Stability

Question 1

Intro to Forces, Moments and Density

What is relative density?

What is the relative density of Salt Water?

Answer 1

The density of a substance compared to fresh water (1000kg/m3)

1.025

Question 2

Buoyancy , Gravity and the Metacentre

What is Metacentric Height?

What is the formula?

Answer 2

GM is the simplest measure of the stability of a vessel. It is the vertical distance between the Centre of Gravity and the Metacentre.

GM = KM (Keel to Metacenter) - KG (Keel to Center of Gravity)

Question 3

Buoyancy , Gravity and the Metacentre

Define Stiff and Tender vessels…

Answer 3

The metacentric height has an effect on the roll period of a vessel. (A larger metacentric height = a greater righting force when the vessel is heeled over by a gust of wind or wave)

Stiff condition - With a greater righting force, the vessel will tend to return upright more quickly (short roll period) once the external heeling force is removed.
Negative characteristics, more violent, uncomfortable.

Tender condition - A lesser GM with its smaller righting moment will come back upright slower (slower roll period). Negative characteristic of loss of stability in heavy conditions.

Question 4

Buoyancy , Gravity and the Metacentre

Define Heel and List

Answer 4

Heel - When a ship is inclined by an external force

List - When a ship is inclined by an internal force

Question 5

Free Surface Effect

How can you reduce free surface effect of your vessel?

Answer 5

Press tanks up full or empty them.

Question 6

Final KG and Loading Tables

How would you go about calculating your vessel’s stability by hand?

Answer 6

I would use the information from the Stability Information Booklet and then use a Loading Table to find the Final KG and Km of the vessel so I could calculate my GM.

Question 7

TPC

What does TPC mean?

Answer 7

Tonnes Per Centimeter.

The weight required to change a vessel’s mean draft (sinkage or rise) by one centimeter.

Question 8

Angle of List

Why does an Angle of List occur?

Answer 8

Due to the transverse shift in the center of gravity of a vessel. This can happen either during loading weights, transferring them onboard, or because of an unplanned cargo shift.

Question 9

Angle of Loll

What is Angle of Loll?

Answer 9

A vessel will heel over to an angle of loll when she has a negative initial metacentric height.

This can be to port or stb depending on environmental factors.

Question 10

Angle of Loll

How can you correct an angle of loll?

Answer 10

G must be bought below M by moving or adding weight further down into the vessel toward the keel (shifting cargo, ballasting in double bottom tanks, remove weight from the vessel located about G). Care should be taken to account for FSE.

When adding or transferring weight to correct AoL, it must be place on the low side - it will heel futher, but turn the AoLoll into AoList to regain a positive GM (adding to high side could result in capsize)

Question 11

Deadweight and Deadweight Scale

What is deadweight tonnage?

Answer 11

The amount that can be loaded onto a vessel from lightweight to bring her to her summer loadline. This includes all weights like provisions, bunkers, cargo, persons and effects.

Question 12

Bending Moments and Sheer Forces

What are the names of some of the stresses that we measure that act on a vessel?

Answer 12

Bending Moments (hogging and sagging)
and
Sheer Forces (localized stresses ex: full next to empty cargo hold)

Question 13

Draught and Bunker Surveys

How can you determine the exact amount of cargo or bunkers loaded onto the vessel?

Answer 13

By doing a Draft/Bunker survey

Question 14

Intro to GZ Curve

What is GZ a measure of?

Answer 14

The Righting Lever

Question 15

Angle of List GZ Curve

Where does an Angle of Lost GZ curve start?

Answer 15

Below Zero on the X Axis

Question 16

Angle of Loll GZ Curve

Describe an Angle of Loll GZ curve.

Answer 16

One that starts at 0 before heading downward below 0 then back up to cross 0 at the Angle of Loll: reaching its maximum GZ value earlier than normal and reaching its angle of vanishing stability earlier, as a result.

Question 17

International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements

What is the minimum GM a vessel can legally have?

Answer 17

0.15m

Question 18

International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements

What is the requirement for area under a GZ curve from 0º to 40º?

and a GZ curve from 0º to 30º?

and a GZ curve between 30º and 40º?

Answer 18

The area under the GZ curve (righting lever) from 0º to 40º (angle of heel) shall be at least 0.09 meter radians.

The area under the GZ curve from 0º to 30º shall be at least 0.055 meter-radians

at least 0.03 meter-radians

Question 19

International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements

Where can you find the IS Code?

Answer 19

IMO Resolution MSC.267(85)

Question 20

International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements

Fill in the blanks…

The righting lever shall be at least ___ at an angle of heel equal to or greater than than 30º

The max righting lever shall occur at an angle of heel not less than ___

Answer 20

0.2m

25º

Question 21

Load Line Regulations

What certificate proves you comply with the Load Line Convention?

Answer 21

International Load Line Certificate

Question 22

Conditions of Assignment and Freeboard

What document would help you prepare for a Load Line survey?

Answer 22

The record of conditions of assignment of freeboard

Question 23

Stability Information Booklet

Where can you find the KM value for any given draught of the vessel?

Answer 23

In the hydrostatic tables in the stability information booklet

Question 24

Inclining Experiment and Lightship Surveys

When is the inclining experiment done?

Answer 24

For a new build vessel or one that has deviation in her expected lightweight condition values compared to those observed during a lightship survey.

Question 25

Dry Docking Stability

When is the critical instant?

Answer 25

The moment just before the whole vessel takes the blocks

Question 26

Bilging

What is parallel sinkage?

Answer 26

The initial increase in the draft that uniformly applies along the length of the vessel as a result of the loss in buoyancy in the bilged compartment

Question 27

Damage Stability Requirements

What is the difference between Probabilistic and Deterministic approaches to damaged stability requirements?

Answer 27

The deterministic approach defines a specific amount of damage a ship must survive

The probabilistic approach looks at the probability of damage in each compartment and models the likelihood of losing the vessel if each compartment were lost and compares the probability of survival against the minimum allowable

Question 28

Ship Emergency Response Services (SERS)

What help and advice is available to vessel’s to help deal with a damaged stability condition?

Answer 28

If subscribed, a number of Class Societies provide a 24/7 Emergency Response Service which are available to help in those situations.

Question 29

Stability Computers and Damage Stability

What regular checks must be performed on the stability computer?

Answer 29

The Master must check the accuracy of the computer calculations annually by applying at least one test condition (other than lightship), typically in the presence of a surveyor.

Question 30

Heavy List Operations Stability

What effect can lifting a heavy object off the quayside with a ship’s crane have on your stability?

Answer 30

The weight of the object will act through the crane head, greatly reducing the stability of the vessel as the CoG of the vessel will move toward the crane head. I would expect this to induce an Angle of List on the vessel as well, due to the transverse shift in the CoG of the vessel.