Antiroll Tanks
Encyclopedia
Antiroll Tanks are tanks fitted on to ships in order to improve their response to roll motion. Antiroll tanks are tanks within the vessel fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side. The tank is designed such that a larger amount of water is trapped on the higher side of the vessel. This is intended to have an effect completely opposite to that of the free surface effect
. They can be broadly classified into Active and Passive Antiroll tanks.
of the roll moments acting on the ship and the resultant liquid motion will be such that it reduces the roll motion. This type of tank was first investigated by William Froude
, but did not receive much attention until the 1950s when it was revived and used in many naval vessels. They have the added advantage that it is possible to vary tank natural frequency by changes in water level and thus accommodate changes in ships metacentric height
. Free Surface Tanks are commonly referred to as "Flume" tanks.
to forward motion. The force required to accelerate sea water outside the ship (which is initially at rest) to the speed of the ship as it enters the ship is a substantial drag
component (momentum drag) as its magnitude increases with the square of ship speed. More recently a variation of these tanks have been used in oil drilling rig applications where forward motion is of little relevance.
-controlled valve system. Since this valve controls the flow of air very little power is required. When valve is closed passage of air from one tank to other is prevented and hence resulting compression of air in tank prevents flow of water also. When valve opens free movement of water and air is possible.
to force the water from one side of the ship to other rather than allowing it to slosh as in passive systems. Webster
(1967) studied the design of such a tank in detail. The main disadvantage to this is that when the pump is operated there is a time lag for a sizeable amoount of fluid to arrive at a tank, thus limiting instant roll stabilisation. Hence compared to fin stabiliser systems this is highly inefficient.
Free Surface Effect
The free surface effect is one of several mechanisms which can cause a craft to become unstable and roll over . It refers to the tendency of liquids — and of aggregates of small solid objects, like seeds, gravel, or crushed ore which can act as liquids — to slosh about: to move in response to...
. They can be broadly classified into Active and Passive Antiroll tanks.
Free Surface Tanks
A single partially filled tank that extends across the full breadth of the vessel. Its shape, size and internal baffles allow the liquid inside to slosh from side to side in response to the roll motion of the ship. The phasingPhasing
In the compositional technique phasing, the same part is played on two musical instruments, in steady but not identical tempo...
of the roll moments acting on the ship and the resultant liquid motion will be such that it reduces the roll motion. This type of tank was first investigated by William Froude
William Froude
William Froude was an English engineer, hydrodynamicist and naval architect. He was the first to formulate reliable laws for the resistance that water offers to ships and for predicting their stability....
, but did not receive much attention until the 1950s when it was revived and used in many naval vessels. They have the added advantage that it is possible to vary tank natural frequency by changes in water level and thus accommodate changes in ships metacentric height
Metacentric height
The metacentric height is a measurement of the static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre . A larger metacentric height implies greater stability against overturning...
. Free Surface Tanks are commonly referred to as "Flume" tanks.
U-Tube Tanks
The use of these tanks were pioneered by Frahm in Germany during the turn of the 20th century and they are often referred to as Frahm tanks. These partially filled tanks consists of two wing tanks connected at the bottom by a substantial crossover duct. The air column above the liquid in the two tanks are also connected by a duct. As in the Free Surface Tanks as the ship begins to roll the fluid flows from wing tank to wing tank causing a time varying roll moment to the ship and with careful design this roll moment is of correct phasing to reduce the roll motion of the ship. They do not restrict fore and aft passage as space above and below water crossover duct is available for other purposes.External Stabilizer Tanks
This was another concept introduced by Frahm and used in several ships in the early 1900s. In this concept the two wing tanks are connected only by an air duct at the top. Water flows in and out of each tanks via an opening in the hull to the sea. This eliminated the need for a crossover duct as in the other designs, but has its own set of disadvantages. This design promoted corrosion to the tanks due to the explicit interaction with sea water. The holes on the hull causes resistanceFriction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
to forward motion. The force required to accelerate sea water outside the ship (which is initially at rest) to the speed of the ship as it enters the ship is a substantial drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
component (momentum drag) as its magnitude increases with the square of ship speed. More recently a variation of these tanks have been used in oil drilling rig applications where forward motion is of little relevance.
Active U-Tube Tanks
This is similar to a U-Tube Tank but the water crossover duct is much larger and the air crossover contains a servoServomechanism
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
-controlled valve system. Since this valve controls the flow of air very little power is required. When valve is closed passage of air from one tank to other is prevented and hence resulting compression of air in tank prevents flow of water also. When valve opens free movement of water and air is possible.
Active Antiroll Tanks
The border between controlled-passive and active stabilisation is not that distinct. Active stabilisation generally implies that the system requires the use of machinery of significant power and the system must be much more effective in reducing roll in order to justify this high cost.Active Tank Stabilizer
This concept utilises an axial flow pumpAxial flow pump
An axial flow pump, or AFP, is a common type of pump that essentially consists of a propeller in a pipe. The propeller can be driven directly by a sealed motor in the pipe or mounted to the pipe from the outside or by a right-angle drive shaft that pierces the pipe.The main advantage of an AFP is...
to force the water from one side of the ship to other rather than allowing it to slosh as in passive systems. Webster
Webster
-People:As a surname:*Augusta Webster , English poet, dramatist, essayist, and translator.*Daniel Webster , American statesman, U.S. Senator from Massachusetts, and U.S...
(1967) studied the design of such a tank in detail. The main disadvantage to this is that when the pump is operated there is a time lag for a sizeable amoount of fluid to arrive at a tank, thus limiting instant roll stabilisation. Hence compared to fin stabiliser systems this is highly inefficient.
External links
- http://www.allatsea.net/article/February_2007/Anti-Roll_Tanks_-_A_Simple_Way_to_Stabilize
- http://www.flume-art.com/
- http://www.hoppe-bmt.de/