Stability Definitions and Basic Principles

Question 1

Define KM
What are the units for KM?

Answer 1

KM is the height of the transverse meta centre from the keel
KM is measured in metres (m)

Question 2

Define KB
What are the units for KB?

Answer 2

Vertical centre of buoyancy above the keel
KB is measured in (m)

Question 3

Define BM
How do you calculate BM?

Answer 3

Vertical distance from buoyancy to metacentre
BM= KM-KB

Question 4

Define buoyancy

Answer 4

Geometric centre of underwater volume

Question 5

Define Metacentre
What do we assume at small angles of heel?

Answer 5

Point of intersection of successive lines of buoyancy force.
We assume the meta centre is fixed. At large angles we do not.

Question 6

Define Transverse Statical Stability

How is it expressed?

Answer 6

Describes the ability of a ship to return to the upright when it has been forcibly heeled by an external force and is momentarily at rest when that ship is floating in still water.

Expressed in terms of meta centric height (GM) and righting lever GZ. Units are m.

Question 7

Define Dynamical Stability
What area is this on the GZ Curve?
What are the units?

Answer 7

Defined as the energy requried to heel the ship from upright to a given angle of heel
It is the area under the GZ curve multiplied by displacement.
Units are tonnes metre radians (tmr)

Question 8

Describe stiff Condition and the associated problems

Answer 8

Large GM
Can lead to excessive acceleration forces and cause damage to ship, crew and cargo.

Question 9

Describe Tender Condition and the associated problems

Answer 9

Small GM caused by KG being too long
Vessel slow and sluggish to return to upright and has a long roll period
Substantial reduction in vessels stability
Can cause an increased risk of water being shipped on deck, strain on cargo and or cargo shift, uncomfortable for crew.

Question 10

What are the different conditions that must be included in a ships stability data booklet

Answer 10

Pax Ship
Fully loaded Cargo DEP/ 100% FO + FW/ Max Pax
Fully loaded Cargo ARR/ 10% FO + FW/ Max Pax
No Cargo / 100% FO + FW/ Max Pax
No Cargo / 10% FO + FW/ Max Pax

Cargo Ship
The same conditions minus the pax and the conditions with no cargo the ship should be ballast

Question 11

What is Archimedes Principle?

Answer 11

An object floating or submerged in a fluid is buoyed up by a force (Bf) equal and opposite to the weight (Wf) of the fluid displaced.
Object floating: Wf = Bf
Object Sinks: Wf > Bf

Question 12

Define the Constant

Answer 12

The constant is the difference between calculated displacement and the true displacement.

Question 13

Define Displacement
What are the units for displacement?
How do you calculate displacement?

Answer 13

The weight of water displaced by a vessel which is equal to her weight and all that is in her
Measured in tonnes (t)
Displ. = Vol. of Displ. x water density
i.e. △ = V x 𝜌

Question 14

Define Draught

Answer 14

Distance from the keel to the waterline

Question 15

Define Freeboard
What are the units?

Answer 15

Distance from the waterline to the uppermost continuous deck
Measured in mm

Question 16

What is the IS Code?

Answer 16

• Provides standards and recommendatory stability criteria to ensure a safe operation, by minimising risk to the vessel, personal and environment.
• There are two parts (A and B). Part A outlines the mandatory criteria. Part B has the recommendations and guidance

Question 17

Outline the contents of IS Code Chapter 5.1: General Precautions against Capsizing.

Answer 17

• Compliance with stability criteria does not provide immunity from capsizing. Master should exercise good seamanship with regard to season, weather forecasts, navigational zone, speed and course with relation to prevailing circumstances
• Follow CSM and if necessary limit the amount of cargo so that ballast weight can be added if required.
• Cargo and cargo handling equipment is properly secured to minimise shifting under effect of acceleration caused by rolling and pitching.
• Vessel engaged in towing should have adeqaute reserve buoyancy to withstand heeling moment from tow line. Deck cargo should not impede the safe working of crew and or operation of vessel. Tow line should have springs and quick release function.
• Slack tanks should be kept to a minimum. Filled pools should also be considered
• Avoid large values of metacentric height (GM i.e stiff condition). This could lead to excessive acceleration forces which could damage and or put strain on cargo securing equipment. Slack tanks in exceptional cases could be used to reduce excessive values of GM
• Regard given to bulk carriers (i.e partially filled holds that could shift or liquefaction)

Question 18

Define Initial transverse metacentric height

Answer 18

The metacentric height (GM) is a measurement of the static stability of a vessel when initially upright. It is the vertical distance between G and M measurd on the centreline.

Question 19

What are the three conditions of equilibrium?
Define the three things required for a vessel to be in equilibrium

Answer 19

• Stable, Unstable and Neutral

For a vessel to float at rest in still water;
1. Must displace her own weight of water
2. B & G must be in the same vertical line
3. No external forces must be acting on the vessel

Question 20

Define a stable equilibrium

Answer 20

A vessel is said to be in stable equilibrium if after being inclined by some external forces she tends to return to her initial position

Question 21

Define a unstable equilibrium

Answer 21

A vessel is said to be in unstable equilibrium if after being inclined by some external forces she tends heel further

Question 22

Define a neutral equilibrium

Answer 22

A vessel is said to be in neutral equilibrium if after being inclined by some external forces she tends to remain in the inclined position

Question 23

Draw a Transverse static stability diagram of a vessel in stable equilibrium (upright and being heeled by an external force)

Answer 23

Question 24

Draw a Transverse static stability diagram of a vessel in unstable equilibrium

Answer 24

Question 25

Draw a Transverse static stability diagram of a vessel in neutral equilibrium

Answer 25

G must coincide with M, GZ = 0

Question 26

Define G

Answer 26

G is the centre of gravity

Question 27

What is the absorption allowance for Timber Deck Cargo?

Answer 27

10%