GSK - Stability Flashcards
Law of flotation
every floating body displaces its own mass of the liquid it floats in
Light Displacement
Empty ship
-no stores, no crew, emty tanks
Reserve buoyancy
Volume of enclosed watertight spaces between the Load line and the Freeboard deck. The reserve buoyancy must prevent the deck line from submerging when the largest watertight compartment is bilged
State the effect that free surface has on the initial transverse metacentric (1) height ‘GM’.
Causes G to rise and therefore reduces GM by the free surface correction.
Centre of Gravity ‘G’
the point through which the whole weight of the body is considered to act vertically down (AND) its position depends upon the distribution of weights within the body.
Centre of Buoyancy ‘B’;
the point through which the buoyancy force acts vertically upwards (AND) it is the centroid of
the underwater volume
Transverse metacentre ‘M’;
point of intersection between force line through B and centre line (AND) at a small angle.
An upright yacht is to discharge a tender from the centreline of the upper deck
onto the quay using her own gear. Explain how the yacht’s centre of gravity ‘G’
would move and the vessel’s response to this transfer.
On first being lifted the G of yacht will move directly towards the head of the gear. This vertical rise in G will reduce the yacht’s GM.
As the tender is swung outboard the G of the yacht will move parallel to the movement of the head of the gear.
This will cause the G of the yacht to be off the centreline and the yacht will incur a list.
As the tender is set down on the quay G of yacht will return to the centreline and the vessel will return to the upright. Additionally the G of the yacht will be lower than its original position (moving away from a discharged weight) and the yacht will have a larger GM than when first starting the discharge.
Gross Tonnage
Internal volume of ship measured at a rate of 2.83 m3 per tonne
Net Tonnage
GT less deductions occupied by crew’s quarters, machinery for navigation, engine room, and fuel
A yacht is to undergo an annual survey. State FOUR items that would be inspected.
weathertight openings (doors, hatches, vent openings side scuttles), stability data, general structural condition.
What ship design features enable water to be cleared rapidly from the
upper deck?
- Sheer
- camber
- scuppers
- freeing ports
- deck drains
Deadweight
Load displacement - Light displacement
TPC (Tonnes per Centimetre)
The Mass required to load / discharge in order to change the mean draft by 1cm
Describe the marine hydrometer and explain how it is used.
Definition:
Made from glass / brassand consists of a weighted bulb
long slender stem
calibrated between 1.00 and 1.025.
Use:
- Fill clean bucket/ jar with dock water from bow, amidships and the stern for more accurate reading across length of ship
- Let it settle
- ensure clean hydrometer
- lower in water and spin to remove air bubbles/break surface tension
- allow to settle
- read as near waterline as possible
- ensure hydrometer not touching bottom of sample jar/bucket
- Dry hydrometer and pack away
What does FWA stand for and what is it ?
Fresh Water Allowance (FWA) is the amount
by which the mean draught changes when a ship passes from salt water to
fresh water,
Define Density
Mass per unit Volume
Define KM
KM is the vertical distance from the Keel (K) to the Metacentre (M)
Free surface Moment
Energy created by free moving liquids in tanks onboard ships
FSM of any compartment is dependent on its width, therefore reducing the width, FSM will be reduced casing the FSC to be less
Free Surface Correction
Virtual Rise of G creating Gv.
Difference between GM and Gv M is called Free Surface Correction
Buoyancy
uppward force created by a fluid that opposed the weight of a partially or fully immersed object
Archimedes Principle
When a body is partially or wholly immersed in a liquid, an upward thrust is experienced equal to the mass of the liquid displaced. This is called Buoyancy Force
Define Displacement
Mass of the vessel at any given moment floating at any draft
Free Surface Effect
Effect of free moving liquid in a tank
Fresh Water Allowance
The number of millimeters by which the mean draft changes when a ship passes from salt to fresh water
Relative Density
density of a substance compared to the density of Fresh water
Minimum GM on arrival
0.15m on arrival for stable vessel
Recovering from Angle of Loll
Reason is that G is at M (same spot)
This is cause by poor loading or fuel / cargo high up.
- Priority for recovery into angle of list is to lower G by adding wight low don or taking weight off high
