Steady State

Advances in motion control technology are making life aboard more comfortable than ever.

Designers and builders have spent lots of time and resources trying to understand the nuances of discomfort on yachts, both underway and at anchor. They’ve commissioned medical studies, interviewed focus groups and built full-scale mock-ups to rock and roll with test subjects aboard.

Have they solved the problem? No. Have they moved the needle? The answer is yes.

This advanced motion control system includes multiple appendages, both active and passive, including rudders, interceptors, centerline and bilge keels, fin stabilizers and canards.

This advanced motion control system includes multiple appendages, both active and passive, including rudders, interceptors, centerline and bilge keels, fin stabilizers and canards.

Unwanted motion is the obvious cause of discomfort on yachts, but the extent of that discomfort varies with the degree and type of motion being experienced. Symptoms range from mild distraction to full-blown mal de mer and are a complex reaction of the human body to actual and perceived stimuli—visual, physiological, gravitational and psychological. When these are in conflict, it is easy for the body to feel out of balance, becoming as confused as the surrounding sea.

Motion control comes in many forms, with price tags that go up as effectiveness increases. Choices span a wide spectrum, depending on the design of the yacht, the area of intended operation and the owner’s requirements. The ultimate—complete motion control—has not yet been achieved, but builders and suppliers are getting ever closer.

Yachts have six possible types of motion along three axes: longitudinal, transverse and vertical. Imagine a single point on the yacht’s centerline, approximately amidships at the waterline. All three of these axes pass through this point. Each axis has both translational (straight line) and rotational (circular) motion. These motions are called surge and roll for the longitudinal axis, sway and pitch for the transverse axis, and heave and yaw for the vertical axis.

With six types of motion to control, it can be a challenge to know where to start, but roll reduction should be primary. As researchers at MARIN, the Dutch marine research facility, noted in a study on yacht comfort, most of the industry’s focus has in fact been on reducing roll, through design and stabilization.

Roll reduction, however, should not be the only step. The MARIN researchers note that heave and pitch, too, “contribute significantly” to the problem. Thus, effective motion control must consist of measures taken in combination.

Early solutions to motion control were passive; that is, once they were installed, you could largely forget about them as they did their job. Examples are fixed appendages such as bilge keels (long, narrow fins that run along the chine or turn of the bilge between the hull bottom and hull sides) to reduce rolling and heave; keels and vertical bow fins to reduce sway and yaw, which in the extreme may lead to broaching; and horizontal bow foils to reduce pitching. Rudders play a part in this, too, which is why you sometimes see fixed stern fins or shallow keels on vessels without rudders, such as those propelled by water jets.

With an internal actuator and an external fin (not shown), stabilizers provide basic roll control. 

With an internal actuator and an external fin (not shown), stabilizers provide basic roll control. 

Semi-passive measures include steadying sails and paravanes, both of which directly reduce rolling; and drogues or sea anchors, which indirectly reduce rolling by keeping the bow pointed into the waves. They require manual or mechanical deployment when conditions warrant, but are largely self-attending otherwise. Paravanes are also known as “flopper stoppers” and consist of weighted planes, or “fish,” that fly underwater at the end of outriggers. Another semi-passive option is a U-shaped anti-rolling tank, such as the Flume system from Hoppe Marine. It uses water moving from side to side, designed to be out of phase with the yacht’s rolling, counteracting the motion. Each of these semi-passive systems has its drawbacks, so you seldom see them on larger yachts.

Active measures, usually electrically or hydraulically powered, have predominated for decades. While more expensive and complicated than passive or semi-passive measures, they are considerably more effective at reducing motion. As noted by MARIN, rolling is the primary contributor to discomfort. It is also the easiest motion to control, so anti-rolling equipment is the most common choice for yachts. Some anti-rolling measures combat heave, as well.

