An anti-roll tank is a device used to minimize the roll of a vessel, making it seem more stable and comfortable for passengers and crew. It consists of a single centreline tank; or a pair of connected water tanks, located to either side of the vessels beam. As the vessel rolls, the transfer of water back and forth will (ideally) be 180 degrees out of phase of the roll, thus counteracting the motion. Anti-roll tanks may be passive ('tuned' by controlling the level of water in the tank or by the fitting of internal baffles) or active (aided by mechanical pump or through air pipes with controllable valves). The actual design of an anti-roll tank is considered a bit of a black art, due to the number of variables involved.
A basic flume tank can be of relatively simple construction and design, though controlled systems add a bit more complexity. They require no forward motion or external appendages (i.e., compared to fins) and can function without power or human input (though, see stability concerns below). Flume tanks can address both stiffness and laziness in a vessels roll characteristics, and can be modified through altering tank levels.
Flume tanks complicate the vessels stability (see below). The purposeful addition of free surface can lead to a vessel capsizing if environmental conditions stray too far from the design conditions. Flume tanks require both volume, often internally in the vessel, and displacement - ~2% of the vessels overall mass. U shaped tanks in particular require an additional cross-duct and potentially overhead air pipes, which will complicate the vessels arrangement considerably. The choice of working fluid is preferentially non-corrosive, such as fuel or fresh water - if a vessels consumables are used, they will be unavailable for the vessel on those conditions in which the tank is in use.
Anti-roll tanks can be constructed in a number of different ways. The methods of operation, monitoring and effectiveness will vary with the construction technique.
Flume-based anti-roll tanks consist of a single tank, open from port to starboard. Flume tanks are controlled by adjusting the volume of water contained. Flume tanks are common on smaller vessels, often with smaller tanks mounted high on the deckhouse. With an externally mounted tank, the contents can be dumped and allowed to run overboard in case of emergency.
U connected anti-roll tanks are two separate tanks, connected by a pipe across the bottom. They may have ventilation pipes as well. U-connected tanks can be moderated by controlling the ventilation valves as well as the water level, allowing for a greater amount of control of the tanks effects. These tanks are generally fitted as deep tanks, lowering their VCG, and providing a greater degree of safety in operation over the flume variety. U-tanks are often fitted with dump tanks underneath, allowing for a rapid drain and lowering of VCG in case of emergency. The dump tanks will be transversely divided as well, to decrease the free surface effect of an anti-roll tank out of phase.
For the greatest effect, anti-roll tanks are often placed where they would induce the highest counter-rolling moment (often, high up on the vessel). The tanks are generally kept slack, as the induced movement of the water is what produces the counterforce. It should be noted, however, that the quantity and location of the water tends to raise the vessels VCG, a detrimental side-effect on most vessels. An anti-roll tank subjected to a continual list or an asymmetrical roll, will have a tendency to increase the roll as water shifts preferentially to the lower side. The tanks only reduce roll motions, they do not increase the range of intact stability, which may be misleading during vessel operations.
When considering stability conditions, the free surface moment for anti-roll tanks may be able to be disregard. Stability conditions generally consider competent masters, and the operational level of the anti-roll tank should be such that the free surface moment is counteracted by the vessels roll period. It is still advised to consider the free surface moment, as this is a more conservative and comprehensive approach. It should also be noted, the free surface moment must be considered whenever an external heeling force is applied - such as during crane operations, or while towing.
Relying on free surface, the effect of the tank is subject to the magnitude and frequency of the vessels roll. Flume tanks will have a design roll period at which it is most effective, the tank response falling in roll periods to either side, increasingly ineffective and quickly becoming detrimental to stability. In practice, a change in the direction of the sea, such as a vessel turning, could put the vessel at risk of capsize if due care isn't paid to the tanks response and the stability characteristics of the vessel.
As the result of the above stability concerns, when anti-roll tanks are installed, a detailed operations manual should be supplied as well. It is generally advisable to dump the anti-roll tanks in situations where stability is reduced; i.e., heavy weather, where the vessel acquires a list due to wind or load, or when loaded in such a manner that it begins to approach the limits of prudent operation.
Ideally, the design of an anti-roll tank, and its operational properties will be determined by model testing prior to operations onboard the vessel. A stability monitoring system may be used to optimize tank operations.