Stabilizers are not new, but the technology has greatly improved in recent years. Early stabilizers were passive, far less complex than modern systems, and although still used commonly today, for the most part less effective. Bilge keels, for example, are paired longitudinal fins affixed to the hull bottom just inboard of the chines; their lateral surfaces serve merely to resist rolling rather than actively countering it. Trawler-type yachts of past decades have used flopperstoppers, towed horizontal planes suspended over both sides, either from the gunwales or from lateral booms overhanging the hull. These may be used to remarkable effect at displacement speeds or at anchor, although they can be tricky to deploy and recover in offshore conditions and are sufficiently heavy to create a risk of damage or injury in the event that they swing free. Sharp-eyed boaters occasionally will spot a trawler equipped with a steadying sail, a small triangle bent onto a diminutive mast and designed to capture lateral wind loads to resist rolling.
For large motoryachts, these systems are impractical or insufficient, especially when compared to hydraulic and electronic technologies in widespread use today. By far the most common of these newer units are hydraulically operated active fins, controlled by a computerized sensor capable of detecting and negating roll motion within milliseconds. Paired on opposite sides of a hull roughly amidships between keel and chine, fins rotate on an axis normally angled downward and outward from their mounting points. In standby mode the fins are oriented fore and aft, and when activated pivot in either direction, driven by one or two hydraulic rams in response to commands from a control unit and a sensor that detects lateral deviation from a level attitude. Underway, these fins behave like rudders, deflecting water flow to one side or the other to counter rolling motion. Newer fin systems with zero-speed capability also can diminish roll when the vessel is at rest, the planes acting more as paddles to attenuate rolling in the absence of water flow.
For their many attributes and popularity among yacht owners, fin systems are not infallible. Yet while fins do produce drag to the detriment of speed and cruise range, they also offer a positive effect that many claim can more than offset any negative result. Western Marine Electronics (WESMAR) President Bruce Blakey cites the experience of his commercial and charter-operator customers who claim that their roll fin stabilizers aid course-keeping to produce a 10- to 15-percent fuel saving compared to their boats’ performance without fins. Naiad Marine, another well-known producer of fin stabilizer systems, reports that its tapered-shaft mounting design allows aligning fins to infinitely small increments, reducing drag to the lowest possible level. Arcturus Marine designs its Trac systems with winglet appendages at the lower end of each fin, not unlike those found on modern jet aircraft wingtips, to reduce or eliminate drag caused by the vortex, or swirling turbulence, common to foil tips.
As underwater appendages, fins are vulnerable to damage from impact with the bottom or floating debris, and while designers habitually configure fins to operate within the dimensional limits defined by a vessel’s beam and draft, accidents do happen, although apparently not very often. Vice President Mike Perkins of Quantum Marine Engineering says that of approximately 250 systems installed over the past 10 years, his company has been called upon to replace perhaps two fins lost or damaged by impact. Fin systems require sizing foils specific to operating conditions—smaller foils for fast hulls, larger for slower vessels—as well as to hull length and displacement. Some are designed to operate only within a specified range of speeds and are programmed to shut down when boat speed exceeds that range. To prevent damage from propeller backwash or steering interference when backing, Trac systems feature a remote locking device that immobilizes fins at the push of a button at the helm.
Consistent with their original design as foils, acting much the same as aircraft wings or ships’ rudders to create lift by deflecting air or water flow over their surfaces, early fin stabilizer designs are effective only when the vessel is in motion. While yachts fitted with these systems deliver an admirably smooth, level ride, at anchor they are still vulnerable to the rolling forces of a beam sea, which equates to uncomfortable nights for motion-sensitive passengers. In response many manufacturers are developing or already have introduced a variation to their fin systems called zero-speed or at-anchor stabilization. Quantum Marine, generally recognized as the pioneer in this technology and a specialist in systems for yachts larger than 130’, introduced the concept in 2000, and since then has installed its Zero-Speed system on dozens of large yachts, much to the delight of their owners and guests who spend most of their time onboard when the vessels are at rest. Zero-Speed technology, says Quantum, “is based on the rapid movement of the fins from ‘lock to lock’ using high acceleration forces to ‘push’ water. The resulting forces essentially nudge the yacht back against the swell and overcome the roll motion.” Because this variant requires a larger fin surface to attenuate roll effectively, at-anchor systems are most at home on displacement vessels, but Quantum more recently has unveiled its new XT fin system, which features a retractable section that nests inside the main fin unit to reduce foil size and minimize drag while the yacht is underway, rendering it compatible with higher-speed operation. In at-anchor mode, the secondary panel swings outward, augmenting fin surface area for greater effect comparable to a larger one-piece fin. The fin extension, moreover, is designed to increase surface area aft of the fin shaft, where it can exert maximum force against roll.
