WHY DO YOU NEED A SYSTEM FOR STABILISING YOUR VESSEL?
Passengers and crew travelling on yachts, ferries, cruise ships, work boats, tugboats and offshore vessels always hope for a smooth ride in great weather, but this is, of course, a wish not always granted.
If you've ever been tossed around on a boat and felt seasick, you recognize the need for a system that stabilizes your vessel. Seasickness is perhaps the worst effect of a rolling, unstable boat, but it's not the only one. In difficult conditions, it can be difficult to move around the boat, and you might find yourself caught between plates and cups being smashed due to heavy rolls. This dramatically reduces onboard security.
Rolling is the most significant problem among all the motions at sea. Hence, solving the physical challenges of roll is a priority. Technological advancements in ship design have already given naval architects a chance to study wave motions around the hull of the ship. This provides a great foundation to create a design that minimizes such effects and ensures a safe and comfortable voyage for passengers and crew.
For work boats operating in rough conditions, an active stabilisation system can be the decisive factor between getting the job done safely or not at all. A stabilised vessel improves the seamen's safety and comfort greatly, as boat roll is almost completely eliminated, both underway and at anchor.
DIFFERENT METHODS OF STABILISATION
Marine stabilisation systems have become big business. Today you can choose between several providers, using various methods to decrease roll. Roll motion stabilisation can be achieved in conventional ships by changing the form of the hull. Reduction in roll amplitudes is also possible by other means. Broadly, we classify stabilisation systems in two categories:
Passive Systems: When no separate source of power or special control system is required. Examples of passive systems are the Bilge keel, anti-rolling tanks (passive), fixed fins and passive moving weight systems.
Active Systems: When the opposing roll is produced by moving masses or control surfaces using power like active fins, Anti-rolling tanks (active), active moving weight and the gyroscope.
On this page, we will focus mainly on the active systems. Depending on the planned use and type of boat, you can mainly choose between three methods: active tanks, gyro stabilisers and active fin stabilisers.
Before choosing the technology when you are designing a new yacht, we recommend thinking carefully through the advantages and disadvantages of the various options. Start by asking yourself these questions:
Where will the ship be sailing? Mediterranean Sea, Caribbean, The North Sea or under conditions demanding an entirely different technology?
What is the primary use of the ship? Will it be used for working in rough conditions, for whale watching, cruising around the islands of Greece or fishing outside the coast of Norway?
Is the hull designed for high speed or displacement?
Will I use the ship to travel long distances at steady speed, for trolling, or mainly to lie still by anchor?
What is my budget for stabiliser systems?
A brief overview of the active systems provided:
Due to the bulky construction and costly assembly, active tanks are mainly used on large bulk carriers, tankers, and LNG carriers. In active rolling tanks, the movement of the water is controlled by pumps or by the air pressure above the water surface. When the water is forced against the power of the waves, it provides stability. Depending on the sophistication of the system, active tank stabilisers have been found to leave an efficiency of 80 % or more in motion stabilisation. Nevertheless, this system requires so much onboard space that it will not be applicable to the vessels we focus on.
Gyro stabilizers can be fitted into smaller boats and are also used by large ships and carriers. In a large boat you might see the need to install more than one gyrostabilizer. Gyro stabilizers are using the physical laws of precession to work against sea motions. Precession is a physical phenomenon that appears when a rotating object's axis "wobbles" while exposed to the effect of an external force. While spinning the gyro tilts fore and aft as the boat rolls, it creates a torque that pulls up on starboard and down on port, or vice versa. This reduces roll. The World War I transport USS Henderson, completed in 1917, was the first large ship with gyro stabilisers (right). It had two 25 ton and nine feet diameter flywheels mounted near the centre of the vessel, spun at 1100 RPM by 75 HP AC motors.
Active fins are fitted outside the hull. Sensing the rolling motion of the ship, a gyro sensor sends a signal to the actuating system. This causes the fins to move in a direction which counteract the roll. The bigger the boat and the longer the lever between the center of gravity and the fin, the more power it gives. The active fins get stronger the bigger and wider the boat is. Fin stabilisers are vastly more efficient at higher velocities, while they lose effectiveness when the ship travels at minimum speed. At low speed or anchor, stabilisation-solutions include actively-controlled fins. The first use of fin stabilisers on a ship was on a Japanese cruise liner in 1933.
The video shows active fins and their impact on the hull's movement in 0,5 – 1 meter high waves, at zero speed.
WHICH STABILISER SYSTEM IS THE RIGHT CHOICE FOR YOU?
Here, we'll list the pros and cons of active fins and gyro stabilisers. Let your preferences decide which system is the best fit for you.
At anchor, the two technologies offer a similar effect on roll and sway. While the fins flap (or swim), they will have a slight impact on moving the boat forward under very light wind conditions. This will occasionally cause some noise from the anchor chain. On the other hand, the gyros are summing at very high speed below deck. Gyros have a certain amount of noise pollution onboard.
The gyro will need about 30-45 minutes startup-time before delivering full effect. Fins are active from the moment you switch them on.
As fins are submerged, this makes repairing them slightly more challenging. This generally means an increase in repair costs. The gyro is located safely inside the hull, with no risk of damage. However, when the gyros need service, you might, depending on the manufacturer's construction, have to take the whole gyro-wheel out.
Because gyro stabilisers do not require protrusions from the hull, they are not subject to damage from grounding or impacted by floating debris.
