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Taming the roll

Jun 20, 2013
An active stabilizer unit changes angle to counteract roll effects.

An active stabilizer unit changes angle to counteract roll effects.

Ralph Naranjo

Active stabilization won’t turn a rough sea ride into the feel of a flat calm, but these ingenious examples of marine engineering will make life aboard much more tolerable.

When a power cruiser is tethered to a marina slip, making way in a flat calm, or anchored in the proverbial millpond, she mimics the stability of a house ashore. Add some chop on the beam or a swell rolling in from the open sea and just six words are needed to define the resulting chaos. Pitch-roll-yaw-heave-sway-surge are terms that describe a vessel’s misbehavior, and they’re exacerbated by hull shape, buoyancy, center of gravity and your boat’s course and speed.

Roll is the biggest culprit and naval architects go to great effort to mitigate the effect. The basic rule of thumb is that narrow round bilge powerboats roll more willfully than their wider, flatter bottomed, hard chine cousins. A low center of gravity lessens the risk of capsize, but it can also shorten the roll period — not always a good attribute. Bilge keels and steadying sails tamp down roll to some degree, but there’s increased skin drag with the former, and added complexity with the latter. Combine this with the fact that some roll mitigating measures, such as a ballast tank on the upper deck, carry a big sea keeping downside, and it becomes clear that further help is needed.

Courtesy Seakeeper

The sportfishing boat at top is equipped with a Seakeeper gyro stabilizer, above, that uses the inertia to combat roll.

Commercial fishermen and some trawler owners tow paravanes on outriggers, and West Coasters long ago learned to deploy larger and larger flopper stoppers in roll-prone anchorages. Such “non-powered” efforts helped to solve the problem — but none offer quite enough roll dampening.

Active stabilization arrived on the scene and it delivered better results. Fin stabilizer systems incorporate roll-sensing electronics linked to servo switches and hydraulic valves that actuate rams for changing fin angle. Like a semi-balanced spade rudder aboard a sailboat, the axis of rotation supporting a stabilizer fin is well aft of its leading edge. This gives the low aspect ratio fin stabilizer some “balance” or force to help make the angle changes. In essence, there’s a power assist created by water pressure acting on the control surface that lies forward of the stock.

The roll dampening effect of these large foil shaped fins stems from the low pressure lift developed on one side of the foil. This force is used to counteract the roll caused by the seaway. The two most important variables are the amount of force generated by the fins and the timing as to where in the roll period it is imposed. If the roll countering force is too small a percentage of the heeling moment generated by the seaway, the damping effect may be insufficient. Even more problematic is the mistiming of a fin angle of attack change. Turning fins at the wrong moment will result in lift and a force that’s in phase rather than in opposition to the roll. Such resonance can make a bad situation even worse. Fin stabilizer designers and engineers have long recognized that timing is a big deal, and they have developed elegant gyroscopic and accelerometer-based roll sensors and linked them to faster and faster processors that engage powerful fin adjusting servo systems at just the right moment.  

Over the years, fin shapes have improved and the lift per square foot of surface area has increased. Digital switching, and more powerful computing have given rise to control systems that learn the roll rate and righting moment of your boat. Imbedded in the software are algorithms that automatically calculate the right time to introduce the roll countering force. Some software packages are capable of delivering more than 200 angle change instructions per second.

The net effect is an uptick in fin stabilizer efficiency, but also greater demand on component parts. The bearing surface supporting the single shaft or stock must handle bending loads imposed by the righting moment of the vessel as well as the torque load developed by the steering servo. Stainless steel shafts and roller bearings are a favored approach. Older technology relied on bushing-type bearings and regular application of waterproof grease. In every case, designers must face the implication of fatigue cycle loading, a destructive trait that responds to every torque input of the servo system and each seesaw induced roll of the vessel. Building in appropriate scantlings to handle the working loads and formidable fatigue cycle are all part of a good installation.

