Stability Flashcards
What do you know about stability? What is the minimum stability required?
Every vessel shall carry a stability booklet
Stability booklets is approved by the Administration (Flag/Class)
Stability booklet shows:
- GA Plans
- All consumable tank capacities, locations and moments
- Minimum international requirements
- GM 0.15m corrected for F.S.E
- The righting lever GZ shall be at least 0.2 m at an angle of heel equal to or greater than 30°
- The maximum righting lever shall occur at an angle of heel not less than 25°
- Minimum area under the GZ curve, given in radians for specific angles of heel. Equal to the energy required to roll the vessel to that angle.
Check draft marks and enter the hydrostatic tables in the Stability information booklet. Use a loading table to find Final KG and KM of the vessel then;
GM = KM - Final KG
What do you know about damage stability?
As per LYC:
Yachts are ONE-compartment vessels for Damage Stability.
(Provided all W/T Doors are closed)
If there is free flooding in one compartment the vessel MUST:
Damage Stability
* Minimum GZ 0.1 meter radians
* Angle of equilibrium less than 7 degrees
* Freeboard not less than 75mm
* Range of down-flooding not less than 15 degrees
* Area under the Curve not less than 0.015 meter radians
Steps prior to dry-docking
-Prepare a systematic, comprehensive and accurate list of all items requiring repair, maintenance or overhaul
-Worklist with HoDs
-Crew Accomodation and shoreside facilities - check SEA obligations
-Inform Flag, Class, Company - organise surveyors, organise contractors
-Conduct a safety meeting
-Conduct a briefing for all crew
-Stability considerations prior to entering
Following to be sent prior to drydocking
5. GA Plans
2. Docking plans - Shows where the strength member are to put the blocks
3. Shell expansion plan (unwraps the outer edge of the shell) how’s all underwater attachments and fittings
4. Plug plan
5. The ships fire fighting arrangement and international shore connection
6. Tank arrangement and levels
7. Relevant stability information
8. Water requirements
9. HVAC requirements
9. Electricity requirements
10. Access to vessel x 2
11. Fire rounds - crew doing it at night time
12. Emergency contact details
13. Security - Crew or hired?
14. Agreed work list
Prior to docking assess the dry dock is suitable and check the keel blocks. Then make all stability preparations
Increase GM
Reduce FSE (press or empty tanks)
Secure all hatches and doors
Ensure all crew briefed in the dry docking plan
Departing Dry Dock / Prior to flooding the dry dock you must have?
- Gave crew meeting / safety meeting / HoD meeting - discuss operation and change of mode of ship operation
- Risk assessment
- Ensure all work complete
- Ensure all certificates received
- Check anodes in place
- Check propellors/rudders/stabilisers
- Check antifoul and have certificate
- Check through hull penetrations
- Check thrusters
- Check anchors / chains
- Dock bottom inspection - signed certificate by foreman of dockyard and representative of the ship owner
- Authority to flood certificate
- Ensure comms, systems online, watertight integrity, all equipment ready.
- Always aim to leave the dry dock in exactly the same state of bunkers, ballast, FW, sewage etc
- Max ‘P’ force is when the bow first lifts off the dock bottom
- Min ‘P’ force is when the last point of the vessel lifts off the bottom
- Trim for Dry Dock is minimum trim allowable as bigger the trim the bigger the ‘P’ force
- Get rid of any unwanted top weight prior to dry docking
- Get rid of FSE
- ## -Ensure passage plan conducted prior to flooding
Health and Safety in dry dock?
- Health and Safety Executive (vessel is no longer a ship when out of the water)
- Slight difference in regulations and procedures from that to the MCA
- Master should look at the vessels own Health and Safety regulations and take the most stringent regulation to enforce for his own vessel and people
Health and Safety on Merchant Ships - UK Flag
- Regulations written by MCA - Authority in Charge
What is an angle of heel?
A temporary condition usually caused by external forces such as wind or waves forcing the vessel to incline for a period.
What is an angle of list?
