| Curves of Stat. Stability | List | Free Surface Effetcs | Trim |
Stability
Angle of Loll
Angle of loll
Consider the following vessel in unstable equilibrium
condition.
As the angle of heel increases, the CB moves out
further until it is directly under G. The capsizing moment disappears now and
this angle of heel at which this condition occurs is called the angle of
loll.
The ship now moves around the angle of loll, but if
the CB does not move out far enough to move directly under G, then the vessel
will capsize.
If the heel increases beyond the angle of loll, the CB
moves out further to the low side and the ship now moves around this angle.
The angle of loll can be on either side depending upon
the external inclining force, such as the wind and the waves.
However, there is always the threat of the G rising
above the M and this will create a situation of unstable equilibrium, thereby
capsizing the ship.
List Caused By Negative Gm
When a ship’s center of gravity moves vertically
upwards and slightly above the Metacenter, the ship
will develop a list (or possibly capsize.) The vessel may also “flop” over,
developing the same list to the other side.
Possible Causes
1. Removal
of low weight
2. Addition
of high weight (ice)
3. Moving
weight upward
4. Free
Surface Effect
5. Free
Flow Effect (if present)
How to Recognize
1. Vessel
will not remain upright and will assume a list to either port or starboard.
2. Vessel
“flops” to port or starboard.
3. Vessel
will have a very long, slow roll period about the angle of list.
4. A
small GM is known to exist plus any of the above.
Corrective Measures
1. Eliminate
Free Surface and Free Flow Effects (if present)
2. Add
low weight symmetrically about centerline.
3. Remove high weight symmetrically.
4. Shift
weight down symmetrically.
List Caused By Off-Center Weight And Negative Gm
The vessel’s stability is reduced by both an increase
in the height of the center of gravity and movement from centerline. A negative
GM condition exists, represented by the “uncorrected” curve. An off-center
weight, represented by the cosine curve, is added and a larger list develops.
Possible Causes
1. A
combination of the previous causes of list.
How to Recognize
1. Vessel
will assume a permanent list either port or starboard (vessel will not flop).
2. Very
slow roll period about this permanent list.
3. The
known off-center weight isn’t proportional to the ship’s list.
Corrective Measures
Correct Negative GM first.
a. Eliminate
Free Surface and Free Flow Effects (if present)
b. Shift
weight down, add weight low, or jettison weight high.
Correct for Gravity Off
Centerline
a. Add
weight to higher side
b. Remove
weight from lower side
c. Shift
weight to higher side
*** ALWAYS correct Negative
GM prior to shifting weights transversely ***