Naiad

Paravanes: An Alternate or Backup Stabilizing System for Trawlers

Bluewater's paravanes running deep at 15 feet.  Photo:  Milt Baker

Bluewater has her paravanes deployed. Photo: Milt Baker

One key to successful long-distance passagemaking is ensuring maximum comfort and safety of the crew.  A factor that contributes to fatigue on long rides is excessive motion.  Granted, sometimes heavy seas make some level of pitch and roll unavoidable, but both hull design and specialized systems can minimize the motion. 

Roll stabilization is now quite common on full-displacement trawlers, but there are two different ways to accomplish the task — active-fin stabilizers and paravanes.  The former are wing-like appendages attached to the hull below the waterline and they move like the control surfaces of an airplane to minimize roll.  Paravanes are also wing-like but they are not part of the boat’s structure and are instead towed alongside the boat, below the surface of the water. 

Milt Baker’s Nordhavn 47, Bluewater, can quiet the rock and roll in two ways. The first line of defense is the active fin stabilization system from Naiad, which was installed on the boat before delivery. The second anti-roll system aboard Bluewater is the paravane system that relies only on a couple of electric winches and the practiced deployment and recovery techniques that he and his wife Judy use.

Paravanes look complicated, with all of the poles and rigging installed, but actually are quite simple. The system involves two “flying fish” made out of metal and weighing about 40 pounds each, for a boat of Bluewater’s size. These are hung from outrigger poles about 15 feet below the surface of the water, well outboard of Bluewater’s midships rail (see photo).

As the boat moves through the water, paravanes resist being pulled up through the water by the rolling movements of the boat. Since one is deployed on each side of the vessel, rolling motions are damped, often by as much as 60 percent of what an active fin-stabilization system can achieve.

Copyright ©  2008 by OceanLines

Posted by Tom in Boats, Technology

Boat Stabilizers: A Smooth Ride

This post is derived from an article that I wrote recently for Mad Mariner

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Milt Baker's Bluewater en route to Horta

Milt Baker's Bluewater en route to Horta

It was mid-morning on June 18, and Milt Baker was enjoying a nap.  His wife, Judy, was at the helm of their Nordhavn 47, Bluewater, when a low-pressure system overtook their little “Med Bound” fleet while it was nearly mid-way between Bermuda and their next destination in the Azores.  Bluewater was on a course of 095 magnetic, traveling at 5.9 knots at 1700 RPM.

“It was all quite comfortable,” Baker wrote in his log, “until Judy awakened me…with a report that we had a stabilizer alarm.  Probably no big deal.”

The deep blue is no place to be in a full-displacement boat without some form of stabilization.  Stabilizer systems are expensive and not every boater needs one.  Plenty of boats successfully traverse large ocean distances without stabilization, but experienced cruisers like Baker consider stabilization not only a comfort feature but also a safety enhancement.

In his case, the starboard potentiometer, which tells the active fin stabilization system aboard Bluewater how that fin is positioned, had failed.  The failure left Bluewater with only one functional fin stabilizer.  Fortunately, as a seasoned passagemaker with a conservative approach to safety, Baker had a backup system of paravanes — small, “flying” delta wing-like devices that can hang down underwater from special outriggers and resist the natural ocean rolling motions.

Without a backup, another member of the Med Bound fleet was less fortunate.  The boat had to turn back to Charleston, S.C., earlier in the cruise when it, too, suffered a stabilization failure.

Baker notes that a stabilized boat significantly enhances crew comfort and rest, which means a rested crew making better decisions.  Other bluewater cruisers echo his sentiments.

A SPINNING TOP

What exactly are these stablization systems?  What can they do, or not do?  And what kind of boats can they work on?  Many of us who have never even seen a stabilizer have actually experienced its benefits directly.

Stabilization made its way into recreational boating when the industry realized many years ago that it had to address stability at sea–and the seasickness that typically results when ships roll too much–to continue to attract new customers.

Early stability systems on commercial ships involved systems of moving weights or ballast water from side-to-side to counter the rolling motion.  These were crude and largely ineffective because they couldn’t react quickly.

The eventual solution was to add special control fins, small wing-like appendages, to the underwater portion of the hulls.  The theory was simple:  Make these fins moveable and program them to act like the control surfaces on an airplane’s wing, moving one way to make the ship lean to the left and the other way to make it roll to the right. 

A sensor package that detects the natural roll of the ship in response to the waves, wakes and the like is the next component.  With the sensors, the stability fins move opposite the natural motion, keeping the ship on an even keel.  When the ship “tries” to roll to the left or right, the fins counter that motion and keep it upright.  This new technology made for much more stable ocean passages and calmed the fears of countless new potential customers. 

