Antiroll tanks are tanks fitted onto ships in order to improve the tank's response to

roll Roll or Rolls may refer to: Movement about the longitudinal axis * Roll angle (or roll rotation), one of the 3 angular degrees of freedom of any stiff body (for example a vehicle), describing motion about the longitudinal axis ** Roll (aviation) ...

motion. Fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side and the reverse, the tanks are designed such that a larger amount of water is trapped on the higher side of the vessel. This is intended to reduce the roll period of the hull by acting in opposition to the

free surface effect The free surface effect is a mechanism which can cause a watercraft to become unstable and capsize. It refers to the tendency of liquids — and of unbound aggregates of small solid objects, like seeds, gravel, or crushed ore, whose behavior app ...

. They can be broadly classified into active and passive antiroll tanks.

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

phasing A phaser is an electronic sound processor used to filter a signal, and it has a series of troughs in its frequency-attenutation graph. The position (in Hz) of the peaks and troughs are typically modulated by an internal low-frequency oscil ...

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 Natural frequency, also known as eigenfrequency, is the frequency at which a system tends to oscillate in the absence of any driving force. The motion pattern of a system oscillating at its natural frequency is called the normal mode (if all pa ...

by changes in water level and thus accommodate changes in a ship's

metacentric height The metacentric height (GM) is a measurement of the initial 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 initial stabi ...

. Free surface tanks are commonly referred to as "flume" tanks.

U-tube tanks

The use of these tanks was pioneered by Herr H. Frahm in Germany at the start of the 20th century and they are often referred to as Frahm tanks. These partially filled tanks consist of two wing tanks connected at the bottom by a substantial crossover duct. The air columns 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 the 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 tank via an opening in the hull to the sea. This eliminates the need for a crossover duct as in the other designs, but has its own set of disadvantages. This design promoted corrosion of the tanks due to the explicit interaction with sea water. The holes on the hull cause resistance 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 has been used in oil drilling rig applications where forward motion is of little relevance.

Controlled passive antiroll tanks

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

servo Servo may refer to: Mechanisms * Servomechanism, or servo, a device used to provide control of a desired operation through the use of feedback ** AI servo, an autofocus mode ** Electrohydraulic servo valve, an electrically operated valve that c ...

-controlled valve system. Since this valve controls the flow of air very little power is required. When the valve is closed, passage of air from one tank to the other is prevented and the resulting compression of air in the tank prevents flow of water also. When the 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 pump An axial-flow pump, or AFP, is a common type of pump that essentially consists of a propeller (an axial impeller) in a pipe. The propeller can be driven directly by a sealed motor in the pipe or by electric motor or petrol/diesel engines mounted ...

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 amount of fluid to arrive at a tank, thus limiting instant roll stabilization. Hence, compared to fin stabiliser systems, this is highly inefficient.

Active tank stabilizer with energy recovery

Instead of consuming energy to control the flow inside the tank, this concept utilises a

water turbine A water turbine is a rotary machine that converts kinetic energy and potential energy of water into mechanical work. Water turbines were developed in the 19th century and were widely used for industrial power prior to electrical grids. Now, ...

to produce electricity using the water that sloshes into the tank as in passive systems. The main advantages to this are operating cost reduction by replacing part of fuel consumption and control of water flow without any mechanical mobile device.Archived a
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References

{{Reflist *''Principles of Naval Architecture'' Vol.III, SNAME, 1989, Pg: 127

External links

*https://web.archive.org/web/20100917163912/http://allatsea.net/article/February_2007/Anti-Roll_Tanks_-_A_Simple_Way_to_Stabilize *http://www.hoppe-marine.com/ *http://www.geps-techno.com/ Watercraft components Control devices