Bilge keels: A bilge keel is used to reduce a ship’s tendency to roll. Bilge keels are employed in pairs (one for each side of the ship). Bilge keels increase hydrodynamic resistance to rolling, making the ship roll less. Bilge keels are passive stability systems.
On commercial ships the bilge keel is the form of a strake, running along much of the length of the hull. They are typically fitted one on each side, low down on the side of the hull, so as not to increase the draft of the vessel. Although not as effective as stabilizing fins, bilge keels have a major advantage in their low impact on internal ship arrangements. Unlike Stabilizer fins, bilge keels do not have any components inside the hull that would adversely affect cargo spaces. Like fins, bilge keels have the disadvantage of increasing the hydrodynamic resistance of the vessel, thus hindering forward motion. This impact is very low and negligible.
To minimize hydrodynamic drag the bilge keel is placed in way of a flowline where it does not oppose crossflow. Ends of the bilge keel are tapered and properly faired into the hull. A bilge keel does not protrude from the hull as seen in the diagram. Bilge keels do not protrude below the baseline either, where they could be damaged or fouled by grounding.
A bilge keel is constructed from flat plate so as to present a sharp obstruction to roll motion. The roll damping provided by a bilge keel is more than that of a barehull ship, but falls short of other roll damping devices. Bilge keels can also be used in conjunction with other roll damping devices. Cost of fitting Bilge keels is negligible.
Double bottom tanks: A double bottom is a ship hull design and construction method where the bottom of the ship has two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship and a second inner hull which is somewhat higher in the ship, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.
The space in between the two bottoms is often used as storage tanks for fresh water or ballast water. Fuel storage in the double bottom is not allowed for newbuilt ships since 2007, due to MARPOL 73/78.
Double bottoms are significantly safer than single bottoms. In case of grounding or other underwater damage, most of the time the damage is limited to flooding the bottom compartment and the main occupied areas of the ship remain intact. For this reason, double bottoms have been required in all passenger ships for decades as part of the Safety Of Life At Sea or SOLAS Convention.
An even more extensive protection is available as a double hull, where the second hull layer extends up the sides of the ship as well as in the bottom.
A double bottom also conveniently forms a stiff and strong girder or beam structure with the two hull plating layers as upper and lower plates for a composite beam. This greatly strengthens the hull in secondary hull bending and strength, and to some degree in primary hull bending and strength.
Some of the advantages of double bottom in ships are:
provide protection to hull in case of grounding
provide great longitudinal strength
can be used as tanks to carry ballast water or fresh water.
helps to prevent pollution in case of liquid cargo (like oil in tankers)
helps to maintain stability of ship
the tank top acts as a platform for machinery and cargo
Peak Tanks: There are 2 peak tanks at fore and aft ends of the ship. Forward is called Fore peak tank and after one is called After peak tank.
Fore Peak Tanks: It is placed forward of the collision bulkhead. Normally it is used for ballast and in some cases for storing Fresh Water. However many modern ships keep it as VOID space. This is done to avoid heavy expenses of replacing worn out metal after 10-15 years of life. When used for Ballast, it helps in correcting trim with minimum amount of water allowing higher amount of cargo.
After Peak Tank: It is identical to fore peak tank and located at stern end. It is forward of Rudder stock and Propeller shaft (Stern tube) passes through it. In earlier days this tank water was used to cool stern tube. Its construction is entirely made up of SOLID floors. Normally it is used for Ballast or for storing Fresh Water.
Wing tanks: This is a broad term and can be further subdivided in “Upper/Lower wing Tanks”, “Top/Lower side tanks”, “Heeling tanks”, Anti-Rolling tanks” etc. Common among all tanks is they are attached to ship sides, means one side of the tank is shell frames and plating (Ship side). Normally they are used for Ballast and in some cases for storing Fresh water. Fuel oil storage is not permitted. Presence of them, covers side frames and gives smooth surface for cargo carriage. Their location allows list/heel to be corrected using minimum weight.
Thing to remember is tanks absorb frames of the ship inside their construction and provide smooth surface for cargo carriage. Smooth surface also makes it easier to clean ship from cargo residues after discharging.
When tanks are used for BALLAST, it is to add weight to the ship to make her STABLE and sink propeller underwater. Without ballast and cargo, ship will float like a PING PONG ball on water and impossible to manage.
Bilges: This term is not to be mixed with other versions of word “BILGE”. BILGE also means
a : the part of the underwater body of a ship between the flat of the bottom and the vertical topsides
b : the lowest point of a ship’s inner hull.
Over here we are talking about Bilge wells. Word is in use for very long time. Earlier ships were made up of wood and use to have small leakages in the hull. In addition some water leaked from decks during bad weather. This water got collected at the bottom of the ship next to Solid Keel on either side. This collected water was pumped out using pumps. Realize that water also got collected in cargo compartments endangering dryness of cargo.
Modern STEEL ships do not have leaks from the hull but may have them from internal pipelines or equipment in Engine room. Hatch covers are Weather-tight and may have minor leakages during bad weather. In addition, there is condensation of water vapour on steel surfaces. Water from both sources drains to the bottom and if not collected, will start sloshing all over the bottom of the ship damaging cargo. Therefore a deeper box (well) is created inside Double bottom to collect this leaked water on port and starboard sides. These wells are called “BILGE WELLS”. They have pumping out arrangement by way of a pipeline that is connected to a pump in Engine room. It also has arrangement to measure volume of collected water by way of Sounding pipe. Modern Bilge wells have liquid level ALARMS. Bilge well is covered with Filter plate (Porous metal plate) that allows only water to drain in the bilge well, keeping solids outside on Tank top. Similarly end of suction pipe also has filter arrangement called Strum box. Bilge wells are also referred as BILGES.
You must be logged in to post a comment.