 of ships length from forward
perpendicular
The collision bulkhead is continuous to upper most continuous deck
The collision bulkhead is 20% stronger than other bulkheads Collision bulkhead is 5 to 8 percent of ships length from
forward.
Q79) WHAT IS BULKHEAD & EXPLAIN DIFFERENT TYPES OF BULKHEAD?
ANS) There are three basic types of bulkhead, watertight, non- watertight and tank.
Different types of bulkheads are designed to carry out different functions.
The watertight bulkhead several important ones;
i. It divides the ship into watertight compartments giving a buoyancy reserve in the event of hull being breached. The number of compartments is governed by regulation and type of vessel
ii. Cargo separation
iii. They restrict the passage of flame
iv. Increased transverse strength, in effect they act like ends of a box
v. Longitudinal deck girders and deck longitudinal are supported by transverse watertight bulkheads, which act as pillars
The number of bulkheads depends upon the length of the ship and the position of the machinery. There must be a collision bulkhead positioned at least 1/20th of the distance from the forward perpendicular. This must be continuous to the uppermost continuous deck.
The stern tube must be enclosed in a watertight compartment formed by the stern frame and the after peak bulkhead which may terminate at the first continuous deck above the waterline. The engine room must be contained between two watertight bulkheads one of which may be the after peak bulkhead.
Each main hold watertight bulkhead must extend to the uppermost continuous deck unless the freeboard is measured
from the second deck in which case the bulkhead can extend to the second deck.
A watertight bulkhead is formed from plates attached to the shell, deck and tank top by means of welding. The bulkheads are designed to withstand a full headwater pressure and because of this the thickness of the plating at the bottom of the bulkhead may be greater than that at the top. Vertical stiffeners are positioned 760mm apart except were corrugated bulkheads are used.
Watertight bulkheads must be tested with a hose at a pressure of 200 KN/m2 . The test being carried out from the side on which the stiffeners are fitted and the bulkhead must remain watertight.
Watertight bulkheads, which are penetrated by pipes, cables etc. must be provided with suitable glands that prevent the passage of water.
Bulkhead definitions Class A
Are divisions forming bulkheads and decks that; Constructed of steel or equivalent
Suitably stiffened
Prevent passage of smoke and flame to the end of one hour standard fire test
Insulated using non-combustible material so that average temperature on exposed side does not rise above 140oC
and point temperature above 180oC. The time the
bulkhead complies with this governs its class A-60 60min A-30 30Min A-15 15Min A-0 0Min
Class B
These are divisions formed by bulkheads, decks, ceilings and lining
Prevent passage of flame for first half hour of standard fire test
Insulated so average exposed side temperature does not rise more than 139oC above original and no single point rises
complies with this governs its class B-15 15Min B-0 0Min
Constructed of non-combustible material and all materials entering the construction are similarly non-combustible except where permitted
Class C
These are divisions constructed of approved non-combustible materials. Combustible veneers are allowed were they meet other criteria
Main vertical zones Divided by Class A bulkheads and not exceeding 40m in length a. Flat Bulkhead b. Corrugated Bulkhead c. Longitudinal Bulkhead d. Transverse Bulkhead. e. Watertight Bulkhead f. Non-Watertight Bulkhead g. Fire Class A Bulkhead h. Fire Class B Bulkhead i. Fire Class C Bulkhead j. Collision Bulkhead. k. Insulated bulkhead