The details include everything from the GRP panel stiffness and strength of the hull skin where the bearing is supported, to the metallurgy of the stock and weldment that forms the foil’s internal armature. If your boat is quite roll prone and you spend a lot of time offshore, it may make sense to scale up the size of the unit you choose. By picking a system that puts your boat in the lower half of its recommended size range you gain increased fin lift and structural durability. It’s always helpful to caucus friends with sister ships and never pass up a sea trial. If you have a chance to steer a seas-on-the-beam course, try shutting down the fin stabilizers to get a feel for what they had been adding to the equation, but first warn the crew and make sure every loose cup and dish has been stowed.

Most fin stabilizer manufacturers now add a fence or end plate to the tip of the control surface. This helps to lessen the parasitic cross flow that can add drag and decrease lift on the low-pressure side of the foil. By changing the fin angle with reference to the vessel’s centerline, lift develops on the low-pressure side of the control surface. This low-pressure region always develops on the side opposite to where the trailing edge of the fin has been angled. For example, if the trailing edge of the fin is to port of the centerline, lift will develop to starboard. The hydrodynamics that cause this effect involve water flow in the boundary layer of a foil. In this case, the acceleration of flow on the starboard side causes molecules of water to spread out resulting in a density disparity and lower pressure on the flow accelerated side (to starboard). At anchor, units with larger control surfaces and fast acting, wide range of movement servos, dampen roll with a paddle-like force countering the roll moment.

Underway, overly large angles of attack can cause excessive turbulent flow on the negative pressure side of a fin stabilizer, and a potential for cavitation does exist. Improved software and precise digital controls lessen the chance of such problems occurring. But the bottom line is clear, fin stabilizers do significantly dampen roll, they can make steering less of a zig-zag net result, and they’ll even add greater fuel efficiency by keeping your power cruiser “on her lines.” Pros we spoke with alluded to ABT-TRAC, Naiad and Wesmar in their descriptions of attributes to look for.
 

Another option

Gyroscopic roll stabilizers, or as technology pioneer Shep Kinney likes to put it “gravity in a bottle,” can also do a lot to dampen the roll and abate the discontent of those on board. In this case, the operating principle is very different from the fin derived stabilization we’ve discussed thus far. With a gyro, there are no fins to worry about, no through-hulls, bearing systems, servos or a roll reacting digital control. The Seakeeper gyro is comprised of a precisely balanced flywheel that runs in an airtight pressure reduced sphere, spinning at 8,000 rpm. The net effect is a turbo version of the top-like gyroscope we all played with as children. Those more familiar with equations rather than words recognize that mass and rotational speed are directly proportional to torque resistance.

A large Seakeeper 8000 gyro, with 17,143 Newton meters of anti-roll torque, will dampen boat roll — both underway and while at anchor. The gyro requires 3 kW of startup current but only 2 kW once it is running at full 8,000 rpm. Resistance to spin is decreased by evacuating air from the chamber in which the gyro spins. Full spin up time takes about 40 minutes, but after about only 20 minutes there’s noticeable effect and the current has dropped to the 2 kW run rate.

Installation involves setting up a load bearing surface that can handle the forces generated as the boat’s roll moment attempts to force the gyro away from the plane on which it currently resides. The nuance here is that instead of developing lift in opposition to the heeling moment, the gyro continuously opposes the roll moment with a fixed torque related to its mass and rate of spin. The specific location of these units is not as crucial as where fin stabilizers must be placed, and their at-anchor contribution to stability is just as powerful as what they deliver underway. Installers must make sure that the bedding stringers will spread torque loads evenly into the hull.  

Subconscious motion awareness is what’s behind all this effort to tame a motorboat’s trajectory. The semicircular canals of the human ear are tuned to sense anomalies to a level plane and constant velocity. And thanks to the circuitous wiring of the vagus nerve, our stomachs are also networked with this inner-ear architecture. In fact it often acts as an alarm bell signaling an increase in unpleasant nautical gyrations. The good news is that active stabilization causes the tiny specs of calcium carbonate known as otoliths to stop flailing about in the semicircular canals, quelling the queasy feeling and putting smiles back on the faces of all of the crew.

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Ralph Naranjo is a circumnavigator, author and freelance writer based in Annapolis, Md.
 

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