Before start to describe the stabilizer system in general, and those that were employed onboard Michelangelo and Raffaello, it is necessary to clearly distinguish the 2 main types of motions that involve all ships.
1) PITCHING: are the movements of the ship in the prow-stern direction.
2) ROLLING: are the movements of the ship in lateral (left-right) direction. Ships extending much more in the length direction than in width, are prone to more lateral movements. The rolling motions are the main cause of the seasickness, especially when travelling in slightly bad sea conditions.
The effect of the rolling in storm, on the liner Rex. The Rex (1932) had not anti-rolling systems.
To reduce the seasickness and improve the comfort of passengers, some liners were equipped with an anti-rolling system, just as some ships today. In reality, anti-rolling systems were invented for quite different reasons, they were employed on military ships.
In fact, reducing the rolling movements of the ship, made it possible to aim the cannons and artillery more easily to the enemy ship. Who aimed better, survived.
Let’s examine the different systems, starting from the older.
LOAD REPARTITION: Originating in the Roman times, but on some occasions where it is needed, it is still used nowadays.
It consists of stowing the load, especially when it is about half or slightly less of the full load, on the sides of the higher decks. In doing so, the weight of the load applies strengths that work as a counterbalance, contrasting the strengths that cause the rolling movements.
This system also affects, though very little, the pitching motions.
ANTI-ROLLING CASES: Working similar to the load repartition system, but in this case the weight is obtained by tanks built inside of the hull and filled with water or mineral oil. These tanks can extend long the entire length of the hull. The disadvantage of this system is that wide liquid surfaces are free to move, consistently reducing the properties of stability of the ship.
PASSIVE FINS: They are slightly extended out profiles, running along the lower edges of the hull, mostly in the central part of the hull. It is the more inexpensive and system, used also on very small size boats. The efficacy increases with the increasing both of the ships speed and the rolling motions speed. Anyway it is not an efficient system. In the photo is visible the right passive wing of the liner “Cristoforo Colombo” the twin of the Andrea Doria.
GYROSCOPIC SYSTEM: It uses the physic principles of the gyroscope, where one rotating mass produces a major opposition against an external strength, working to alter its motion direction.
It is realized by 2 or 3 enormous spinning “tops”, of which their rotation axles move under the directions coming from a sensor that detects the lateral inclination of the ship. The force of inertia of the spinning tops create an opposition strength to the motion of the ship.
The first passenger ship having this stabilizer system was the Italian liner “Conte di Savoia”, the brother of the famous “Rex”, in 1932.
It was composed of 3 spinning tops 100 tons each in weight, located toward the prow section of the hull.
This system had several limits. Firstly, the hulls of mercantile ships (different than military ships) are not built to house so highly concentrated weights. These abnormally heavy and concentrated loads cause enormous torsions strengths on the hull, during the rolling and pitching movements, also altering the stability of the ship. The hull of Conte di Savoia emitted frightful creaks in the presence of bad seas.
Additionally, in rough and irregular sea, when quick and sudden movements of the spinning rotation axles was required, the mechanism was not able to work with sufficient speed. This brought the system into anti-phase, increasing the rolling motion instead of working against it.
The low efficacy, the high costs and the wide spaces required, soon brought to its end.
2 of the 3 spinning tops onboard the liner “Conte di Savoia” under construction, and one group photo with some 1st class passengers visiting them.
ACTIVE STABILIZER FINS: It is the more advanced and efficacious system, it was installed on Michelangelo and Raffaello as well on several ships nowadays. It is based on the hydrodynamic thrust generated by one or more couple of “fins” protruding from the hull under sea level. Their extension can be from 1/2 to 1/4 of the hull width, and their width vary from 0,4 to 1,2 meters. The fins work similarly to the fins of an airplane, but their inclination is commanded by a sensor that detects the lateral bending of the ship.
Naturally, the efficacy of the fins is proportional to their surface and to the speed of the ship.
The weight of the complex is about 1/4 of the gyroscopic system. As was on Michelangelo and Raffaello, the fins could be retracted inside of the hull when not necessary. This decreased the water resistance and the fuel consumption.
SCHEME OF ONE RETRACTABLE FIN, type “Denny-Brown”, mounted on the liner Leonardo Da Vinci