You might have often seen videos or pictures of ships tipping from one side to the other and getting destabilize while moderate waves crash against the ship. Most of the times, this is because of a failure in one of their systems, the stabilizers. We’ll find out how do these stabilizers actually balance a ship in the wavy waters and also see the impact on the ship if the stabilizers fail to do their job.
The stability of a ship is determined by the centre of gravity and the centre of buoyancy. As long as the two centres are perfectly lined up in their correct place, the ship is going be upright no matter what. Smaller waves are unlikely to displace the two centres, but as the wave length and wave amplitude increases, the rolls start to become significant. The ship will move from side to side, following the rhythm of the wave. Most of the time the ship will just continue to ride the waves from side to side, but with the exception of synchronous rolling, where the frequency of the waves match the frequency of the ship’s roll. This results in rapidly increasing motion of the ship until it finally capsizes.
Gentle rolling isn’t an issue on cargo ships but on a cruise ship, the customers won’t be happy when the glasses of champagne fall down the table. These ships use stabilizers to prevent this from happening. The goal of the stabilizer is to make the ship stay upright while the waves roll underneath the ship. They achieve it by generating a force which counters the tipping force provided by the waves. Unfortunately that tipping force isn’t constant. To solve these problems, the industry has come up with a quite a bit of solutions.
We have two types of stabilizers. Active and passive.
A good example of passive stabilizers is bilge keel. These are protrusions in the form metal strips from the hull. All they do is dampen any roll. They produce turbulence when the ship’s hull moves up and down. Although bilge keel is not much effective, they only dampen the rolling motion and not completely stop it. These are relatively cheap and easy to install, and are mostly found in cargo ships, as they don’t do enough to be put on cruises.
Another example of passive stabilizers is anti-roll tanks. These are tanks of water constructed on extreme sides of the ship and connected with a crossover pipe. Normally with the ship upright, the amount of fluid inside the tanks will be same. But when the waves interact, the fluid starts to flow to other side from the crossover pipe. Using the weight of the extra fluid in the other tank, the ship actually does stabilize itself, but it isn’t instant.
We can add a gyroscopic motor to the crossover pipe to pump water to either side and make it an instant stabilization. This system with pumps then falls into the category of active stabilization. It does help in cruise ships but it takes time for the water to fill in or flush out of the tanks, adding a little delay between stabilizations.
The most common active stabilization system in cruise ships is the fin system. It consists of a hydraulic operated wing sticking out from the hull underwater. It looks similar to the bilge keel, but the fins are able to change the angle of deflection. These fins work on the same principal as the rudder of a ship, but only sideways. With correct gyroscopic control, these fins can produce, almost instantly, the perfect amount of counter force to prevent the rolling motion of the ship. The fin stabilizers also can counter forces other than waves, like heavy winds, the internal movement of people or cargo.
So, next time you sit in a cruise sipping champagne in high sea, just think about the technology that goes into keeping your glass from sliding off the table.
Fun Fact: The fin stabilizers only work when the ship is moving (they need a water flow across them) and has a speed above 6 knots. Fortunately, cruises have a speed of well above 6 knots and they can benefit from these fins. Cargo ships don’t move at such high speeds and they therefore cannot use the fins as effectively.
Authored By:- Cdt. Shubham Shivne, TMI