On The Level
New roll-stabilization systems can put smooth sailing in your grasp.
By George L. Petrie — July 2004
As even the hardiest salts will admit, uncomfortable motions in a seaway (especially roll) can put a damper on almost anyone’s cruising pleasure. Naval architects and yacht designers place a high priority on achieving good motion characteristics, and for decades roll stabilizers were stalwarts in their arsenal. The concept was simple: Movable fins on either side of the hull could be manipulated to produce forces in opposition to the wave forces that produce roll motion. But executing the concept was not so easy, and until recently all that could be achieved was a modest reduction in roll motion, even with the best systems.
There are several limitations with the older systems: First of all, because fins require the boat to be moving forward to produce a force (like the flaps on an airplane wing), their effectiveness is reduced at low speeds and virtually nil when the yacht is at rest. Ironically, at high speeds (when the fins are most efficient), the older analog-control stabilization systems can’t respond quickly enough to counter the roll motion. And finally, older systems rely on gyroscopes that sense roll rate; therefore, they can react only to roll motions that have already begun, unable to prevent those motions from occurring in the first place. At best, older technologies can be thought of as “roll damping” systems, capable of reducing but not eliminating roll on yachts running at displacement or semidisplacement speeds (typically 8 to 12 knots or 12 to 18 knots, respectively).
Several recent advances have significantly improved the situation, making nearly total elimination of roll motion in seas of three to four feet or more possible, whether sitting at anchor or cruising at speed. Different manufacturers have competing views on what’s the best approach, so here’s a compilation of the new technologies that some of the leading companies have embraced.
Control systems. Solid-state inertial sensors respond to roll angle as well as roll rate, allowing high-speed digital processors to react almost instantly to very small angles of roll. This lets the stabilizing fins stop the roll before it begins, without having to fight the momentum of a rolling hull.
The good news here is that, if you have an older system, you may be able to significantly improve its performance by simply upgrading your electronic control system. Check with the company that built yours to see if a retrofit is available.
Stabilization at anchor. Since many owners spend more time at anchor than underway, the desirability of a system for roll stabilization at zero speed is self-evident. But if there’s no flow across the fin, how can it generate a force that will stabilize the yacht? VT Naiad Marine pioneered a solution for the 231-foot Boadicea.
The solution lies in the fact that any change in the angle of a stabilizer fin requires that the trailing edge of the fin move up or down (like the flap on an airplane wing) and that the vertical movement through the water produces a vertical reaction force. Ordinarily the fin would be configured to minimize this force, because it increases the power needed to control the fin. But to achieve stabilization at zero speed, it is necessary to maximize the vertical force by making the fin longer in the fore and aft (chordwise) dimension and moving the pivot axis a bit farther forward on the fin. The control system then angles one fin up and the other down; as the yacht begins a roll to either side, the controller quickly flips both fins to their opposite orientation, producing a pair of vertical forces that act in opposition to the roll motion.
This article originally appeared in the June 2004 issue of Power & Motoryacht magazine.