Steve Zimmerman walks through the pro's and con's of various methods of stabilization.

Stabilization options for cruising powerboats have evolved from steadying sails and bilge keels to active fins and gyro stabilizers. These new technologies offer far more than incremental improvement over the older methods. While active fins and internal gyros dominate the market, a number of other choices can also be effective for stabilizing your powerboat. We’ll begin by exploring the pros and cons of each type of system and then we’ll examine the maintenance needs of fins and gyros.

But first let’s set expectations up front. Stabilizers excel at reducing roll and this makes for a much more pleasant cruising experience overall. Keep in mind, however, that your boat will still rise and fall with each wave—it just won’t rock as much. And when beating into head seas the benefit of stabilizers is minimal, if at all—it can still be a miserable experience.

Stabilization Options

A Paravane stabilization system on a Sam Devlin design.

A Paravane stabilization system on a Sam Devlin design.

Paravanes

A paravane system uses two wing-shaped weights (often called “fish” or “birds”) that are dragged through the water on cables. Long booms, usually aluminum, hold the fish out some distance from the hull and related rigging holds the booms in position. These systems are fairly simple and can be quite effective if designed correctly for the boat. We once set up a Willard 30 with paravanes and took the boat comfortably to Bermuda and back. On the other hand, the system does take more effort to deploy than other means of stabilization, and we once repaired a 50-foot trawler after one of the fish flew through the saloon window due to mishandling by the crew.

Paravanes must be designed by someone familiar with their requirements. The length and diameter of the aluminum poles, the size of the fish, and the location of the rigging attachments must all be properly engineered.

Pros: A paravane system is simple, low tech, reliable, and reasonably effective.

Cons: Paravanes take some effort to deploy and require a wide clearance (not suitable in harbors or close quarters). Drag created by the fish impacts speed and fuel economy.

Flumes

An intriguing option, a flume arrangement controls the flow of water through a chamber to offset the motion of the vessel. Like paravanes, they offer a simple, low-tech option for stabilization. These systems have been used effectively on ships but they are rare on cruising boats. Over the years I’ve met a handful of people who have a flume and all of them seemed pleased. You can learn more and find some videos at www.hoppe-marine.com.

Pros: Simple, low tech, and reliable, a flume system creates no drag as nothing protrudes from the hull.

Cons: The flume tank adds weight and requires space. The flow rates must be adjusted for the particular conditions. And as these systems respond much more slowly than gyros or active fins, flumes are most suited to open-water situations where waves come in a regular rhythm.

Active Fins

Fin and mechanical detail of a NAIAD active stabilizer fin. 

Fin and mechanical detail of a NAIAD active stabilizer fin. 

Until recently, active fins dominated the cruising boat market and a number of companies compete in this arena, including ABT-TRAC, Naiad, Side-Power, WESMAR, and Gyro-Gale. A gyroscope senses the motion of the vessel and hydraulic actuators rotate the fins under the hull. The movements are quick and compensate for motion with remarkable effectiveness underway. Fins can be powered by hydraulic pressure, compressed air, or electric power. Until recently, active fins only provided stabilization while underway. Most manufacturers now offer some version of stabilization at rest. In each case, the power demands require running a generator to energize the fins.

Pros: Active fins are highly effective and provide quick response. They require significantly less space to install than gyros and aftermarket installation is possible on most cruising boats.

Cons: Active fin systems are complex and require expensive components to be exposed to the elements. In addition to the sensing mechanisms, the hydraulic components include an engine-driven hydraulic pump, a hydraulic fluid tank, seawater circulation to cool the hydraulic fluid, actuators, seals, and more. In addition, the fins protrude from the hull, adding the risk of incurring major damage underway.

SeaKeeper's line of gyro stabilizers.

SeaKeeper's line of gyro stabilizers.

Gyro Stabilizers

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Gyros are the prime alternative to active fins. The basic concept is illustrated by those pull-the-string spinning gyros many of us had as kids: The rotational force produces stability. The gyro stabilizers excel at keeping a boat steady, both underway and at rest.

Pros: Gyro stabilizers are highly effective at anchor as well as underway. No fins protrude from the hull, minimizing the risk of damage.

Cons: These systems are power hungry. And while they can be installed as a retrofit, finding the space for installation and engineering the gyro support structure can be a challenge.

Choosing the Right System

Let’s get this out of the way: There is no best system. The best system for you depends on your budget, the space you have available to install a stabilization system, and your need for stability at rest. If, for example, you place a priority on minimizing roll at anchor or in an exposed slip, the gyro will be your best bet. For faster planing hulls, gyros excel, but for slower full displacement trawlers active fins will be more effective. If you are crossing oceans and not concerned about coastal cruising and if you have a tight budget or place a priority on simplicity, paravanes or a flume would be the way to go.

Another consideration is speed. As boat speed increases, forces against the bottom create dynamic stability that reduces roll. On a semi-displacement or planing hull, increasing speed usually improves comfort. Full displacement trawlers do not benefit from this factor and have a greater need for stabilization. In addition, trawlers tend to be heavier and the added mass creates the need for greater stabilization forces. On lighter and faster boats trim tab systems—like those made by Humphree or Zipwake—now provide stabilization as well.

