Stability Flashcards
How does free surface effect G
It creates a virtual rise in G. See picture
What is the angle of lol?
A Vessel with an initial negative GM will become unstable when slightly inclined. The angle of lol is the point to which the vessel heels over where g and b are in equilibrium. The vessel will roll between the LOL angles on either side
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 is Gross tonnage and Net tonnage?
Gross tonnage is the total of all enclosed space of a vessel. It is a measure of volume and not weight. 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 a vessels DEAD WEIGHT and how to work it out?
A vessels total the dead weight is the total weight of cargo it can carry. It is the difference between the loaded Displacement/summer load line (∆) and the Light ship (∆).
What is a vessel’s LIGHT SHIP DISPLACEMENT
Light ship is the displacement of the vessel when ready for sea without all stores, fuel, water, person, ballast and cargo. It does include water contained in the boilers if a vessel is equipped.
What is the 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 draft.
What is the centre of buoyancy (b)?
The centre of buoyancy is the point where the force of buoyancy is considered to act vertically upwards with a force equal to the vessels weight of water displaced. It is also the centre of gravity the of the UNDERWATER SECTION of the hull.
What is the inclining experiment used for?
In order to do any stability calculations the vessels initial conditions need to be know. Th light ship experiment is used to determine the vessel’s Light ship displacement, VCG (vertical centre of gravity)or KG and the LCG (longitudinal centre of gravity).
How is the incline test performed?
The test must be done when the vessel is a close to completed as possible and in its lightest conditions. A Plumb line is positioned along the centre line of the vessel amidships. If additional plumb lines are used then they will be placed forward and aft. These line will hang vertically. Large weight are then moved transversely across the vessel forcing a list. The angle of deflection on the plumb line is measured.
What preconditions need to be met before inclining experiment can be conducted.
- No wind, or directly astern or directly ahead.
- Vessel needs to be in lightest condition. Full boilers, no fuel, no stores, no crew, no ballast, no fresh water.
- Vessel to be floating freely and not attached to any other vessel.
- Mooring lines to be slackened as much as possible so that they do not effect the listing of the vessel.
- Deep enough water so that at any point of list the vessel will not touch the bottom.
- Any loose weights need to be secured in position
- No Free-surfaces
- Vessel must be upright.
When does an inclining experiment need to be undertaken.
On any newly built vessel and on any vessel that has undergone any substantial modifications.
What is trim?
Trim is the difference in draft forward and aft. It is the longitudinal stability of a vessel. If a ships forward draft is deeper than the stern it is said to be “trimmed by the bow” and if it is deeper at the stern it is said to be “trimmed by the stern”. If the vessel draft is equal forward and aft the it is said to be “on an even keel”
What is the critical point and critical period
Critrical point is the mooment the stern post of teh vessel takes the first block.
The critical period is the time from the critical point until the vessel has taken all the block.
Damage stability requiremnts
1 Compartment flooded waterline no less than 75mm below the freeboard deck/weather deck
The angle from the upright should not exceed 7deg.
At least 15 Degrees of Positive stability
