Stability and seaworthiness Flashcards
What is meant by seaworthiness?
Seaworthiness is whether a ship is safe to set out on a voyage and deal with the perils of the sea. It is a criminial offence under the Merchant Shipping Act to put to sea a vessel which has:
* Defective structures
* Defective/inadequate equipment
* Undermanned
* Overloaded
Other points:
- Under the Marine Insurance Act, in a voyage policy there is an implied warranty that the vessel is “reasonably seaworthy in all respects”.
- Under charter contracts, the shipowner must provide a seaworthy ship which complies with the charter party description
What is meant by watertight integrity?
Watertight means having scantlings and arrangements capable of preventing the passage of water in any direction under the pressure of water.
- Watertight doors
- Shell doors closed and sealed
- Vents & machinery space openings can be closed
- Underwater valves
Preparations when heading to sea with regard to watertight integrity
Master:
* Check the weather forecast is acceptable.
* Check the passage plan is suitable with regard to the capabilities of the vessel.
* Stability book ready and ship is in an acceptable condition.
* Ensure the vessel complies with the condition of assignment.
* Fill out the OLB.
* We are going to check our load line certificate and markings comply and post form FRE13
Chief Officer:
* Hatchways are shut.
* Watertight doors closed.
* Opening of the ships side below the freeboard deck.
* Deadlights fitted.
* Storm shutters fitted.
* Freeing ports Scuppers water freeing arrangements.
* Ventilators and air pipes.
* Load lines markings.
* Jacuzzi swimming pools drained and open.
* Tenders secured etc.
Actions when encountering bad weather
Navigation and vessel
* Consider re-routing.
* Find a lee shore.
* Verify the vessel position.
* Update weather report.
* Plot storm position often.
* Engage manual steering.
* Reduce speed.
* Be ready to alter course and speed to make reduce risk of broaching, pooping damage to hull or superstructure.
* Check stability and ballast down.
* Potentially go to sea if at anchor or in port
Crew and passengers
* Make a public announcement to stop all work on deck and restrict access.
* Warn all departments to secure their work area.
* Chef to prepare food in advance
* Re-arrange working hours to ensure adequate bridge cover.
Chief Officer:
* Check deck lights
* Loose gear removed from deck
* Hatches, scuppers, freeing ports etc clear
* Watertight doors are closed
Engineers:
* Engine checks
* Steering gear checks
* Engine room secure
After a long passage, how would your stability be affected?
It would be less stable because fuel has been burnt on passage. Therefore GM will have decreased, causing a longer, more sluggish roll period.
Angle of List vs Angle of Loll and how do you correct?
Angle of List
Horizontal shift in centre of gravity. Corrected by either:
- Removing the weight
- Moving weight to the centreline
- Adding weight to the opposite side
Angle of Loll
Vertifcal shift in the centre of gravity so that G=M (neutral equilibrium), resulting in no righting lever. Corrected by:
- Removing weight high up
- Adding weight low down - ballasting etc.
What is found in hydrostatic curves?
Provides hydrostatic parameters at a given draught. Includes:
- Displacement
- Longitudinal centre of buoyancy
- Longitudinal centre of flotation
- Moments to change trim by one cm - MCTC
- Tonnes per cm immersion - TPC
What is a GZ curve?
It is a graphic represenatation of a ships transverse statical stability and is a measure of the ability of a ship to return to its upright. It can be used to calculate:
* GZ at any angle of heel.
* Max GZ and angle at which it occurs.
* Range of positive stability and angle of vanishing stability.
* Approximate deck edge immersion.
Explain free surface effect
Causes a virtual rise in G and a loss of GM.
- Centre of gravity in the tank moves as the vessel heels over (g1)
- Center of gravity of the ship moves towards g1
- Righting lever (GZ) decreases as a result and will be the same length as it would be if G was much higher - virtual rise in G
Contents of a stability book
- General description of the shio
- Table of contents
- Instructions for the use of the booklet
- General arrangement showing watertight compartments, vents, downflooding angles etc
- Hydrostatic curves
- GZ curves
- Plans showing capacity and centre of gravity for cargo spaces
- Plans showing capacities, free surface data and centre of gravity for tanks
- Loading restrictions
- Precautions for unintentional flooding
- Inclining test report
What is freeboard?
Distance between summer load line and the deck line
What is reserve buoyancy?
The volume of the area between the waterline and the deck line - below which all openings in the sides of the ship are fitted with permanent means of watertight closing.
How would you conduct an incline experiment?
Used to calculate the GM for a vessel.
- Fitted with a wire plumb in lightweight condition. Plumb bob is immersed in a liquid to dampen its motion
- Known weights are added at known distances from the centreline, creating listing moments
- List is measured using the pumb against a graduated batten, and used to calculate KG, KM and GM
Preparations:
- Little to no wind
- Moorings slacked
- Draught noted forward & aft
- Water density noted
- Free from unnecessary personnel
- Tanks etc either pressed or empty
- No loose items onboard
Preparations for entering dry dock
- Dock Master has docking plan (shows hull structure and locations of log, echo sounder, areas to be kept clear etc)
- Stability caluclations completed ensuring adequate GM
- Slight trim by the stern
- Good briefing and agreement of everyone’s roles
- Crew in correct PPE
- Remove Free Surface by emptying of filling tanks
- Engine and steering checks prior to arrival
- Watertight doors closed
- Cranes secured
- Arrange 2 means of access, power, fire lines, etc.
Preparations before flooding dry dock
- All plugs and fittings are reinstalled and the hull is watertight
- Anodes replaced
- Antifoul repainted
- All underwater repairs completed
- Underwater surveys completed
- Sufficent crew onboard
- Mooring lines in place
- Stability calculated and adequate
Damage stability requirements as per LYC
If flooding of any one compartment
- Area under the curve no less than 0.015m radianns
- Freeboard no less than 75mm
- Angle of equilibrium no more than 7 degrees from upright
- Range of downflooding no less than 15 degrees
