Stability

Question 1

What is the relative density of Fresh water and salt water?

Answer 1

RD of SW 1.025

RD of FW 1.000

Question 2

Distinguish between the terms density and relative density ?

Answer 2

Density is mass divided by volume and has units of t/m3 or kg/m3

Relative density is a ratio
and is comparison between the density of a substance and density of fresh water. As a ratio it has no units

Question 3

Describe the marine hydrometer and how it is used?

Answer 3

It consists of a weight bulb and long stem

• Obtain bucket of water from side of the vessel well below surface
• lower hydrometer into the bucket
• once settled read RD of the scale
• take average of 3 readings from forward mid and aft

Question 4

Explain the term KG

Answer 4

KG

Is the distance from the KEEL to the CENTRE OF GRAVITY of the vessel

Question 5

State the effect that free surface has on vessels stability

Answer 5

Free surface effect
Apparently Pushes G upwards which
Apparently reduces GM

Question 6

Define the term meta centre and meta centric height

Answer 6

Meta centre is the point where the force of buoyancy intersects the centre line of the vessel

Meta centric height is the distance between the centre of gravity and the meta centre otherwise known as GM

Question 7

What are the three states of equilibrium

Answer 7

Stable
Neutral
Unstable

Question 8

State three items of information given on the hydrostatic data and explain their use?

Answer 8

TPC - tonnes per center meter. Gives the number of tons required to sink or raise a vessel by 1 cm.

KMT - gives the height of the meta centre above the keel. This can be used with the KG of the vessel to determine the GM

Displacement - give the mass of the vessel at given drafts. It can be used to determine the change in draught when masses are added or removed from the vessel

Question 9

Describe the motion and list the advantages and disadvantages for a vessel said to be Stiff

Answer 9

large GM

Will have fast roll periods to relatively small angles of heel

Advantages-
Less tendancy for water to break over weather decks.
Resistance to heel, small movements of G will have minimal effect, loss of stability due to a compartment flooding will have a smaller effect of the vessel’s stability.

Disadvantage
Uncomfortable
Severe Racking stresses

Question 10

Describe the motion and list the advantages and disadvantages for a vessel said to be Tender

Answer 10

Small GM

Advantages - roll motion is slow so more comfortable for passengers, less violent motion
Racking minimal
Less likely to have cargo shift

Disadvantages - little resistance to heel, small movements of G will have a greater effect on the overall stability of the vessel.
Flooding of a compartment will have a serious impact on the stability of the vessel.
More tendency of water breaking over weather deck.

Question 11

Explain how the effect of free surface can be reduced?
A)at the design stage?
B)when at sea

Answer 11

By designing tanks containing liquids to be long and narrow rather than short and wide

When at sea drain down swimming pools and jacuzzis

Question 12

Define light displacement

Answer 12

The mass of their vessel when completely empty

WITHOUT
Crew, passengers, stores, fuel or water

Question 13

Define load displacement

Answer 13

The mass of the vessel when

FULLY stored ready for sea and floating at the summer load line or datum load line (100%loaded condition)

Question 14

Define load deadweight

Answer 14

The difference between the load and light displacements. The “lift” capacity of the ship

Question 15

Define Archimedes principle

Answer 15

Any body fully or partially immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the body.

Question 16

Equation to find the mass of water referring to Archimedes principle?

Answer 16

Mass of water =

Volume x density

Question 17

Deadweight = (as an equation)

Answer 17

Load displacement - light displacement

Question 18

Define TPC immersion

Answer 18

tonnes per cm immersion - the mass required to be added (or removed) to cause 1cm sinkage (or rise)

Sinkage (or rise) =

Mass added (or removed) divided by TPC

TPC has units of tonnes per cm, and mass is measured in tonnes.

What will the units of sing

Question 19

Define Center of buoyancy (B) where is it located ?

Answer 19

The point trough which the force of buoyancy is considered to act vertically upwards.

Is located at the overall centre of the underwater volume.

Question 20

Where is the centre of gravity located ?

Answer 20

Located at the overall centre of mass of the vessel and her contents .

Question 21

Three vital rules with the movement of the center of gravity

Answer 21

When mass is ADDED to the yacht, G moves TOWARDS the added mass

When mass is REMOVED to the yacht, G moves AWAY from the added mass

When mass is MOVED to the yacht, G moves in the SAME DIRECTION the added mass

Question 22

Ways to reduce free surface effect?

Answer 22

Press tanks! (Or empty)
empty swimming pool, spas
Long narrow tanks
Wash head bulkheads (bulkheads with ovals/circles cut in them

Question 23

Equation to find the righting moment ?
RM
(The moment of statically stability?)

Answer 23

GZ multiplied by displacement
(Value in tonne meters TM)
(∆ x GZ)

Question 24

Neutral stability is when

Answer 24

Centre of Gravity(G) and Metacenter (M) are at the same place

Question 25

Define Centre of Gravity (G)

Answer 25

The point through which the entire weight of the ship and her contents are considered to act vertically downwards.

Question 26

Conditions of Assignment

All Harley Dudes Support Vacations Airports + Fun

Answer 26

All openings - to be weather tight
Hatchways- at least 2 locking devices
Doorways- should be watertight
Skylights, side scuttles- equivalent strength of structure
Ventilators- permanent means of weathertight closure
Air pipes greater than 10mm in diameter- permanent means of closure
Freeing ports- adequate number