Stability Flashcards
What is the relative density of Fresh water and salt water?
RD of SW 1.025
RD of FW 1.000
Distinguish between the terms density and relative density ?
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
Describe the marine hydrometer and how it is used?
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
Explain the term KG
KG
Is the distance from the KEEL to the CENTRE OF GRAVITY of the vessel
State the effect that free surface has on vessels stability
Free surface effect
Apparently Pushes G upwards which
Apparently reduces GM
Define the term meta centre and meta centric height
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
What are the three states of equilibrium
Stable
Neutral
Unstable
State three items of information given on the hydrostatic data and explain their use?
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
Describe the motion and list the advantages and disadvantages for a vessel said to be Stiff
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
Describe the motion and list the advantages and disadvantages for a vessel said to be Tender
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.
Explain how the effect of free surface can be reduced?
A)at the design stage?
B)when at sea
By designing tanks containing liquids to be long and narrow rather than short and wide
When at sea drain down swimming pools and jacuzzis
Define light displacement
The mass of their vessel when completely empty
WITHOUT
Crew, passengers, stores, fuel or water
Define load displacement
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)
Define load deadweight
The difference between the load and light displacements. The “lift” capacity of the ship
Define Archimedes principle
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.
Equation to find the mass of water referring to Archimedes principle?
Mass of water =
Volume x density
Deadweight = (as an equation)
Load displacement - light displacement
Define TPC immersion
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
Define Center of buoyancy (B) where is it located ?
The point trough which the force of buoyancy is considered to act vertically upwards.
Is located at the overall centre of the underwater volume.
Where is the centre of gravity located ?
Located at the overall centre of mass of the vessel and her contents .
Three vital rules with the movement of the center of gravity
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
Ways to reduce free surface effect?
Press tanks! (Or empty)
empty swimming pool, spas
Long narrow tanks
Wash head bulkheads (bulkheads with ovals/circles cut in them
Equation to find the righting moment ?
RM
(The moment of statically stability?)
GZ multiplied by displacement
(Value in tonne meters TM)
(∆ x GZ)
Neutral stability is when
Centre of Gravity(G) and Metacenter (M) are at the same place
