Stability

Question 1

What do you know about stability? What is the minimum stability required?

Answer 1

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 and moments
• Minimum international requirements
  - GM 0.15m corrected for F.S.E
  - MAX GZ not less than 0.2m - Not before 25 degrees
  - Areas under the GZ curve
  1. 30* -> 0.055 m.rad /
  2. 40* ->0.09 m.rad /
  3. between 30* and 40* -> 0.03 m.rad)
• Pro forma’s so that the GM can be calculated
• GZ curve proving that all states of load of the yacht will result in adequate stability, typically these curves will be for 0%, 10%, 30%, 50%, 90%, 100% Load

Question 2

What do you know about damage stability?

Answer 2

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:

• Float at a level not less that 75mm below the freeboard mark
• Not have a list more than 7 degrees (unequal flooding)
• Positive stability for at least 15 degrees after list angle

Question 3

What is a small GM and What is a large GM?

Answer 3

Whatever you consider to be the GM for your own vessel - half that and that is what you would consider a small GM, and if you double it that is what you would consider a big GM

Question 4

What information do you provide prior to dry-docking

Answer 4

1. 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. 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

Question 5

Prior to flooding the dry dock you must have?

Answer 5

• Dock bottom inspection - signed certificate by foreman of dockyard and representative of the ship owner
• 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

Question 6

Health and Safety in dry dock?

Answer 6

• 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

Question 7

What is an angle of heel?

Answer 7

A temporary condition usually caused by external forces such as wind or waves forcing the vessel to incline for a period.

Question 8

What is an angle of list?

Answer 8

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.

Question 9

What is free surface effect?

Answer 9

” 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.

Question 10

What happens during the dry-docking process?

Answer 10

1. Virtual rise in ‘G’ (Loss of GM)
2. Max GZ reduced
3. Point of vanishing stability reduced
4. Range of positive stability reduced
5. Areas under curve reduced
6. Deck Edge Immersion later

Question 11

Dry-Docking Stability - What is ‘P’ Force?

Answer 11

‘P’ force is the actual rise in ‘G’ due to the loss of buoyancy when pumping out water from the dry dock….

‘Weight of the vessel is no longer supported by it’s buoyancy. Support by the block. Seen to be a virtual discharge of weight from the ship to the block’

As you lift out of the water you have less water supporting you.

Question 12

What is the critical period & critical point?

Answer 12

Critical Period
- First part of hull touching the first block…

Critical Point
- 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.

Question 13

What is contained in the stability booklet?

Answer 13

• Curves of static stability = GZ Curves
• 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
• GZ Curves for various loads on the vessel

Question 14

Stability Miscellaneous

Answer 14

• Angle of down flooding will be calculated and shown in the stability booklet
• 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
• Calculations for Dock Water Allowance
• DWA = FWX x 1025 - DWD / 25 (FWA is in your ILLC)