In rough seas, a vessel's uncontrolled rolling motion is not only is not only uncomfortable for her crew and passengers, but may be dangerous for the craft's hull and its conents. In addition, the yawing motion normally amplified by the roll alters the vessel's course, making it difficult to steer accurately, reducing effective speed and increasing fuel consumption.

Fortunately there are several means for minimizing roll and its effects. These go under the collective name of roll stabilizers and may be divided into stationary and active systems. Stationary systems, which resist the roll but do not counteract it, come in a variety of shapes and sizes, all of which operate on the same principles.

Types of stabilizers
Some yacht designers employ a deep central keel as means of creating resistance to roll, while others use bilge keels mounted at the turn of the bilge and extending for almost the entire length of the water line. Many fishing crafts and some trawler-type yachts rig steadying sails, which of course rely on wind rather than water resistance for their effect.

Flopper-stoppers are a third type of stationary stabilizer. In essence, these consist of submerged plates suspended from booms extended over the sides of the vessel. The plates ride beneath the surface of the water as the craft moves at low speeds and creates resistance to rolling.

All stationary stabilizers are partially effective, in that they remove the snap from the rolling motion. By their design, however, they begin to respond only after the vessel has already begun to roll. Activated stabilizers, the subject of this article, are designed to anticipate the magnitude of roll and begin to counteract it before it starts.

Activated roll stabilizers are controlled through a rate gyro which continuously positions two or more aerofoil shaped fins projecting from both sides of the vessel. The fins on one side are angled to develop upward lift, while the opposing fins develop downward lift. The combinanation produces the required stabilizing torque to counteract the rolling force induced by the waves action.

The stabilizing systems are operated either pneumatically or hydraulically-electrically and consist of the four primary elements. The rate gyro and control unit, the fins and driving mechanism, the interconnecting piping and filiters and the power supply (compressor or pump and electricity).

Elements of the system
Perhaps the simplest way of understanding the functions of the activated stabilizer system is to compare it to the human body. The gyro and control unit represents the brain, the power supply source is the heart. The piping stands for the veious and nervous systems, the fins are the legs and arms and the vessel is the body itself.

The gyro-control unit is designed so that any roll causes a corresponding deflection of the gyro. It should be noted, however, that gyro deflection is proportional to the rate or speed of the roll, rather than to the angle or amount of roll. The gyro anticipates the magnitude of the roll and sitions the stabilizing fins accordingly before the roll has actually begun. Consequently, a small fin can do a big job, even on a large vessel. Once the roll has begun, however, the inertia created will require a very large fin to stop the motion and bring the craft level again. Obviously, a properly designed stabilizing system requires quick response and speedy action in the driving mechanism to level the vessel before roll inertia gets beyond the capacity of the fins.

The gyro's deflection its mechanical response is converted through the control unit into pneumatic or hydraulic pressure equivalent to the magnitude of deflection, and this pressure in turn activates and positions the fins.