Stability Flashcards
Stable equilibrium?
G below M- Positive GM
Neutral equilibrium?
G and M same height
Unstable equilibrium?
G above M - Negative GM
What is a stiff vessel?
Large GM
Larger righting lever
Difficult to incline initially
Returns to upright quickly
Uncomfortable for crew
Undue stress on vessel and cargo
Racking stress
What is a tender vessel?
Small GM
Easy to incline initially
Smaller righting lever
More comfortable for crew and passengers
DANGERS OF SYNCHRONOUS ROLLING
Chance of Dangerous Gv through fuel consumption
Stiff vs Tender vessel?
Stiff comes upright more aggressively while tender has a smoother roll
Free surface effect?
Freedom of movement of liquid within a body i.e a tank.
Creates a free surface moment which will counteract our righting moment
Will cause a virtual rise of G and reduce GM
We can calculate free surface correction and take it away from our solid GM and it will give us actual GM
How to reduce free surface effect?
FiIll or empty tanks or use long skinny tanks
Watertight integrity (Scuppers, freeing ports, fairleads, vent covers, hawse pipe covers etc)
Preparations for drydock? (9)
- Discuss docking plan with the yard
- Remove or reduce free surface effect as much as possible
- Stability calculations for the rise of G when the vessel takes the block to ensure enough GM
- Consult dock authorities on draught of vessel and trim required
- Sound all tanks to be aware of quantities
- Make sure doors and hatches are shut
- Lock up ship’s lavatories
- Rig up enough fenders
- Man mooring stations
What is a dry dock plan?
Underwater profile of vessel , shows recommended positions for blocks.
How do we prepare for bad weather?
Watertight integrity
Ensure doors & hatches are closed,
W/T closed,
Hawse pipe & vent covers on,
unnecessary sea valves closed
Vessel security
Scuppers & freeing ports are clear,
extra lashings and stowed correctly
reduce free surface effect
fill ballast tank if possible,
check emergency comms
Crew safety
Reduce access to deck,
rig lifelines and safety harnesses, modify watch routine,
prepare food in advance
Storm shutters & deadlights
Storm shutters goes on the outside of a window and deadlight on the inside, eg over your porthole
What is GT?
Internal volume of ship
What is NT?
Earning capacity of the ship (spaces available for cargo, crew quarters, engine room)
What is reserve buoyancy?
Volume from waterline to freeboard
What happens to G when you add a weight?
G moves towards the added weight
What is fresh water allowance?
The amount of decrease of draught when moving from saltwater to freshwater
What is list?
Vessel tilting to one side due to uneven weight distribution
What is angle of loll?
State of a ship that is unstable when upright and therefore takes on an angle of heel to either port or starboard.
What is the metacenter?
Point of intersection between force line through B, and centre line at a small angle
What brings a vessel back over?
Righting lever (GZ) will create a righting moment that brings vessel back over
The bigger the GM, the bigger the GZ, the bigger the righting moment
How do I correct an angle of loll?
Move weight from the high-side to below where it is lolled which will create an angle of list instead
