For current and aspiring superyacht owners, Sam Fortescue details everything you need to know about marine stabilisers for your yacht...
What is stabilisation?
Simply put, stabilisation is any design feature of a yacht intended to reduce or combat its tendency to roll in seas.
What causes rolling?
Even very large yachts are affected by swell and waves on the surface of the sea. In the trough of a wave, the hull receives less support and drops. The crest of the wave has the opposite effect, applying lift to that bit of the hull. To some extent, waves in line with the bow and stern are cancelled out by the length of the yacht, because several sets of waves will be acting on it simultaneously. The problem is much more acute when the seas are acting on the narrower beam of the yacht.
What is the difference between passive and active stabilisation?
Passive stabilisation is nothing more than fixed features of the hull, such as a keel or ballast, which help to keep the boat steady. Active stabilisation refers to a device which moves or reacts to combat roll. This can be a movable fin, such as the Quantum XT, a rotating arm such as Quantum’s MAGLift, a foil or a gyroscope-style stabiliser. Active devices have a much greater effect than passive ones, but also consume power, use space and generate noise. In the case of Quantum, its systems eliminate at least 80 per cent of a ship’s roll.
They rely on an electronic sensor to detect roll angle, velocity and acceleration. “Thirty years ago, we had a gyroscope spinning with some electrodes on it and that was considered high tech,” says Quantum Marine Stabilizers co-founder John Allen. “Look at what we have today: incredible sensors the size of quarter with three wires coming out of them the breadth of a human hair."
“All of the manufacturers have their own qualified control algorithmns, and from that they generate the signals to move their stabilisers around under the ship. We believe our control systems are the best.”
What is zero speed stabilisation?
This is a key concept, and one that has driven a lot of recent development in the market. It is the ability of the stabilisation system to combat roll when the boat is not moving through the water – such as when lying at anchor. Whereas some systems rely on the lift generated as water moves past a foil or a fin, zero speed stabilisation can generate its own righting forces. “It creates a counter force when the boat rolls – like grabbing the keel and pushing in the other direction,” explains Allen. “In the case of a fin, you have to do it very quickly and create a force for a very short period of time.”
What options are available?
Broadly speaking, there are four different types of commercial stabiliser. Some work better at rest or at slower speeds, while others are more effective under way. Besides performance, choice will also depend on budget and the space available, with retractable systems typically requiring more of both.
The most popular is a stabiliser fin mounted on the hull below the waterline, such as Quantum’s semi-retractable XT system. The fin rotates on an axis perpendicular to the hull, giving it the ability to create very high up-down forces for a fraction of a second. The Quantum XT unit has an extendable element to the main fin, which dramatically increases its effectiveness at zero speed, while stowing away for minimal drag under way. “It’s like a paddle under water,” says Allen.
Then there’s the rotating-type stabiliser, which harnesses the so-called Magnus effect to create lift. A composite horizontal cylinder rotating rapidly underwater develops low pressure on top, with high pressure underneath, to give lift. Mounted on a swinging arm, Quantum’s MAGLift can apply very precise lift to counteract roll at speeds from stationary up to around 15 knots. It also has the benefits of being totally retractable at higher speeds, and has a relatively small internal footprint for tight installations.
A foil-based system generates lift using a hydrofoil that deploys from the side of the hull. Under way, the principle is simple, but at anchor and in Zero speed mode, the foil itself can be swung in and out rapidly to create lift. In the case of Quantum’s Dyna-Foil system, this can be 150 per cent more effective than a standard fixed fin of the same size. It can either be retracted into a pocket for minimum drag or rotated parallel to the hull.
Finally, gyroscopic stabilisers rely on a heavy flywheel being spun at great speed under a vacuum to transform sideways rolling motion into longitudinal pitching motion. They rotate at close to the speed of sound and take time to get going. They are more effective in smaller yachts.
Can these systems be retrofitted?
Yes, any type of stabiliser can be retrofitted, although choice may be determined by the available space on board. “If it’s a smaller boat below 60m length, then it goes in the engine compartment, otherwise in general it has its own compartment,” says Quantum’s John Allen. “Two sets of fins is generally not as efficient as one big set, but sometimes there’s no other option.”
How loud are they?
Active stabilisers always have moving parts and are hydraulically powered, so there is the noise of the machinery itself, plus the hydraulic pumps and the generators driving them. Their positioning needs to be carefully worked out to minimise hull-borne noise, and the power units must be well designed and very well insulated. “Noise is extremely critical,” says Allen at Quantum. “First there’s the design of the hydraulic equipment – this has to be custom built. Then you have to address the structural design of the boat itself and the acoustic damping in the compartment around the equipment. Steel work and compartment design have to be looked at. We are often consultants on that account.”
How much does it cost?
Well, how long is a piece of string? For new equipment, costs at Quantum range from around $300,000 up to $5-5m, depending on the size of the yacht and the scale of the system. Retrofits may simply involve updating an old fin with a newer one, in which case costs are lower, because the hydraulics may already be in place. Installation and sound proofing costs would come on top of that.