Theory Questions Flashcards
Dangers of stiff vessel
Cargo shift
Structural damage
Uncomfortable for crew and passengers
Dangers of tender vessel
Large angles of heel
Deck edge may immerse and effect stability
Could become unstable due to fuel and water consumption
State purpose of the inclining test
The value of the metacentric height (GM) is obtained directly from the experiment, since the height of M can be calculated from the ship geometry, the height of the centre of gravity (KG) can be obtained
KG = KM-GM
Describe how an inclining test is conducted on a superyacht and state the
information that can be calculated from this action
To carry out the experiment the vessel is positioned initially upright and then known weights are shifted measured distances transversely over the deck. The resulting angle of list noted.
The angle of list may be measured by means of an instrument called a stabilograph or by means of a plumb line suspended from a point on the centreline.
The experiment is carried out several times both to port and starboard to ensure an accurate result is obtained.
From these results the GM can be obtained and subtracted from the KM to give the lightship centre of gravity height above the keel (KG)
KG = KM - GM
The lightship KG is required by law to be provided onboard.
Describe the precautions that must be taken during an inclining test to ensure that the result is as accurate as possible
- No wind, if any head wind
- Moorings should be slack, vessel can list freely
- The draught and density should be noted to accurately determine W and KM
- Vessel must be upright at commencement
- Only persons concerned with test onboard
- Weight shifted should give vessel about 4 degree list
- if possible no free surface effect
- No loose weights onboard
Cause of angle of list
Angle of List - is caused when the internal distribution of weights causes a shift of the Centre of Gravity horizontally from the center line: G to G1
Cause of angle of loll
If the initial metacentric height of the ship is negative, the ship is not stable in its upright condition, leading to a heeling moment, as shown in the diagram below.
As a result of the negative righting lever (GZ), the ship heels further until it reaches an angle where the righting moment and righting lever, both, become zero. The Transverse Metacentre (M) has moved up to coincide with the Centre of Gravity (G) resulting in a Metacentric Height (GM) of zero. This angle at which this condition is achieved is called Angle of Loll.
List circumstances which may cause an initially stable and upright yacht to come to an angle of loll
The following will cause a rise of G that could result in a negative GM:
-Consumption of stores, fuel and water from below the ship’s initial CoG.
-Free-surface effects in partially filled tanks.
-Collapse of a longitudinal division/bulkhead in a partially filled tank of liquid.
-Icing up of superstructures.
-Loading cargo in upper reaches of the vessel.
-Water entering the ship through badly maintained hatches and flooding the tween decks.
-Water landing on the deck from the sea in heavy weather conditions.
-Raising of a weight from a deck using a mast and derrick.
-Raising a weight low down in the ship to a higher position within the ship.
-Timber deck cargo becoming saturated due to bad weather conditions.
-A blockage of freeing ports or scuppers on the upper deck.
-Passengers crowding on superstructure decks at time of departure or arrival
-Adding weight at a point above the ship’s initial CoG.
-Discharging a weight at a point below the ship’s initial CoG.
An unstable vessel lying at an angle of loll to starboard has an empty double bottom tank subdivided into two watertight compartments of equal width. (as shown on the sketch)
The tank must be ballasted to return the vessel to a safe condition.
Describe with the help of attached sketch:
How you would correct the vessel lying at an angle of loll and the vessels response to your sequence of ballasting at each stage of ballasting?
To correct the angle of loll, start by filling the bottom tank (B). This will cause the Center of Gravity (G) to move downwards towards the gentre of gravity of the tank (g1) - when the tank is filled, the ship’s center of Gravity will have moved off the centerline to G1. The vessel may take up a greater angle of list, but this will be corrected when the high-side tank (A) is filled. The ship’s Centre of Gravity will now move towards the centre of gravity of the fluid in the tank (g2). When the tank is filled, the ship’s Centre of Gravity will have return to the centeline at G2.
Describe how the centre of gravity (G) of an upright stable vessel will move and the effect this will have, if any, on the angle of list AND metacentric height (GM) for the following scenario
Weights are moved from the port to the std side (Kg remains constant).
Horizontal shift of weight.
G will move parallel to and in the same direction as the shift of the centre of will move to std.
GM - The transverse weight shift resulting in the shift of G has done nothing to alter the vertical height of G nor the displacement, therefore there is no change in the initial GM.
List- The vessel was upright before the move. Weight has been shifted to the STB side The vessel will list to STB
Describe how the centre of gravity (G) of an upright stable vessel will move and the effect this will have, if any, on the angle of list AND metacentric height for the following scenario
A weight is lifted from low down on the port sided and relocated to a position on deck on the starboard side.
G move towards the centre of gravity of the weight, up and to STB
GM - The diagonal weight shift, up and to STB, results in a reduction in GM
The vessel was upright before the move. Weight has been shifted to the std side therefore vessel will list to STB.
Describe how the centre of gravity (G) of an upright stable vessel will move and the effect this will have, if any, on the angle of list AND metacentric height for the following scenario
A weight is loaded on the centreline and at height above the keel equal to the vessel KG
G will move directly towards the COG of the weight loaded, but as the weight will be loaded at the same KG, G will not move.
GM will not change as G has not moved.
The weight is loaded on the CL so therefore no list.
How will an angle of list affect the draught of a box shaped vessel
Draught will increase on the side shes listing towards
Explain the influence of freeboard on the GZ curve, and hence explain how the GZ curve
would change if the freeboard was increased?
The more freeboard that a vessel has, then the greater the angle to which she can be inclined without immersing the deck edge.
As the freeboard increases the following takes place:
- GM increases
- GZ increases, so Stability increases
- Deck edge becomes immersed at a greater angle 02 instead of 01
- KB decreases because drafts are less in value
- BM increases because volume of displacement is less.
- KMincreases.
- Range of Stability increases, giving greater dynamical stability.
Explain how the GZ curve would change if the KG of the vessel was reduced (but assume that the freeboard does not change).
The size of the righting lever (GZ) is directly proportional to the metacentric height (GM), this is because: GZ = GM × Sin 0
If KG is reduced, then GM is increased - this will result in an increase in the righting levers GZ for all angles of heel
The GZ curve produced for a vessel that has reduced KG (resulting in increased GM) will be steeper and have a larger range of stability
