Everything you need to know about yacht stabilisers

Modern technology can remove as much as 90 per cent of the rolling motion of a yacht, greatly improving comfort and safety and even saving fuel - and technology is improving all the time. BOAT explores the options.

"It isn't motion itself that causes travel sickness," explains Ronnie Skuan, CEO of Norwegian stabiliser specialists, Sleipner. "You are fine moving in a straight line, but it is the acceleration and deceleration in between each change of direction that affects a person's balance. A yacht not only pitches and rolls in a seaway, but it also corkscrews, an uncomfortable motion known as yawing. What stabilisers do is largely cancel out the rolling motion, damping the yaw in the process. Any slight pitching motion that is left is usually well below the frequency that makes people ill, so they probably wouldn't even notice it. As a result, boating becomes a pleasure again."

Stabilisers must prove equally effective both underway or at anchor (known as 'zero-speed'). When underway, the water moving past the hull can be deflected up or down by powerful electric or hydraulic fins, providing an immediate counterforce to the rolling motion. When anchored or drifting, and with no water flow to harness, stabilisation becomes more of a challenge. As such, many yacht builders will fit a combination of technologies, such as fins and gyros, which work well together. Fins will kick back and forth to minimise a roll, while gyro stabilisers - big, powered examples of the spinning flywheel toys we probably all played with at school - will exert a counterforce on the hull as they try and stay upright. A large yacht could have several gyros fitted, interceptors, fins, or a mix of all of them.

"Stabilisation isn't just about comfort, it is also about safety," says Jonathan Ridley, associate professor of marine engineering at Solent University. "Combatting acceleration is very tiring for even the most experienced crews, as they are constantly expending energy to oppose the changing dynamics. Even sitting down requires effort to stay in one place. For the guests, being seasick is a miserable experience that can put them off boating for life. A steady ship also prevents damage to inventory from excessive movement. There are even fuel savings to be made from improved directional stability underway. As such, stabilisation is seen as a standard fit on most modern motor yachts above 25 metres, although is far less common in sailing yachts. Whilst self-contained gyros are still used at anchor, underway the press of sails and the long keels tend to have a powerful damping motion, although some sailing yachts will still carry a fin system for long passages under motor."

All the stabilisers we feature here are "active" units, meaning they have moving parts that "push back" against the sea. Most will operate with very little noise, another consideration if anchored peacefully at night. Although there are variants and technology is improving all the time, here are the five main types:

FINS

Mimicking the profile of a fish, an opposing pair (or two pairs on larger vessels) of pivoting fins is placed under the hull as close to midships as possible and fitted with powerful actuators. The fins respond to sensors that measure the roll of the vessel and react quickly, providing a force that is equal and opposite. They are shaped to offer minimal drag going forward, but maximum effect when moving from side to side.

Stabilising a yacht while it is motionless or at anchor requires a different type of fin motion. Today, most companies offer an at-rest solution as well - either by using the same fins in a different mode or by installing a second set of fins.

Sleipner has taken the concept further by giving the fins a swept upward curve, which makes the thrust far more effective at anchor. The shape also minimises 'fin walk' where the kicking movement gradually propels the yacht over its anchor. Some designs, such as those from Naiad Dynamics, also have a retractable inner blade that expands the surface area in rougher conditions.

In the past, most fin stabiliser systems were hydraulic, but nowadays there’s a growing trend among manufacturers to offer electric versions. CMC Marine was the first company to do this and it leads the market in supplying systems to all the major Italian shipbuilders.

Electric fin systems are well suited to installation in yachts that also have other electric-powered equipment and can run off the vessel’s house system, eliminating the need to accommodate hydraulic piping on board. They also create less noise than some older hydraulic systems, which is very desirable to boat owners.

PROS: Fins are very effective, quick acting, powerful and usually offering zero speed functions. Less space is needed with the latest units due to the compact size of motors and hydraulics.

CONS: Vulnerable to damage from submerged objects.

GYROS

A relatively new technology compared to fins, gyro stabilisers spin a heavy metal flywheel at up to 17,000rpm and then use the gyro's desire to remain vertical to push back against the forces trying to tip it over. This proves very effective, especially at anchor. The rolling and yawing is instead turned into more of a bobbing motion, which is very easy to live with.

Gyros are usually delivered as a self-contained unit and can be installed wherever there is room, but ideally on strengthened bearers along the centreline. They operate almost silently, mainly because the flywheels are often spinning in a vacuum to minimise resistance and heat. Gyros come in a wide range of outputs, from the superyacht-sized Veem VG1405D to the Seakeeper 1, installed on a centre-console sportsboat. Gyros do require a considerable amount of space below, a steady power source and strong points to push against.

PROS: Very effective at anchor. Easy to install. Multiple units can work together. No need to place it on the centre line.

CONS: Not as effective as fins underway, but very useful in tenders, especially for sports fishing. Require a reasonable amount of space and provision of strong points.

ROTARY STABILISERS

Spin a cylinder in water or air, and you develop a low-pressure area on one side. This generates lift, known as the Magnus Effect, which can be harnessed to counteract a roll. Rotary stabilisers have proved effective both at anchor and underway as they can be articulated backwards and forwards. Skippers deploy them at 90 degrees at anchor for maximum leverage but angle them backward to reduce drag when cruising. The speed of the spin can be quickly altered to create different levels of lift depending on the conditions. Some types can be retracted into the hull to keep them safe and reduce drag to a minimum.

PROS: Relatively simple in operation. Quite easy to install. Fast acting. Good for tenders.

CONS. Some drag when deployed underway, so only effective for displacement vessels.

WINGS

As actuator technology advances, the hydrodynamic wing is evolving fast. This is a melding of fins and trim tabs, so when underway, the wings behave just like those of an aircraft, presenting at right angles to the flow and twitching up and down to exert the necessary opposing forces. At anchor, they sweep backward and forward like giant paddles, generating the lift required to cancel out rolling. When not needed, they retract neatly into the hull.

PROS: Effective and stowable, so ideal for ice-class vessels wishing to minimise protrusions.

CONS: More complex than most systems. Needs modified space in the bilge area for the retracted wing and the actuators.

INTERCEPTORS AND TRIM TABS

Only effective for vessels underway, and particularly useful on faster yachts and tenders, interceptors and trim tabs are found on the transom and act on the very rear of the vessel. Interceptors are guillotine-type blades that drop vertically into the wake, whereas trim tabs are horizontal flaps. Both are usually motorised, and thanks to modern sensors and actuators, can react in a split second. The force they can exert cancels out a lot of the yawing movement underway.

PROS: Fast acting, effective and helpful in saving fuel by establishing correct trim when planing. Minimal internal space needed.

CONS: Only effective at speed.

Some companies, such as Humphree, make both fins and interceptors, such as this Lightning mode (below), complete with colour change underwater lights. The unit is intended for tenders up to about 14 metres.