How Do Fin Stabilizers Work?

Adjustment of an Angle

The pressure dictated by the control unit is exerted on the driving mechanism’s cylinders which defect the fin tail on one side upward and produce a downward lift on the other. The amount of lift force produced by a given hydrofoil depends upon the angle of deflection of the fin and on the vessel’s speed. Thus, to obtain maximum stabilization from a vessel’s system, the boat speed and fin deflection angle must be coordinated. This way, the lifting force can remain constant over a wide range of speeds. In other words, as the vessel’s speed increases, the fin angle should decrease proportionally, and vice versa, regardless of the intensity of the rolling motion.

Without continuous speed/deflection-angle coordination, a stabilizer will be fully effective at one speed only. For example, if a stabilizer system were designed for a 20-knot craft, it would be only one quarter as effective when boat speed drops to 10 knots which is normally the time when maximum stabilizer effectiveness is required. On the other hand, if a boat’s speed through the water exceeds the designed value, there is a danger of overspreading the fin post.

One method of achieving speed/angle compensation is by utilizing the compressibility of air in the pneumatic cylinders that activate the stabilizer. As air pressure is applied to the cylinders, the fins deflect until the pneumatic pressure inside the cylinders is balanced by the hydrodynamic pressure on the fins themselves. If the vessel’s speed increases, the hydrodynamic pressure also increases, catching the angle of deflection of the fins to reduce as the air inside the activating cylinders compresses.

At the same time, if the boat speed is reduced, the hydrodynamic force on the fins also reduces, and the air inside the cylinders expands with the decreased pressure, yielding a greater angle of fin deflection. This balancing of the hydrodynamic and pneumatic pressures ensures that the lift force produced by the fins remains almost constant regardless of the intensity of the rolling, thus, producing maximum stabilization over a wide range of operational speeds.

Elements of the Fin Stabilizer Systems

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 venous 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 positions 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 are 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.

Stress on the Hull

Obviously, roll stabilizers stress the vessel’s hull, particularly at the fin locations, and the larger the fins, the greater the stress. Yachts longer than about 50 feet, equipped with a single pair of large lins located amidships, will also endure high twisting stresses. This fact has led to the introduction of multiple smaller fins, two or three per side, instead of the single large pair of the same total area. For instance, a 50-footer cruising at 10 knots requires 4.5 sq. ft. of fin area on each side of the hull. This may be arranged as a single fin or as two fins of 2.25 sq. ft. each. Not only do smaller multiple fins reduce stress on the hull, but they have also proved to be more efficient as stabilizers and less obstructive for navigation. Moreover, if one fin is damaged, the remaining three would produce 75 percent stabilization, as compared with 50 percent when one of a two-fin system is lost.

Traditionally, the stabilizing fins have been made of fiberglass over a metal frame, attached to the driving mechanism by a shaft passing through the hull via a watertight fitting. The shaft must carry both the bending stresses exerted by the water pressure as well as the torsional stresses created by the drive mechanism. These combined stresses have dictated the use of a large diameter stainless steel shaft that is unlikely to break or bend if the fin strikes an object.

Because of the likelihood of hull damage in case of an accident, some fins are designed to retract upon collision, and several breakaway models have recently been introduced. The most intriguing so far calls for the separation of bending and torsional stresses, which is accomplished by installing the fin on a stationary tubular post firmly attached to the outside of the hull. Proportional deflection of the fin is achieved by a solid shaft inside the tube and attached to the fin, which is free to rotate orotund the post.

In this arrangement, the stationary post carries only bending stresses while the solid shaft bears only torsional stress. The diameters of both shaft and tube are considerably reduced from the single-shaft requirement and an impact with a submerged object will easily shear the tube and bend the driveshaft. A breakaway seal prevents seawater from reaching the interior of the vessel if the fin is damaged.

The power supply for a pneumatic system is obtained from a compressor, which is belt-driven or directly gear-driven off the main engine. In some installations, an independent compressor and driving motor are employed. Power supply for a hydraulic-electrical system is obtained from a hydraulic pump which is belt-driven off the main engine or independently driven from a separate electrical power source.

Stabilizer systems may be installed in existing as well as new vessels and the work generally takes three to five days. In general, multiple-fin arrangements are simpler, as they do not require hull reinforcement at the fin locations and the units involved are lighter and easier to work with.