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Why tiny active stabilizers have a large effect on ship roll motions

Originally published on dsaocean.com

What do an elephant and a manatee have in common? It might sound like the start of a good joke, but it is pure biology. The answer is they have muscular hydrostats on their faces. The elephant’s trunk and manatee’s highly prehensile snout are both examples of this fantastic mechanical structure. Both animals can use them for gripping and holding things. A muscular hydrostat has no bones to speak of but can make incredibly complex shapes and motion – and even exert tremendous forces. But how do they do this without any bones?

The key is in small groups of muscles. These groups of muscles are clustered and oriented in specific ways, and they each react against each other. When they get their timing right – by contracting and relaxing in harmony – they change the shape of the hydrostat and create this wide range of motion and force. Tiny components work together to have a large effect.

It takes a particular set of circumstances for something tiny to have a large effect. But there are examples everywhere, and the world of ship motion is no exception. Many ships can have dangerously large roll motions in certain ocean conditions. But ship designers have several ways to deal with this problem. One way is with active roll stabilizers. These tend to be extremely tiny devices, especially compared to the size of a ship itself. But when these small components work together and get their timing right, they can have a large effect – and bring roll motions down to much safer levels.

What are active stabilizers?

Active stabilizers typically look like tiny wings on the hull below the waterline. Often, most hull appendages are fixed on the hull and don’t move around. But in this case, these are referred to as active because they have an actuator that can quickly change their pitch angle.

Active stabilizers reduce uncomfortable or dangerous ship motions

The most severe ship motions and accelerations are often caused or exacerbated by roll. This is because many hull forms are long and slender, without a lot of capability of damping or slowing down this motion. Often, fixed hull features like bilge keels can add some drag to increase damping.

But depending on the requirements of the use of the vessel, it may not be enough. Severe roll motion can lead to large accelerations that cause sea sickness and injuries. Equipment on the ship or the ship hull itself can be damaged, too. So how exactly do active stabilizers work?

Active stabilizers have a foil profile just like airplane wings

Active stabilizers look like tiny wings for a reason: they use the same principles of lift to do their job. Fortunately, because water is so dense compared to air, the lift forces generated are significant, so the stabilizers’ size doesn’t need to be massive to be effective.

The water flowing over the foil generates a powerful lift force. Because the foils jut outward from the hull below the waterline, these lift forces create a sizable torque that can directly affect the ship’s motion in roll. So the real trick with active stabilizers is not about how much force they make but the timing of the pitch angle.

The pitch angle of the foil is critical. The pitch angle of the foil means that the “lift” force on the foil can be directed upwards toward the sea surface or downward toward the seabed. It’s this timing that the active stabilizer needs to get right to make sure the lift force is always creating an anti-roll torque that resists and damps out roll motion on the ship.

Timing the anti-roll torque is what’s key to these small stabilizers being so effective

They are substantially better when you compare them to fixed hull appendages like fixed foils or bilge keels that can’t change how their forces are applied. An active stabilizer produces the right amount of stabilizing torque at the right instant in time to keep roll motions under control. Active stabilizers can do a lot, but what kind of alternatives are there?

Active stabilizers aren’t the only solution

There are other tools that ship designers can use to control ship motions, like slosh tanks or U-tube tanks. There are other active devices, too, like gyroscopic stabilizers. Certainly, controlling roll is essential for rounded monohull ships. But certain hull types are much more inherently stable – like multi-hull catamarans, for instance.

How do you find the right size of active stabilizer?

Just because they are small, it doesn’t mean sizing the system is a no-brainer. It is always a good idea to verify the performance of a specific foil and control system. One way to verify this relatively quickly is using a seakeeping analysis tool. In ProteusDS, there are features for the numerical evaluation of active stabilizers. This is one way to show ship motion in the worst-case scenario but also in a typical sea state with and without active stabilizers.

When do active stabilizers fall flat?

There are a few disadvantages to active stabilizers. Because they work based on hydrodynamic lift, they always need some relative flow in the water, so usually, the ship has to have some forward speed. On top of this, the faster the ship moves, the more effective the active stabilizers work, but the inverse is true, too – the slower the forward speed, the less effective they are.

Active stabilizers may be a slam dunk depending on the specific needs and requirements of the ship design. Still, it may also be a good idea to check out other options like U-tube tanks or gyroscopic stabilizers.

Example time

We can illustrate the effect of active stabilizers on the Generic Frigate using the ProteusDS ShipMo3D toolset. Once the Generic Frigate ship model is set up and time-consuming radiation and diffraction calculations are completed, the active stabilizers can be added and adjusted as needed to see the impact on RAO and ship motion. A pair of active stabilizers at midships with a basic roll velocity control reduces the peak roll RAO substantially. The peak RAO in roll is reduced by almost 30%.

You can see how to set up these active stabilizers using the Stabilizer Generic Frigate project in this video tutorial from our YouTube channel. The Stabilizer parameters are based on the same parameters as in the ShipMo3D validation report.

Summary

Active stabilizers are small but effective roll control devices – one among many at the disposal of ship designers. While they are effective, they are not always a slam dunk and require careful consideration and sizing for the ship type.

If you are feeling left out because you don’t have a muscular hydrostat on your face like an elephant or a manatee, don’t worry. It turns out you already have one in your mouth – your tongue!

Next step

Read more on the active stabilizer scenario with the Generic Frigate in the ShipMo3D validation report here.

PS

Read more on the prehensile manatee snout (with pictures!) here.