Stability

Question 1

What is the relative density of salt water?

Answer 1

1.025

Question 2

What is a moment of force?

Answer 2

It is the turning effect produced by a force acting at a distance from a pivot point

Question 3

What is the standard unit for measuring force?

Answer 3

Newtons

Question 4

When using a hydrometer, why do we not take the measurement where the water touched the stem?

Answer 4

Due to surface tension the water can be higher and give incorrect reading

Question 5

Define the metacentre?

Answer 5

The point on the vessels centreline where a vertical line crosses through the centre of buoyancy when heeled over at small angles

Question 6

What is ‘GM’?

Answer 6

It is the vertical distance between the Centre of Gravity and the Metacentre

Question 7

What is considered stiff vessel?

Answer 7

A vessel with a large metacentric height and therefore short roll period

Question 8

What is a considered tender vessel?

Answer 8

A vessel with a small metacentric height and therefore a long roll period

Question 9

Will a vessel have a deeper draft in fresh water than in salt water?

Answer 9

Yes. Fresh water is less dense than salt water and therefore provides less buoyancy force

Question 10

What is ‘GZ’?

Answer 10

The horizontal distance between the centre of gravity (G) and the point Z. The distance acts like a lever arm

Question 11

How can you reduce Free Surface Effect on your vessel?

Answer 11

1. Pressing tanks up full or emptying them.
2. Emptying swimming pools our jacuzzis if fitted
3. Ensuring scuppers on deck are clear of blockages

Question 12

How does Free Surface Effect affect the centre of gravity?

Answer 12

It lads to a ‘virtual’ rise in the centre of gravity called ‘Gv’

Question 13

What is Free Surface Correction (FSC)?

Answer 13

The correction made to stability calculations to account for the total free surface moment

Question 14

What do we call the GM when the Free Surface Correction has been applied?

Answer 14

GM1

Question 15

If there is too much Free Surface Effect, what can happen?

Answer 15

The virtual reduction in GM can result in a negative metacentric height

Question 16

How would you go about calculating your vessels stability by hand?

Answer 16

I would use the information from the Stability Information booklet and then use a loading table to find Final KG and Km of the vessel so I could calculate my GM

Question 17

What is the formula for calculating GM?

Answer 17

GM = KM - Final KG

Question 18

Where can you find the KM for a vessel?

Answer 18

In the hydrostatic particulars from the stability booklet

Question 19

What is the formula for calculating final KG?

Answer 19

Final KG = Final moment / final displacement

Question 20

What does TPC mean?

Answer 20

The wight required to change a vessel’s mean draft (sinkage or rise) by one centimetre

Question 21

Why is TPC useful to know?

Answer 21

It can inform you how much cargo, fuel, ballast etc, can be taken onboard before reaching a desired draught or load line

Question 22

Will TPC be higher or lower in salt water, compared to fresh water?

Answer 22

Higher. Salt water has a higher density than fresh water, and therefore provides a greater buoyancy force

Question 23

Why does an angle of list occur?

Answer 23

Due to the transverse shift in the centre of gravity of a vessel this can happen wither during loading weights or transferring them onboard or because of an unplanned cargo shift

Question 24

What happens to the centre of gravity when a mass is loaded off the centreline?

Answer 24

It moves towards the load

Question 25

Why does a vessel stop listing when a mass is loaded off the centreline?

Answer 25

When the centre of the buoyancy and the centre of gravity are aligned, the vessel returns to equilibrium

Question 26

Would you leave a port with an angle of list?

Answer 26

no

Question 27

What is an angle of loll?

Answer 27

A vessel will heel over to an angle of loll when she has a negative initial metacentric height. This can be to port or starboard depending on environmental factors

Question 28

What can cause an angle of loll?

Answer 28

1. Loading weight too high 2. Consumption of Fuel and Water 3. Free-Surface Effect 4. Shift in Bulk Cargo 5. Liquefaction of Bulk cargo 6. Vessels Transporting Timber Deck Cargoes 7. Heavy Lift Operations

Question 29

How do you recover from an angle of loll?

Answer 29

By loading further down the side already heeled over and brining G down so you regain positive stability and turn the loll into and angle of list

Question 30

What are the signs of an angle of loll?

Answer 30

- Sudden heel to one side - Unstable rolling - Inconsistent Heel

Question 31

What is deadweight tonnage?

Answer 31

The amount that can be loaded onto a vessel from lightweight to being her to her summer load line. This includes weights such as provisions, fuel water, crew and personal effects as well as as cargo

Question 32

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

Answer 32

Bending Moments and Sheer forces

Question 33

What is 'hogging'?

Answer 33

Hogging often occurs when the ship is loaded unevenly, with more wight concentrated towards the bow and stern, leaving the middle section relatively lighter

Question 34

What is 'sagging'?

Answer 34

Sagging often occurs when the ship is heavily loaded in the middle, with less weight at the bow and stern

Question 35

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

Answer 35

By doing a Draft/Bunker Survey

Question 36

What is the GZ measure of?

Answer 36

The righting lever

Question 37

What does the 'Angle of Conflecture

Question 38

What is the Final KG?

Answer 38

It's the calculated KG based on the lightship displacement of a vessel, plus all loaded/discharges weights