Stability Flashcards
Why does a vessel return upright?
GZ - righting lever : produces a righting moment.
GM - metacentric height :
+ GM = stable
- GM = unstable
0 GM = if inclined to a small angle of heel it will tend to remain at that angle. (neutral)
law of flotation states..
“a body wholly of partially immersed in a fluid will experience an upward force equal to the weight of the fluid displaced.”
Whats free surface effect?
an apparent rise in G, leading to a reduction in GM.
due to the movement of the CoG of a liquid
explain a tender vessel
small positive GM therefore smaller righting lever (GZ)
ship roll angles and motions are long and gentle.
no risk of structural damage but
risk of shifting stores and capsizing in rough conditions.
explain a stiff vessel
The larger the value of GM, the more ‘stiff’ (stable) the vessel. A larger righting lever (GZ)
roll angles and motions are short + aggressive.
risk of structural damage AND risk of shifting stores.
‘List’ is..
ship inclined by forces within the vessel
‘Loll’ is…
vessel unstable when upright due to a negative metacentric height ( - GM )
and will take on an angle of heel without any external force
‘Heel’ is…
ship inclined by external force
Duties for dry docking
(list 6)
- engine and steering checks.
- crew briefed and agreement made with yard.
- dry dock plan sent to dock authorities
- all hatches and cranes are secured
- free surface in tanks reduced or removed.
- stability calculation made to ensure adequate GM to take into account rise of G when the vessel has taken the blocks.
- adequate fenders rigged
docking plan
includes:
-profile, plan and sections of vessel aligned with dock drawings
-any projections from the hull of the vessel
How do we prepare for heavy weather?
WATERTIGHT INTERGRITY
hull openings closed
watertight doors closed
sea valves closed
scuppers and freeing ports clear
VESSEL SECURITY
check emergency comms
stability increased
free surface effect reduced
deadlights and storm shutters on
CREW SAFETY
Check liferafts
Rest crew
Prepare food
restrict deck access
lifelines rigged
Crew wearing safety harnesses
What’s GT?
measurement of vessels internal volume
What’s NT?
the earning capacity of the vessel. obtained by deducting non-cargo spaces from the GT.
What’s FWA?
fresh water allowance
calculated to allow for the change of draught when the vessel moves from SW to FW. (less dense, therefore sits lower in water)
