Stability Flashcards
Whats Free surface effect
The movement of the centre of gravity of the liquid, and that of the ship, will be outward in the same direction as the heel, and upward, relative to the ship’s centre of gravity. The movement will reduce the righting moment, and thus reduce the stability. This is called the ‘free surface effect’ and is expressed as an apparent (or virtual) rise in G, and thus a reduction in metacentric height GM.
On heeling, the centre of gravity of the liquid in the tank shifts from G to G1.
This shift will mean that the ship’s centre of gravity also moves; it moves parallel to the movement of G.
So, the ship’s centre of gravity moves from G to GT (true centre of gravity). This has the effect of reducing the lever arm of the righting moment, and thus making the ship less stable. The new righting arm will be the same for a ship’s centre of gravity at GT or GA, directly above it on the centre line. So, GA (apparent centre of gravity) and the apparent metacentric height, GAM, are used as a measure of the free surface effect, and of the reduced stability.
Explain a tender vessel
Explain a stiff vessel
List is:
List is when it is inclined by forces within the vessel, e.g. movement of weight within the vessel. A horizontal movement of G for instance loading a tender on one side of the vessel the centre of gravity is off centre to either port or starboard.
To correct add weight to the high side or remove weight from the low side.
• An angle of list is caused by uneven distribution of weight.
• In port, this may be corrected by distribution of cargo or ballast operation.
• At sea, the transferring of bunkers or ballast or ballasting to the high side or ballasting
the low side does this.
LOLL is
LOLL
The term “loll” describes the state of a vessel, which is unstable when upright, and which floats at an angle from the upright to one side or the other, caused by a vertical movement of G.
If an external force, e.g. a wave or wind, changes this state, the vessel will float at the same angle to the other side.
To correct for loll the following procedure should be observed: You must lower the centre of gravity.
You can take ballast. If you do so (and if your vessel has ballast tanks that you can fill), then you should begin by pressing up tanks on the low side first. This will initially make your angle of loll worse because you are adding weight on the side to which the vessel is leaning, and you are introducing a free surface (if you are ballasting on an empty tank). This is still safer than ballasting the high side first, because that could cause the vessel to flop-over to the other side, and possibly capsize.
The second option open to you is to remove the cause. A loll does not suddenly occur. It is a result of decreasing stability that is caused by the progressive rising of the centre of gravity of the vessel. This can only occur if you are loading weights on deck and using fuel or water from low down in the hull (where most tanks are located anyway). In these circumstances you may have to jettison weight from above G.
Duties for dry-docking
Chief Officer’s Duties
Preparation and Precautions for Entry
Prior to arrival in the dry dock, prepare for a critical manoeuvre via engine checks, steering checks good briefing before arrival agreement on who is doing what with the yard!
• All hatches and beams should be in the stowed position to ensure continuity of strength throughout the ship’s length.
• All derricks and cranes should be down and secured, not flying.
• Any free surface in tanks should be removed or reduced to as little as possible, either
by emptying the tank or pressing it up to the full condition.
• Stability calculations should be made to ensure adequate GM to take into account the
rise of ‘G’ when the vessel takes the blocks.
• Inform dock authorities in plenty of time of any projections from the hull of the
vessel, as indicated by drydock plan.
• Sound round all tanks once the vessel has taken the blocks, to ensure a similar
stability state when leaving the drydock.
• Lock up ship’s lavatories before entering the dock.
• Ensure adequate fenders are rigged for entry into the dock and that dock shores are
correctly placed against strength members once the vessel is positioned. If it is the custom in the graving dock, arrange for fo’c’sle head party to position shores on one side and the stern party to deal with the other side.
• If required, endeavor to have the vessel cleaned and scrubbed as the dock water is pumped out.
• Inform dock authorities where to position extra shores or blocks to take account of additional stresses caused by the weight of cargo aboard.
• Obtain telephone/electricity/and water pressure fire line garbage and sanitation facilities as soon as possible.
Docking Plan
Docking plan
Docking Plan
The Docking Plan includes profile, plan and sections of the vessel which is aligned with dock drawings to ensure that the vessel can be accommodated and locate the position of the keel blocks, bottom and bilge blocks, together with side shores. When placing shores on the ship side, it is important that the end of the shore is positioned over a frame or at the intersection of a frame and a deck stringer.
Usually, the vessel will be trimmed by the stern, which enables the stern to set down onto the blocks and then be used as a pivot to align the keel along the blocks. As water is pumped from the dock or the lift platform is raised, part of the buoyancy of the water is transferred to the keel blocks, this is called the Upthrust (P). In effect, this can be considered similar to a weight pushing up from the keel aft, and the Upthrust causes a loss of GM. Caused by a virtual rise in G!
How do we compensate for bad weather?
Navigation and Bad Weather
• Consider re-routing.
• Verify the vessel position.
• Update weather report.
• Plot storm position often.
Fred’s Web App Notes M200 FW04
• Engage manual steering.
• Reduce speed.
Gross Tonnage
Considered as the measurement of the vessel’s internal volume below the upper deck less certain exemptions. Other spaces include wheelhouse, bridge, chartroom, radio-room, galley, etc., measured above the tonnage deck. (a gross ‘tonne’ is a unit of volume containing 2.83m3).
Net Tonnage
Represents the earning capacity of the vessel and is obtained by deducting non-freight spaces from the gross tonnage (i.e. Master/crew accommodation, chain lockers, machinery spaces, workshops, ballast tanks, etc.). Port and harbour dues are assessed on net tonnage.
Reserve Buoyancy
The volume of the entire watertight part of the hull is known as the buoyant volume; that portion below the waterline-buoyancy and that above the waterline is called Reserve Buoyancy. An approximate measure of this reserve is indicated by the ‘freeboard’ i.e. the height above the waterline of the highest continuous watertight deck.
The Fresh Water Allowance
The fresh water allowance is calculated to allow for the change of draught when the vessel moves from FW to SW (RD 1.025). It is the amount one may submerse the appropriate saltwater mark when in freshwater. Since FW is less dense than SW, the vessel must displace a greater volume of FW to achieve the same weight, (by sinking deeper in the water.
