Stability

Question 1

3 types of equilibrium & explain each.

Answer 1

Stable - Positive GM
~> will return upright

Neutral - GM coincide
~> no righting moment

Unstable - Negative GM
~> will topple over

Question 2

Difference between stiff and tender vessels:

Answer 2

Stiff:
Navy destroyer
Returns upright quickly
Large righting moment (GZ)
Large GM

Tender:
Pleasure yacht
Returns upright slowly
Smaller righting lever (GZ)
Smaller GM

Question 3

Explain free surface effects & ways of mitigating it:

Answer 3

Virtual rise in G causing a reduction in the GM making the vessel unstable, eg Water on decks, half empty tanks.

Emptying tanks / pressing full
Long, skinny tanks

Question 4

C/O duties for drydocking:

Answer 4

Steering & Engine checks

Hatches / WT + fire doors clossed for continuity of strength

Stability calculations

Free surface effect minimised

Cranes & davits secured

Docking plan & dock authorities

Sounding tanks

Fenders / protection

Extra support

Connection to shore power, phone, internet, water, sewage etc

Question 5

Docking plan:

Answer 5

Locate positioning of keel blocks, slings & safety shores (along frame or stringers!)

Stern usually trimmed down to allow keel to pivot

When water is removed UPTHRUST on keel blocks

Question 6

Explain upthrust

Answer 6

When water is removed the force acting on the keel blocks (P) causes a virtual rise in G ~> reduction of GM making the vessel unstable.

Question 7

External examination of hull:

Answer 7

Props - Pitting / cracks

Grids checked

Cracks - welds / prone to slamming and vibration

Corrosion - Anodes replaced / dissimilar metals joined

Question 8

Preparations before refolating:

Answer 8

Watertight integrity
Underwater repairs complete
Anodes refitted
Surveys complete
Crew onboard
Stability confirmed

Question 9

Damage Stability requirements:

Answer 9

Damage to 1 watertight compartment from mild hull damage vessel must float not less than 75mm below weatherdeck.

Vessel sinks upright (not exceeding 7•)

Question 10

Compensation for bad weather:

Answer 10

BPG

• WT integrity
• Crew safety
• Vessel security

Question 11

WT integrity for bad weather:

Answer 11

Hull openings closed
Hawse pipes closed
Deadlights / storm shutters
Unused valves closed

Question 12

Vessel security for bad weather:

Answer 12

Scuppers & freeing ports clear
Minimise free surface (drain / fill tanks)
Increase GM (ballast)
Lashings / stowage
Check comms and radar

Question 13

Crew security for bad weather:

Answer 13

Restrict access to decks
LSA checked
Inform galley
Watch schedules for prior rest
Lifelines

Question 14

Net tonnage:

Answer 14

Earning capacity of vessel, cargo spaces.

(Get it from deducting non-freight spaces from GT)

Question 16

Explain reserve bouyancy:

Answer 16

Between waterline & highest continuous deck (— mark on hull)

Watertight part of the hull is known as bouyant volume.

Question 17

Explain: Heel, List & Loll

Answer 17

Heel: Ship leans over because of external force applied (waves, wind)

List: Uneven loading

Loll: vessel unstable when upright because of rise in G.
When an external force (wind, waves) is applied the vessel will float at that angle.
Can be removed by taking ballast or removing weight higher up which caused the rise in G.