Caused by movement of masses within the vessel which cause and inclining moment producing a list. Caused by an asymmetric disposition of internal weights about the center line.
What is free surface effect?
” Free movement of a liquid within the hull of the vessel creates a virtual movement of ‘G’ outwards and upwards”
“Actual reduction in the righting lever which is equated to a virtual rise in G”
NOTE: If the upward movement creates negative stability the vessel will take an angle of loll.
FSE - If you have a stiff vessel and are sustaining damage due to violent movements, you can introduce free surface effect to obtain the virtual rise in ‘G’ - (reduced GM = less stiff vessel)
Stop water generators
Only take a small amount of slack in the tanks to introduce FSE (10-15%)
If a vessel is very tender, consider reducing the FSE of the vessel.
Dry-Docking Stability
‘P’ force is the virtual rise in ‘G’ due to the up thrust it creates once the keel lays on the blocks. It increases as the water lowers until the full length of hull is supported by the blocks - p is distributed across all blocks and ship is supported.
‘Weight of the vessel is no longer supported by it’s buoyancy. Support by the block.
As you lift out of the water you have less water supporting you.
- Virtual rise in ‘G’ (Loss of GM)
- Max GZ reduced
- Point of vanishing stability reduced
- Range of positive stability reduced
- Areas under curve reduced
- Deck Edge Immersion later
Critical Period
- First part of hull touching the first block…
Critical Point
- Just as the whole keel takes the last block.
Least amount of water supporting the vessel. Most likely time to capsize
You will be advised of a minimum trim when going into dry dock - must go in with minimum requirement by the dry dock.
What is contained in the stability booklet?
Required for all vessels >24m and every passenger ship
- General Particulars
- Load Conditions for the vessel
- Tank Capacities
- Tank Plans
- Moments of tanks with different loads
- Notes for the Master on stability and how to maintain a good stability
- International requirements for Stability
- Curves of statical stability - GZ Curves
- Cross curves of stability - KN curve
- Trim tables
- Inclining test results
- Damage stability - cross flooding diagrams
- Angle of down flooding will be calculated and shown in the stability booklet - angle of heel progressive down flooding will occur due to an opening
Inclining Experiment
-Ensure vessel is in calm conditions
-Mooring lines slacked and allow unrestricted movement
-Tanks are either pressed or emptied to minimise FSE
-Density of water known
-Contents and weights of all ships compartments should be known
-Vessel clear of unneccesary personnel
-Inclining carried out by shifting known weights around the vessel
-Sufficient to incline vessel by 1 - 4 degrees
-Pendulum on the centreline are used to measure angle of heel
-Confirming lightship KG (GM=KM-KG)
Archimedes Principle
Bouyancy = Displacement
Any object wholly or partially placed in a body of water is acted upon by an upthrusting force called buoyancy which is equal to the weight of the fluid that the object displaces
Angle of Loll
An angle of which an initially unstable vessel will come to rest - due to the righting lever created created by the movement of B ( hull form )
Corrected by lowering centre of gravity - adding weight downwards and towards the low side. Or removing weight above the COG.
Different representations fo the righting arm.
GZ Curve
KN Tables
GZ curve (Static Stability Curve) - tracks the stability of a ship (GZ) for a range of heel angles
Key points on the curve:
-Initial GM-
-Deck immersion angle (at curve inflection point)
-Maximum righting lever and angle at which it occurs
-Range of positive stability (point line dips below 0 GZ) - point of vanishing stability
KN curves (Cross-Curves of stability)
Represents relationship between condition of loading, GZ and angle of heel.
Helps visualise how the righting arm changes with increasing angles of heel with different loading conditions.
Cross curves of stability developed so that in any condition of loading (where KG is already known), the values of GZ (righting lever) can be obtained depending on the angles of heel.
GZ = KN - KG sin heel
Hydrostatic Tables
Hydrostatic data table or hydrostatic curves, is a tabulated set of data that provides information about the buoyancy characteristics of a ship at various drafts or immersion levels.
It provides displacement, TCPI, LCB, LCF, MTCT and other data at various drafts and densities of fresh water and salt water.
