Ships Stability Flashcards
(9 cards)
Describe, with the aid of a sketch, a freeing port
INSERT SKETCH
- A Freeing Port is an open hole cut into a bulwark that allows rapid draining of sea water and collected rain water from the weather deck.
- Continuous drainage of shipped water prevents any change to GM allowing for stable stability.
Explain how freeing ports assist in maintaining the stability of a ship
- Freeing Ports assist in maintaining the ships stability by maintaining a positive metacentric height (GM), they do this by continuously draining any shipped water therefore preventing any change to GM.
- Prevents Free Surface Effect which would have a massive affect on the overall stability of the ship.
+ Clearing water from deck, reduces the chance of capsizing.
+ As a result, the stability of the ship is maintained.
Explain the term Free Surface Effect
- Happens when liquids or loose materials can move freely in a tank or compartment.
- As the vessel heels or rolls, the liquid shifts to the lower side, moving with the motion of the ship.
- This shift causes the Centre of Gravity (G) to move sideways.
- The result is a reduction in the ships metacentric height (GM), which means less stability.
+ The effect is worse in wide, shallow tanks where liquid spreads out more.
+ Can lead to increased rolling, listing or even capsizing if uncontrolled.
+ Common in partially filled tanks or compartments.
Explain why free surface effect is important in ro-ro ferries and other ships having large enclosed deck areas.
- Ro-Ros have large open vehicle decks with very few internal bulkheads.
- If water enters, it will spread out freely and move with the ships motion.
- This creates a severe free surface effect, reducing the Metacentric Height (GM)
- Ro-Ros also often have a small freeboard, which means water can entre the deck much more easily in rough seas or even if the bow doors fail.
+ A small amount of water can quickly cause a dangerous list or even capsize.
+ Free Surface Effect is one of the leading causes of Ro-Ro ferry accidents, like the Herald of Free Enterprise.
with reference to the accumulation of water on the open decks of ships
Explain the importance of ensuring that there is no accumulation of water
- Water accumulation on a ships deck has a danger of Free Surface Effect.
- The fluid moves with the vessel shifting the Centre of Gravity (G) which has a negative effect on the ships stability.
- G moves up which then introduces the Metacentric Height (GM)
+ if serve enough, this will cause the vessel to capsize.
with reference to the accumulation of water on the open decks of ships
Explain, with the aid of a sketch, how it is prevented
INSERT SKETCH
- A Freeing Port is an open hole cut into a bulwark that allows rapid draining of sea water and collected rain water from the weather deck.
- Continuous drainage of shipped water prevents any change to GM allowing for stable stability.
with reference to transferring large quantities of fuel from low tanks, such as double bottom storage tanks, to higher wing bunker tanks
Explain the possible effects on the stability of the vessel
- Wing Tanks are located higher than Double bottom Tanks.
- Transferring fuel upward raises the ships Centre of Gravity (G).
- A higher G Results in a lower Metacentric Height (GM).
- Reduced GM = Reduced Stability (Listing)
- Stability is negatively affected as weight is moved from low to high position.
- The situation worsens if the double bottom tanks are emptied further without compensating.
To maintain better stability, use fuel from the wing tanks first, keeping weight lower for longer.
with reference to transferring large quantities of fuel from low tanks, such as double bottom storage tanks, to higher wing bunker tanks
State the important checks to be made before and during the fuel transfer operation
Explain, with reference to stability, why, when vessels has full bunkers, fuel stored in higher tanks, such as wing tanks, is generally used before fuel stored in lower tanks, such as double bottom tanks
- Fuel is taken from wing tanks (higher) before double bottom tanks (lower) due to stability reasons.
TAKEN FROM WING TANKS FIRST
- Removing fuel from the higher tanks first will lower the Centre of Gravity (G).
- A lower Centre of Gravity (G) increases the Metacentric Height (GM).
- An increased GM means better stability.
- Lowering G also Increases the Righting Arm (GZ) when the ship heels .
- A Longer GZ gives a stronger righting moment, helping the ship return upright more effectively.
TAKEN FROM DOUBLE BOTTOM FIRST
- Weight is removed from low in the ship, so G rises.
- Keel of Centre of Gravity (KG) increases so GM decreases = Reduced Stability.
- Higher G also shortens GZ.
- Shorter GZ = Weaker Righting Movement
