stability Flashcards
BOUYANCY
An upward force exerted by fluid that opposes the weight of a partially or fully immersed object.
CENTRE OF BOUYANCY (B)
The centroid of the ships underwater volume.
GRAVITY (G)
A downward force that attracts a body towards the centre of the earth.
KM
Vertical distance from the keel (K) to the metacentre (M)
KG
Vertical distance from the keel (K) to the centre of Gravity (G)
DENSITY
The density of a substance is defined as its mass per unit volume
DENSITY = MASS / VOLUME
LAW OF FLOTATION
Every floating body displaces its own Mass of the liquid in which it floats.
ARCHIMEDES PRINCIPAL
when a body is wholly or partially immersed in a liquid, an upthrust (apparent loss of weight ) equal to the mass of liquid Displaced. This is called Buoyancy Force (BF)
DISPLACMENT (W) OR Δ
underwater volume x RD of the water
Mass of the vessel at any given moment floating at any draught between light displacement and summer draught marks.
LIGHT DISPLACEMENT (W OR Δ)
Mass of the vessel when complete and ready for sea but with no passengers, stores , fuel or cargo on board. An empty ship
LOAD DISPLACMENT (W OR Δ )
Mass of the vessel fully loaded with cargo etc. floating at her assigned freeboard
DEADWEIGHT
Difference between lightship displacement and displacement at any given draught.
Tonnes per centimetre immersion (TPC)
The mass required to load / discharge in order to change the mean draught by 1cm.
RESERVE BOUYANCY
Volume of the enclosed watertight spaces between the load line and the free board deck.
The reserve buoyancy must prevent the deck line from submerging when the largest watertight compartment is bilged
RIGHTING MOMENT (RM)
Is the result of the ships displacement (buoyancy force) acting on the end of the righting lever (GZ) . Righting moment
FREE SURFACE EFFECT
Free movement of liquids, which may or may not be confined in tanks
FREE SURFACE CORRECTION
The virtual rise in the ships centre of gravity creates a new virtual Gv.
The difference between GM and GvM is called free surface correction.
MARINE HYDROMETER
The hydrometer is used to obtain the density of dock water so that a dock water allowance (DWA) may be calculated so as to not submerge the relevant load line when at sea. The hydrometer is made from glass and brass. it features a weighted bulb and a long slender stem. calibrated between 1.00 FW and 1.025 SW
HOW TO USE A MARINE HYDROMETER
- Use a clean bucket of water
- Fill with dock water
- Allow to stand
- Take hydrometer from case and ensure clean and dry
- Lower into water and spin gently to remove air bubbles
- Allow the water to settle
- Read from the bottom of the meniscus
- Remove and dry the instrument and replace case
ARCHIMEDES AND THE LAW OF FLOTATION
Any body wholly or partially immersed in a liquid experiences an upward thrust equal to the weight of the volume of liquid displaced.
CALCULATING DISPLACMENT
Calculating displacement =
L x B x D x RD ( of the water in which you are floating )
FRESH WATER ALLOWANCE (FWA)
it is the amount by which the summer mark can be immersed when in FW
STABLE EQUILIBRIUM
G is below M. The vessel has a positive GM and therefore a positive righting moment
A stable vessel will return to the initial position when the external heeling force is removed.
NETURAL EQUILIBRIUM
G is at M. The vessel has zero GM therefore no righting lever.
This vessel when heeled by an external force will not return to the upright when the heeling force is removed but will remain at the angle of heel achieved.
UNSTABLE EQUILIBRIUM
G is above M. When heeled by an external force a capsizing lever is created.
An unstable vessel may reach a position of neutral equilibrium at a certain angle of heel when B moves out to bring M into the same position as G.
RECOVERING FROM LOLL
- The reason for a vessel lolling is that G is above M.
- The priority for recovery is to lower the centre of the gravity by adding weight low down or removing weight up high.
- This could be caused by using fuel or stores, water on deck, poor loading or damage.
CHARACTERISTICS OF A STIFF SHIP
- Large GM
- Short roll period, violent/jerky uncomfortable nature, racking stresses
- Difficult to incline initially
- Reserve of stability
- Large righting lever (GZ) easy to keep relatively upright
CHARACTERISTICS OF A TENDER SHIP
- Small GM.
- Easy to incline initially.
- Long slow lazy roll period, comfortable , less movement and less stress on the ships structure.
- Small reserve of stability.
- Small GZ, less able to return to upright once inclined.
RELATIVE DENSITY OF FW
1ton or 1000kg/m3
RELATIVE DENSITY OF SW
1.025ton or 1025kg/m3
RELATIVE DENSITY OF DW
Between 1.000 and 1.025
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
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
A solid block material has a mass of 6.2 tonne and occupies 2.4m3
calculate the 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 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
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
Three vital rules with the movement of the centre 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