These and other refinements are steadily expanding the speed range at which fin systems are effective, but even newer technologies also are emerging to give owners and designers a broader choice of systems to meet specific needs. Of these, gyro-based stabilizers appear to be attracting the most attention. Physics students who have ever grasped the hubs of a spinning bicycle wheel during a lab demonstration have experienced the remarkable force with which the wheel resists any deviation from the axis of its rotation. That phenomenon corresponds to the dynamics of gyro stabilizers. Typically fitted in the lower confines of a hull, a gyro stabilizer unit comprises an electrically driven flywheel (of considerably greater mass than the humble bicycle wheel) enclosed in some form of housing and capable of turning at a high rpm around its vertical axis. Because this technology operates on the principle of angular momentum instead of water flow, the entire gyro stabilization unit is contained entirely within the hull; while this attribute eliminates both drag and the threat of damage to or fouling of external components, and frequently translates to easier installation, the physical bulk of the gyro unit—especially where multiple installations are specified—typically is greater than the combined interior volume requirements of fin systems. In any case gyro stabilizers operate without regard to hull speed, performing at rest or underway. In order to prevent damage to gyro bearings, these units are designed to shut down automatically in conditions that produce severe pounding or slamming.
Dutch builder Heesen Yachts has been cooperating with Voith in the application of the German manufacturer’s cycloidal rudder as a replacement for mid-ship fin stabilizers. For decades cycloidal propellers have powered commercial tractor tugs acting as more or less conventional rudders, but Heesen identified their potential as a multi-purpose technology providing everything from roll damping at low and zero speed, helping with close-quarters maneuvering and useful as a dynamic positioning tool. They also can produce thrust, powered by a single generator to provide silent, low-speed propulsion, and in passive mode can serve as a high-speed stabilizer.
Beyond fins, manufacturers are exploring other technologies relying on a variety of physics principles. The Ferretti Group, in 2004, started offering yachts equipped with the ARG (Anti-Roll Gyro) system, which was developed by Mitsubishi Heavy Industries, and today offers that technology in its Ferretti, Mochi Craft and Custom Line ranges. The system is also available as on option on Bertram sportfishing boats 50’ and larger. Units currently are available in two models, the 660-lb. MSM 2000 and the MSM 4000, at 1550 lb. delivering double the roll-damping force. Maryland-based Seakeeper Inc. offers a number of refinements to the fundamental principles of gyro stabilization, including a spherical wheel that spins in a near vacuum, allowing it to achieve higher operating speeds to produce a comparable roll-damping effect with a unit of considerably lower weight. The Seakeeper system, which is available on Azimut yachts thanks to a partnership between the two companies, also features an active control unit that regulates fore-and-aft tilt of the gimbal-mounted gyro housing for maximum effect in varying sea states. As a result, says company president Shep McKenney, “The Seakeeper 7000 Gyro is capable of exerting a maximum anti-rolling torque of 3.5 tons down on one side and 0.5 ton up on the other.” To date, gyro stabilizers have been mounted mostly on yachts 120’ or smaller since larger yachts require four or more units to achieve optimum effect. For the moment at least, fin stabilizers remain the system of choice aboard larger yachts.
For vessels designed to operate at speeds less than 15 knots, another emerging stabilizer design offers intriguing promise. Produced and marketed by Quantum Marine as the MagLift system, this design operates according to the principles of the magnus effect, an arcane phenomenon of fluid dynamics that can be observed in the path of a major-league curve ball. The magnus effect is described on answers.com as “The force perpendicular to the forward motion of a spinning object moving through a fluid,” and in a yacht stabilization application is achieved by way of a slender, retractable cylinder that moves forward with the hull and spins, creating either an upward or downward force, according to direction, to generate a stabilizing effect. One of the earliest MagLift installations was completed about three years ago on the 95’ Inace Impetus, a steel-hulled expedition yacht whose owner, according to Quantum Marine’s Perkins, reports dramatic improvement in ride comfort. Because roll reduction using this system requires that the cylinder move through the water as it spins, Quantum has designed its system to swing the cylinder back and forth through a 120-degree arc, giving it effective at-anchor capability as well.
Owners who haven’t specified a modern stabilization system for their next yacht or as a retrofit on their current one (and there can’t be many) would do well to give the issue serious thought, especially if they plan to offer their vessels for charter. Indeed, agencies indicate that charter clients routinely pass on those yachts, steadily declining in number, whose equipment lists do not include stabilizers, specifically those with at-anchor capability. As one observer noted, “If you no longer have to endure discomfort, why do it?”
