Stability Flashcards
A solid block of material has am ass of 6.2tonne and occupies 2.4m3
Calculate its density, show working
Density= Mass / volume
6.2t/2.4m3 = 2.6tonnes/m3
A steel plate 5m x 2m x 5cm and density 7.8t/m3 is to be lifted by a crane calculate the mass of the plate?
Show working
Volume = length x breadth x depth Volume = 5mx2mx0.05m= 0.5m3
Mass = density x volume (tonnes) Mass = 7.8tonnes/m3 x 0.5m3
Mass = 3.9 tonnes
What is the density of Fresh water (FW)?
1t/m3
What is the density of sea water (SW)?
1.025t/m3
(In practice the density of sea water will vary with many factors. A standard value has been adopted and this should always be used unless told otherwise)
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
Oil has a RD of 0.89. Give the density in kg/m3 and t/m3
890kg/m3
0.890t/m3
A rectangular tanks is 12m wide, 8m long and 4m deep. Calculate the mass of oil (RD 0.84) in the tank when it is 98% full?
Show working!
12x8x4x0. 98x0.84=
316. 1t
Describe the marine hydrometer and how it is used?
It consists of a weight bulb and long stem
Which, for marine use, is graduated from just above and below the density of sea and fresh water respectively
The RD is measured by placing the instrument in the liquid and noting the level which it sinks.
The denser the water the higher the hydrometer will float. The correct reading is that corresponding to the plane of intersection of the horizontal liquors surface and stem, I.e allow for the meniscus effect
(Measure from the bottom of the meniscus)
Explain the term KG
KG
Is the distance from the KEEL to the CENTRE OF GRAVITY of the vessel
Explain the term LCG?
LCG is the horizontal distance of the centre of gravity measured from the aft perpendicular
explain the term Kg
Is the height of the centre of gravity of an object or mass on the vessel, measured vertically up from the keel
Explain the term KB
Is the distance from the keel to the centre of gravity
State the effect that free surface has on vessels stability
Free surface effect
Pushes G upwards which
Increases KG therefore reduces GM (IMPORTANT**)
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-
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.
Describe the motion and list the advantages and disadvantages for a vessel said to be -
Tender
Small GM
Will roll to large angled but have a longer roll period
Advantages - roll motion is slow so more comfortable for passengers, less violent motion
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.
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 (NEED TO KNOW!)*****
Any body wholly or partially immersed in a fluid experiences an upward force equal to the weight of the fluid displaced.
Equation to find the mass of water referring to Archimedes principle?
Mass of water =
Volume x density
Density =
Mass divided volume.
Density = p (Greek letter)
Steel has a density of 7.8t per m3
Dock water density range?
1.001 - 1.024 t/m3
1001 - 1024 kg/m3
The force of buoyancy is equal to (Archimedes principle)
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
TPC formula
Mass divided by TPC
Where is the center of buoyancy located ?
Is located at the overall centre of the underwater volume of the vessel (IMPORTANT)
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
Formula to find GM (metacentric height)
KM - KG
Distance from keel to the meta centre
Minus
The distance from the keel to the centre of gravity
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
Gravity and metacenter are