To get a better idea of how these systems work, think about a spinning top.  The faster it spun–and the more massive it was–the better it stood up straight and resisted tipping.  This is the gyroscopic effect.  A spinning sphere tends to counter any motion away from the vertical.  It can be used to design a sensor that detects those forces that would tip it over left or right.  With that sensor input, the stability fins are programmed to assist the gyro in staying upright, fighting the natural roll motions of the ship. 

EARLY RELIABILITY PROBLEMS

The original fin stabilization systems used bulky mechanical gyros that connected to hydraulic systems to move the fins.  Simple analog computers translated the mechanical forces on the gyro into control commands for the fins.  They were effective but relatively limited by the slowness of the mechanical sensors and the computer.  As long as the ships and megayachts traveled at 8-12 knots, the systems worked well enough, taking out about 60 percent of the roll motions.  But they were not all that reliable.  Mechanical parts wore out.  Older hydraulic systems leaked.

The next generation of stabilizers benefitted from technology developed in the aerospace industry.  High-speed aircraft and spacecraft used similar gyroscopic systems for control but requried much faster sensors, computation and control surface movement.  Aerospace industry development of solid-state, electronic gyros and optical sensors allowed new systems to take sensor measurements hundreds or even thousands of times per second, catching even the slightest rolls and couterracting them before they became large movements.

Newer lightweight, high-pressure hydraulic systems were capable of driving fast-motion actuators to take advantage of these quick sensors to actually drive the fins.  By this point the performance of stablization fins had eliminated up to 90 percent of roll motion in all but the worst sea conditions.  These newer, electronically controlled fin stabilization systems also work with the boat’s autopilot and can even “learn” the current wave patterns and anticipate corrections.

Faster sensors and new digital control systems also made stability systems possible aboard high-speed vessels traveling 30 knots or greater.  Obviously, higher speeds require the faster reaction times of the electronic systems. Ironically, the faster speeds also mean smaller fins, since the water flowing over the fins, which is what generates the control forces, is so much greater.  At high speeds, even newer stabilization technologies come into play too.

NEW TECHNOLOGIES

One of these is called “ride control.”  It works by lowering fins into the water at the boat’s transom in a way that not only controls rolling motions but a significant element of the pitch-motion–the up and down movement of the bow.  At low speeds these systems are much less effective and have not been installed on smaller, slower recreational boats.  They are installed on some of the bigger and faster sport-fishing boats.

Sometimes, when multiple pairs of active fins are installed, they too can be used to control a combination of both roll and pitch.  A single fin, or pair of fins, however, can only control roll motions.  Companies like Quantum and Seakeeper offer ride control systems for yachts.

Baker’s Bluewater had a backup stabilization system, known as paravanes.  Sometimes referred to as “flopper-stoppers,” paravanes are useful because they require no real mechanical systems to control them.  Simple block and tackle can deploy, tow and retrieve the “fish.”  They do, however, require long outrigger poles to gain leverage on the boat and the fish themselves can weigh enough that losing control of one in heavy sea conditions can be somewhat perilous.  They also have a drag penalty; most users note a speed reduction of some fraction of a knot.  It’s not much, though the longer the passage, the longer the delay.

Do you really need stabilizers?  If you’re never leaving the bay or the sound, not taking overnight voyages and never leaving port for more than a few hours a day, the answer is clearly no.  If, however, if really want to cruise, spending several days continuously at sea or cruising between overnight stops, and you venture where the water is not still, stabilizers might fit the bill.  They can be fitted to boats as small as 30 feet, assuming inside the hull has room for the actuator and control mechanisms and someplace to generate hydraulic power.

A COSTLY ADDITION

They are not cheap.  Even a modest system aboard a 40-foot boat, using fins of 2.5 square feet, will probably cost north of $15,000, including installation and related costs.  Bigger systems will naturally cost more but only incrementally.  Outfitting megayachts with multiple pairs of fin stabilizers will cost quite a bit more.  A paravane installation will cost less but perhaps not much less given the need for extensive rigging.

Once you’ve decided your boating lifestyle could benefit from a stabilizer system, the next question is how much it’s worth to have a rested, comfortable crew that is not suffering from pervasive seasickness.

Robert Beebe, one of the earliest proponents of power passage making, believed that some kind of stabilization capable of eliminating at least two-thirds of the rolling motions was an absolute necessity.  He described his thinking and experience with various stabilization schemes in “Voyaging Under Power,” a classic text now in its third edition.  It includes updates written by Nordhavn’s chief designer, Jim Leishman.

