By William Huang
If you’ve ever been on a boat, big or small, in the sea or ocean, you probably know that the voyage isn’t the smoothest form of transportation with winds, tides, and waves causing constant up and down, side to side motions. However, those motions pale in comparison to what could be reality without one essential part of all modern vessels -- roll stabilization systems.
There are a variety of roll stabilization systems for ships, from fins and rotors to anti-rolling tanks. However, they all serve one vital purpose to water transportation: preventing significant rolling of ships due to winds, tides, and waves, making voyages across bodies of water much less turbulent. By making the rolling effects of the water less prominent, passengers are less prone to seasickness and other health related issues.
There are two broad categories of stabilizers: passive and active systems. Passive systems do not require separate sources of power or control systems to function. Contrarily, the more modern active systems provide more stabilization through the controlled movement of masses, which usually require separate control systems, such as gyroscopes and hydraulic systems, to function. Modern boats, especially large ships, such as civilian cruise ships, cargo ships, and naval vessels, often utilize both passive and active systems for increased stability. Below, we will discuss the main types of stabilization systems and how they function.
Photo of the Littoral Combat Ship USS Independence in a floating drydock. This is a trimaran ship with active fin stabilizers on the center hull .
There are two main passive stabilizers: bilge keels and fixed fins, both of which function in similar fashions.
The use of bilge keels is one of the simplest and earliest methods of ship stabilization, with the first ships being equipped with the structure in about 1870. These structures are still the most popular methods of stabilization and are installed in most ships. Bilge keels are thin, flat, long strips of metal that project from the turn of the bilge of the ship and run along much of the length of the outer hull, usually in pairs with one keel on each side. They are usually attached to the lower sides of the hull to prevent increasing the draft, or distance between the waterline and bottom of the hull, which determines the minimum water depth at which the vessel can travel in. In naval battleships, bilge keels are often larger than in commercial ships and are used for torpedo defense. Because of the structure’s flatness and protrusion out of the hull, it acts as a sharp obstruction to the rolling motion of turbulent water and increases the ship’s hydrodynamic resistance. However, because of the increased resistance, the vessel’s forward motion can be hindered, so a balance between keel size to stabilize the ship and ship speed is always considered during design and construction. One advantage to bilge keels is that they do not impact the internal design of the ship as they do not have any internal components unlike other forms of stabilization. Because of this, bilge keels are vital to naval architecture and are often used in conjunction with other roll stabilizers whenever possible.
Similar to bilge keels, passive fins are fixed and fitted onto both sides of the lower outer hull at the stern (back) and/or bow (front) of the boat. Fins function in the same way as bilge keels but often protrude farther from the boat, allowing for more hydrodynamic resistance and decreased rolling. The trade-off, however, is reduced maneuverability and increased required docking clearance.
These two passive systems are commonly used in vessels due to their simple design and relatively simple implementation. However, most modern ships also include active systems to enhance stabilization.
Photo of a bilge keel and passive fin on the hull of a large ship 
Active stabilizers are technologically more advanced and complex than passive stabilizers but are able to better counter the rolling motion of water. As mentioned before, these systems involve controlling the movements of masses with control systems, usually a gyroscope or hydraulic/pressure system. The two commonly used active stabilizers are active fins and active anti-roll tanks.
The outer structure and placement of active fins are similar to passive fins; however, active fins can be adjusted by control systems inside the boat according to wind and sea conditions. Much of the fin actuating machery is in what’s often called the fin box and includes multiple complex systems, such as hydraulic power units, local control units, roll motion sensors, and more. In general, a gyroscope system measures the orientation and angular velocity of the ship to sense its rolling motion. A signal is then sent to the control systems to move and rotate the fins in a direction to oppose the roll. When docking, some ships are also able to retract the fins into the hull to decrease the required docking clearance.
Photo of an active fin stabilizer 
Anti-roll tanks are quite different from bilge keels and fins in that they do not include any protruding structures from the boat. Instead, the stabilization system is inside the hull and consists of large, water-filled tanks in the sides of the hull with a system of pumps, air pressure controls, and roll sensors. In general, an accelerometer or gyroscope system is used to detect the rolling motion of the ship, then a signal is sent to the pumps and air pressure controls to control the flow of water inside the tanks. If the waves tip the ship towards the port side (left side facing the front of the boat), then the water in the starboard tanks (right side facing the front of the boat) will rise to counter the waves’ motion. The same logic applies when the ship tips toward the starboard side. Depending on the complexity of the system, active anti-roll tank stabilizers have been found to be about 80% efficient in stabilizing rolling motions.
Diagram of an anti-roll tank stabilization system. Water is contained in large tanks on the sides and is transferred from side to side through pumps and pipes .
Over many years, more advanced stabilization systems have been developed, from bilge keels to active anti-roll tanks. Without these systems, travel by water would be extremely turbulent, inefficient, and unenjoyable. Thus, you can see how important ship stabilization systems are to a smooth cruise in the ocean.
Sinha, T. (2019, October 19). Different Types Of Roll Stabilization Systems Used For Ships. Retrieved from https://www.marineinsight.com/naval-architecture/roll-stabilization-systems/.
Koenig, K. (2014, August 18). Everything You Ever Wanted to Know About Marine Stabilizers. Retrieved from https://www.powerandmotoryacht.com/maintenance/everything-you-ever-wanted-know-about-marine-stabilizers.
Grant, R. (2015, June 11). How Cruise Ship Stabilisers Work. Retrieved from https://cruisedeals.expert/how-cruise-ship-stabilisers-work/.
What is Bilge Keels of a ship ? (2019, April 8). Retrieved from https://www.worldmaritimeaffairs.com/what-is-bilge-keels-of-a-ship/.
Anish. (2019, October 11). Understanding Ship Fin Stabilizer and Its Operation. Retrieved from https://www.marineinsight.com/tech/ship-stabilizer/ship-fin-stabilizer/.
Knupp, C. (2019, July 13). Warship Stabilization Systems : Warship Tech. Retrieved from https://www.navygeneralboard.com/warship-stabilization-systems-warship-tech/.