Stability Flashcards
Why does a vessel return upright?
GZ - righting lever : produces a righting moment.
GM - metacentric height :
+ GM = stable
- GM = unstable
0 GM = will remain at displaced angle
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 dispersed.”
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.
ship roll angles and motions are long and gentle.
no risk of structural damage but risk of shifting stores.
explain a stiff vessel
The larger the value of GM, the more ‘stiff’ (stable) the vessel.
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…
a vessel unstable when upright, caused by the vertical movement of G.
‘Heel’ is…
ship inclined by external force
Duties for dry docking
-engine and steering checks.
-crew briefed and agreement made with yard.
-adequate fenders rigged
-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.
docking plan
includes:
-profile, plan and sections of vessel aligned with dock drawings
-any projections from the hull of the vessel
how do we compensate for bad weather?
consider re routing
verify vessels position
update weather report
plot storm position often
engage manual steering
reduce speed
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 on draught when the vessel moves from FW to SW. (more dense, therefore sits higher in water)