And one of the most important factors that is often overlooked is how much time, effort, and money it will take you to maintain the system over the years. While some of the systems on the lowtech-end require minimal maintenance, active fins and gyros are complex systems with their own cooling systems that need to be maintained.

Maintenance Requirements

Close-up view of the outer hull-to-fin shaft seal and inner bearings. Each manufacturer has a specified replacement interval. 

Close-up view of the outer hull-to-fin shaft seal and inner bearings. Each manufacturer has a specified replacement interval. 

This fin stabilizer shaft has been damaged from failure to replace the outer seal. When the seal fails, seawater enters the bearing assembly, diluting the lubricant and causing corrosion on the shaft.

This fin stabilizer shaft has been damaged from failure to replace the outer seal. When the seal fails, seawater enters the bearing assembly, diluting the lubricant and causing corrosion on the shaft.

Low-tech solutions are low maintenance. Other than inspection of cables and attachment points, paravanes require very little upkeep, and the same applies to flumes.

Active fins fall at the other end of the maintenance spectrum, requiring attention to the cooling system, hydraulic system, and fin seals. Maintenance intervals and details depend on the manufacturer but should be followed, especially when it comes to replacing the seals. In the boatyards, we often see avoidable damage and unnecessary expense incurred when boat owners neglect the seals on their active fins. Attached to a shaft that passes through the hull, each fin requires an inner seal to retain lubrication and an outer seal to keep out seawater. Neglecting the outer seal will allow seawater into the assembly. The seawater will then dilute the lubricant, eventually damaging the inner seal and possibly the shaft itself. Replacing the outer seals might cost you $1,500 per side, but if you ignore them too long the cost for replacing inner and outer seals plus the shafts themselves will run upwards of $4,000 per side.

The cooling system for the active fins mirrors the main engine seawater loop: intake strainer, water pump, heat exchanger. The same procedures used for maintaining engine cooling systems apply here (see “Troubleshooter” April 2017 and July 2018). The hydraulic system includes an oil reservoir, an engine-driven hydraulic pump, and a filter. Other than monitoring the fluid level, which should be done as part of your routine engine room check, these components should generally be serviced on an annual basis. Coolant pump failures are one of the more common issues and this critical component must not be overlooked.

Many active fins rely on hydraulics. As the hydraulic oil passes through the system, it heats up and must be cooled. On the right is a heat exchanger that works just like the one on an engine: seawater circulates, removing heat. Like the one on your engine, this one has anodes that must be maintained. At the top, a meter with a spinning vane visually confirms the flow of seawater. 

Many active fins rely on hydraulics. As the hydraulic oil passes through the system, it heats up and must be cooled. On the right is a heat exchanger that works just like the one on an engine: seawater circulates, removing heat. Like the one on your engine, this one has anodes that must be maintained. At the top, a meter with a spinning vane visually confirms the flow of seawater. 

This actuator moves the fin. Hydraulic fluid enters at the bottom of the image. The hat-shaped cap in the center covers the upper end of the shaft. At the top is the locking pin. This pin locks the fins when the engine is put in reverse or when “LOCK” has been selected on the control panel.

This actuator moves the fin. Hydraulic fluid enters at the bottom of the image. The hat-shaped cap in the center covers the upper end of the shaft. At the top is the locking pin. This pin locks the fins when the engine is put in reverse or when “LOCK” has been selected on the control panel.

If you have active fins, one more situation must be considered. The hydraulic system relies on a pump attached to the main engine or transmission. If this pump starts to overheat due to poor lubrication or failed cooling by the hydraulic fluid, the pump can seize. If the overheating situation cannot be resolved, the pump will need to be removed. In this case it is helpful to have on hand the original plate that covered the opening so you can run the engine or transmission without the pump attached. This plate comes with the engine or transmission and is removed (and unfortunately often discarded) when the pump is installed. If you don’t have the cover plate it would be wise to obtain or fabricate one to have on hand.

Gyro-Gale, a 40-year-old company based in Stuart, Florida, offers a variation on the active fin system. By using compressed air instead of hydraulics, their system avoids some of the complexity by eliminating the fluid reservoir, the power takeoff (PTO) pump, and the lines full of hydraulic oil. Instead, this system cools and compresses the air and moves it at a high speed. Gyro-Gale has a patented fin design with an articulated trailing edge that enables these systems to use smaller fins more effectively. They also tend to use more fins—for example, a system installed on a 48-foot trawler might use four fins. According to Gyro-Gale, the extra fins improve roll control and help reduce pitch. While this system still relies on seals, the elimination of the hydraulic system and related cooling components reduces maintenance needs. However, these compressed-air systems do have other maintenance requirements, such as biannual air filter replacements.

Three-dimensional rendering of the gyro mechanism inside a Seakeeper stabilizer. 

Three-dimensional rendering of the gyro mechanism inside a Seakeeper stabilizer. 

Like the hydraulic active fin systems, gyro-stabilizers also depend on a cooling system with a heat exchanger, and this component has similar maintenance needs. Although the gyro does not require seal replacement, it does have service requirements at 2,000 hours for the brake mechanism and hydraulic components.

Whichever system you choose, pay close attention to the recommended maintenance intervals and you will get many years of reliable service. 

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