Stability Flashcards
Why does a vessel return to the upright position?
When the combination of the center of gravity “G” pulling down coupled with the center of buoyancy “B” pushing up creates a righting action that forces the vessel back to its upright position.
What is free surface effect?
Free movement of liquids, which may or may not be confined in tanks.
What is a tender vessel?
- Easy to incline initially.
- Small reserve of Stability, GM.
- Small GZ, less able to return to upright once inclined.
- Long slow lazy roll period, comfortable, less movement of weights & less stress on the ship’s structure.
What is a stiff vessel?
- Difficult to incline initially.
- Reserve of Stability, large GM.
- Large righting lever, GZ, easy to keep relatively upright.
- Short roll period, violent/jerky uncomfortable nature, racking stresses.
What is list?
A vessel is said to be listed when it is inclined by forces within the vessel, e.g. movement of weight within the vessel.
What is heel?
Whena vessel is inclined by external forces e.g. wind and waves
What is Gross Tonnage?
Gross tonnage is the total of all enclosed space of a vessel. It is a measure of volume and not weight.
What is Net Tonnage?
Net tonnage is the total enclosed space available revenue generating cargo including passengers. This will exclude non-revenue generating areas like bridge, engineering spaces and crew quarters.
What is FWA?
Fresh Water Allowance
It is the amount that the draught of a vessel will change when a ship passes into fresh water while floating at the loaded draught.
What is the meta centre and metacentric 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
What is light displacement?
Mass of the vessel when complete and ready for sea but with no passengers, stores, fuel, or cargo on board. An Empty Ship.
What is load displacement?
Mass of the vessel fully loaded with cargo, stores, etc. floating at her assigned freeboard.
What is TPC immersion?
The mass required to load/discharge in order to change the mean draught by 1cm
What are some ways to reduce free surface effect?
Ship design: Longitudinal watertight divisions
During a passage: Press full, empty, use narrow width tanks.
How do you find the righting moment?
Displacement x GZ (∆ x GZ)
What is reserve buoyancy and what is the formulae to calculate it?
It is the amount of enclosed watertight space above the water line to the freeboard deck. It provides a safety factor to the vessel in the event of damage. Reserve buoyancy = L x B x Fbd(freeboard = depth - draft) x Cb
What information do you provide prior to dry docking?
Prior to arrival: Send Docking plan, dockmaster to return arrival conditions, insurance, class, flag , managment company details
Prior to dry docking: Upright, no free surfaces, trim as required by the dock master, minimum ballast, no loose equipment on deck, increase GM. All water tight and bulk head doors closed to ensure continuity of strength through vessels length. Conditions need to be recorded so vessel can be refloated with same characteristics.
Risk Assesment, Pilot, Tug and mooring operations, Weather, Tides.
Meeting with dry dock master: Discus dry docking plan which would include the placemnet of blocks and shores, location of any hull protrusions, sea chests, tank plugs and tranducers.
Post lift: Hook up firefighting services, water, electricity
What does a stability booklet contain and which vessels must carry one?
- Vessel over 24m
Gives information of the vessels stability at different loading condition. e.g. Curves or table of GM vs draught and data necessary to maintain stability after damage.
What information can be found on the docking plan?
- Locations of: Beams and frames, through hulls, sea chests, transducers, stabilizers, rudder posts, propellors and shafts, thrusters, protrusion.
- Positions of: Blocks shores
- Tank locations
- Docking conditions