Fins can correct small steady/static heels or list due to beam winds, off-center weight or extended period rolls running downwind. This is due to their inherent ability to apply a steady lift force under the hull while the vessel is in motion. In contrast, gyros cannot supply constant forces, which means they cannot correct a heel or list angle over longer time periods.
The gyro stabiliser has its own power impulse - firmly defined. The force from an active fin is the exponential result of speed (V^2), meaning you will see very high power at high speeds.
Fins don't need a lot of space for installation. Modern system actuators have a low built height, which allows you to spread the gear around the boat. Gyro stabilisers come as one big box, which is quite space-demanding.
Professional yacht broker Jimmy Rogers has written this article summing up the pros and cons of fins and gyros: Gyro Stabilizers and Fin stabilizers, what you need to know
Fin stabilisers come with the only system that can handle both cruising and "at anchor" use. The "at anchor" stabilisation force determines the size of the fins. Larger fins provide higher stabilisation force, but also create more drag. This equals higher fuel consumption, an element that is often decisive for most boat owners. Larger fins also mean that the internal components grow in size, making it more difficult to install them in an optimal position. This can result in negative side-effects like causing yaw (steering effects) and sway (side-way movements). Thus, the overall situation often results in a compromise in fin size, meaning most fast boats today do NOT have the stabilisation their owners want.
The most advanced fins today have a much better force angle than the older flat fin systems, providing a lot less drag as they also create lift. The improved force angle reduces the unwanted side effects of yaw and sway for installations that are not perfectly situated on the hull. The increased efficiency of these new designs means that smaller fins can achieve the same stabilisation force as bigger flat fin systems, while simultaneously reducing the internal component size necessarily.
BENEFITS OF STABILITY BY ACTIVE FINS
As captain of Anna J., Richard Kennedy was searching for a stabilisation system to increase his passengers' comfort while onboard. In this video, he shares his experience with the Side-Power stabiliser system.
INSTALLATION AND PLANNING OF STABILISER SYSTEMS
Whether you are planning to get stabilisers for a newly built vessel or a retrofit, you should be aware of some key factors. Firstly, you must make sure the hull covers the basic stabilisation requirements, meaning it has to be prepared for the extra load and strain.
As a naval architect, you have the freedom to design the hull as you wish in relation to stiffness and rolling. If you are going to have a stabilisation system onboard, you can plan for a softer hull. Normally, a softer hull means more rolling. With stabilisers, however, you should only comply with the minimum requirements. This will provide greater freedom and a more fuel-friendly hull.
With Side-Power's installation guide, it is easy to do the right measurements while on the drawing table.
John Maxey is the owner of a 78 feet long Fairline Squadron. In this video from 2011, he shares his experience with the Side-Power stabiliser system.
RECOMMENDATIONS AND PRACTICAL USE
The Side-Power Vector fins™ improve the roll reduction efficiency by some 50% in "at anchor" mode, and 30% in cruising situations compared to flat fins. At the same time, they dramatically reduce undesired yaw and sway motions caused by active fins.
The vast benefits in efficiency and the reduced side-effects make them particularly ideal for today's modern, fast cruisers. Traditionally, standard fin stabilisers have not satisfied the desired roll reduction without causing too many side-effects. This is particularly true for light-weight cruisers below 20 meters. Due to their shorter roll time periods, they are particularly difficult to stabilise. The Vector fins™ unique advantages make them the preferred choice for smaller, fast boats, a market that previously lacked ideal overall solutions for both cruising and anchor stabilisation.
WHAT IS THE PRICE OF A FUNCTIONING STABILISER SYSTEM?
A stabiliser system will cost about 2-4 % of the boats total cost. Complete installation on a 70 feet long new motor yacht will cost around GBP 70.000. The price range of active fins is smaller than for gyros. Because the gyros weigh four times more than an active fin-system, you have to strengthen the hull and give up space for the two-ton gyro-box.
Be aware that you usually don't install fins on boats smaller than 50 feet. The gyros will fit in vessels as small as 30 feet. Both systems can be retrofit in a motor yacht, which is great news for a proud boat owner who will rather install a stabiliser system on his ten-year-old rolling lady than buying a new motor yacht.
The direct benefit of the stabilisation systems is clear when it comes to comfort and safety aboard. Consequently, this allows usage of the vessel when it otherwise would have stayed in port. For work boats, charter boats and other vessels that generate an income by the hour, a stabilisation system can be a smart financial investment. If you are buying or selling a boat second hand, it is worth noting that stabilisation gets a lot of attention in the market. We already notice a significant advantage in the market for vessels with a good stabilisation system onboard, and we see no reason to expect this to change in the future.
HOW TO CHOOSE THE BEST STABILISERS MANUFACTURER FOR YOUR VESSEL?
With several manufacturers on the market and various technical solutions available, it is worth doing some research to find the stabilisation system best suited for your vessel. Some systems claim to be better at anchor, while others claim they're better underway. The final choice often comes down to the boat owner's intended use of the vessel, the design or hull limitations, and whether it's for a new or retrofit boat.
This checklist of important features/performance points might be helpful in your considerations:
Reaction time "AtAnchor" (for smallest waves)
Handling of long waves in cruising
Dynamic heel in turns
Speed and fuel consumption
Weight vs. stabilisation force
Space to install
Instant ON and instant OFF
Stabilise while reversing
Can operate without generator when cruising
Typical "AtAnchor" noise level
Ease of service/repair if needed
Geographical service points