The main anti-rolling choices are fin stabilizers, Magnus-effect stabilizers, and gyroscopes, all of which can be designed for underway and at-anchor operation. Fin stabilizers from Quantum, Naiad Dynamics, ABT-Trac and others are essentially flat or foil-shaped underwater wings. They are mounted in pairs, port and starboard, on rotating shafts that penetrate the hull. Most yachts have only one pair of fins, but two or more pairs may be mounted, with forward-mounted fins referred to as canards. Fins are powered by internal actuators and controlled by an electronic system that senses the rate and extent of the yacht’s rolling. Magnus-effect stabilizers from Quantum and Dynamic Marine Systems are similar, except the fin is replaced by a rotating cylinder that, like a fin, delivers force in a direction perpendicular to the mounting shaft.

Building on its Mag-Lift technology, Quantum recently introduced Dyna-Foil, a high-aspect-ratio fin, long and slender for better efficiency, more like an airplane wing than many other fins. It extends much farther from the hull, but is retractable for docking and use in shallower water.

“This newly patented technology offers solutions for zero- and high-speed stabilization in one compact, high aspect foil system,” says John Allen, president and CEO of Quantum. “Essentially, it checks all of the boxes: more lift, with less drag and less power at any speed.”

Allen says Quantum has signed the first contract for a Dyna-Foil on a 230-foot (70-meter) project.

Stability fins on echo yachts’ white rabbit G

Stability fins on echo yachts’ white rabbit G

Active hull vanes (highlighted in red) work together to reduce roll, pitch and heave, the three motions found by research to be the most significant in motion-induced discomfort.

Active hull vanes (highlighted in red) work together to reduce roll, pitch and heave, the three motions found by research to be the most significant in motion-induced discomfort.

Gyroscopic stabilizers from Seakeeper and Mitsubishi require no hull penetration or equipment external to the hull, and they operate on a totally different principle than fins or rollers. A gyroscope’s main component is a heavy wheel spinning at a high rate of speed, the mass and rpm combining to create inertia, or resistance to movement. Small yachts may need only a single unit, which can be mounted anywhere within the hull, but larger yachts require more inertia, which means more weight is necessary. That can be provided by a single larger unit or by multiple smaller units for redundancy and flexibility in arrangement.

Pitching, again as noted by MARIN, is another significant factor in motion discomfort. Active countermeasures are readily available, the most common being trim tabs, interceptors, and horizontal fins at the bow and stern. Trim tabs, such as those from Bennett and others, as their name implies, were originally designed to adjust a yacht’s trim (running angle) by setting the angle of a tab, or horizontal plane, at the stern. As control systems became more sophisticated, tabs were given more powerful actuators so they could actively reduce pitching motion. Interceptors, from Humphree, Naiad and others, operate on the same principle, but utilize vertical plates or rotating vanes that are lowered to “intercept” the water flow. Tabs and interceptors mounted in pairs to port and starboard, and controlled independently, also can be effective in helping to reduce roll.

These individual pieces of equipment each address one, or at most two or three, types of motion. An ideal motion control system would address all six. That’s not yet been invented, but it’s still the goal.

Naiad Dynamics is the current leader in this technology, having worked with navies around the world to create the steadiest mission platforms possible. That same technology is now available for yachts through Naiad’s Total Ride Control system. It addresses five of the six types of motion, excepting only surge. Naiad selects active components including, as appropriate, T-foils, trim tabs, interceptors, lifting foils, fins, canards and rudders, and then uses analytical software specific to each yacht to coordinate all the equipment for the best motion control and the utmost in comfort aboard. According to Naiad, that analysis includes extensive FEA (finite element analysis) engineering design, CFD (computational fluid dynamics) hydrodynamic design, naval architecture and structural analysis, and highly correlated proprietary ship modeling and simulation.

While it’s unlikely any of these efforts will completely solve the motion churned up by the angry sea, they’re advancing the cause and making for fewer angry—and seasick—guests on board. 

This article originally appeared in the Winter 2020 issue of Yachts International.