For the average coastal cruiser taking proper care of them, these stabilization systems are quite reliable.  For the ocean-crossing voyager, there may be more to consider.

N47 Bluewater and her Med-Bound Crew

N47 Bluewater and her Med-Bound Crew

“In my experience, stabilizer failures have many causes, including lack of maintenance, poor maintenance, poor installation, and under–sized units,” Baker says. “In some cases, components fail and you can attribute at least some of that to poor quality control and some of that to specifying under–sized systems.

“Consider that by most any yardstick, an ocean crossing constitutes a commercial duty cycle for stabilizers. They are working hard 24/7,” he says. “On my Atlantic crossing this summer, for example, my Nordhavn 47 was underway for more than 25, 23–hour days with only brief stops in between.”

ENTER THE GYRO

Baker chose over–sized fins from VT Naiad Marine for Bluewater. He says that having a good company standing behind its product is just as important as the product itself. His system was still under warranty and Naiad flew a technician across the Atlantic to fix the problem.

“In my experience both Naiad and TRAC/ABT go to great lengths–often extraordinary lengths–to take good care of their customers,” Baker says.

As materials, processing and controls improve, they sometimes mix with old technology to produce new options for mariners. That’s what happened with the gyroscope. Yes, the same equipment once sensed the movement of the boat and drive the fin stabilizers is now being used itself to stabilize some boats. In this case, the gyro is big–two feet or more in diameter. And it is heavy––weighing several hundred pounds. It also spins at extremely high speeds.

Mounted low in a boat’s hull, these gyroscopes will themselves reduce a boat’s roll significantly. Today’s control–moment gyros are spun up inside a vacuum to eliminate air resistance and lower power requirements. One such system is the Seakeeper GYRO, which has been fitted on some large sport fishing boats.

Ferretti, the luxury Italian boat builder, has licensed a similar gyro–based system from Mitsubishi Heavy Industries and introduced it in the company’s 630 model, calling it the “anti–seasickness boat.” It’s now offered as an option on Ferretti yachts, from the smallest to the 780, and comes standard on the 830 and 881. While the “anti–seasickness yacht” may be a bit of an exaggeration, a good gyro system can effectively reduce roll motions. In megayachts, the installation sometimes includes several gyros, and the effect is noticeable. Ferretti calls its licensed system the Anti Rolling Gyro, or ARG system. The gyro itself weighs about 150 pounds and takes about 45 minutes to spin up to full operating speed.

SOME IDEAS

What to do? These are just recommendations; your mileage will vary, and you should talk to as many people and manufacturers as possible before making a decision.

Most trawler–type cruisers probably will want to consider active fin stabilizers as their first line of technology. There are several companies offering the systems, all with excellent products, design assistance and warranty service. If your full displacement or semi–displacement hull is not currently stabilized, you will be stunned at the improvement in ride and the way you feel after a long day in heavy seas. They will put a dent in your wallet but I’ve yet to meet a captain who didn’t think it was worth it for long–distance cruising.

If you run a high–speed sport fisherman, you might consider either a gyro system or a ride control system of stern–mounted tabs; or perhaps a combination. The gyro systems are good because they don’t require mounting external fins and generally need only electrical power and room to mount. The ride control systems are normally hydraulically actuated, but they can be electrically driven, too, and these will require mounting the fins/tabs and their actuators on the boat’s transom.

What about while the boat is at anchor? Some good strategies are out there to prevent rock–and–roll while the boat isn’t moving, usually called zero–speed stabilization. They use either a version of the “flopperstopper” or specially programmed and designed active fins that move quickly, displacing a large volume of water and keeping the hull from rolling. This type of fin movement is more like a bird flapping its wings than the normal fin action when the boat is moving through the water.

The “flopperstopper” solution is to use the paravane rigging to deploy a special piece of gear that will sink easily and without resistance but will resist being pulled up, thereby reducing the roll motion of the boat at anchor. It can be quite effective but again involves the same somewhat complicated rigging as the paravanes.

If you need zero–speed stabilization, the options include an enhancement to your existing fin–stabilization system to enable at–anchor control; a gyro system, which can also be used at speed; or a flopperstopper, which might be adequate enough for smaller boats in quieter anchorages.

For backup, you can take Baker’s route and add paravanes to take over for a primary fin–stabilization system. Or, you can carry replacement parts and learn how to service your own stabilizers. Your own comfort level will dictate which belt and suspenders you choose.

Copyright ©  2008 by OceanLines

Posted by Tom in Technology