How does a Seakeeper work?! Most of the explanations go shy on details, because the physics take a little effort to understand. So it’s time for the full explanation. How is the Seakeeper so effective at roll mitigation?
How does a Seakeeper work?! The marketing videos show this strange ball that pitches back and forth inside the ship. Somehow, this ball of magic stops the roll motions for the boat. How you ask? People give a vague explanation; they mention that the ball contains a spinning flywheel. And then they desperately hope that you don’t have any follow-up questions.
Truthfully, the Seakeeper takes a little effort to fully explain. It isn’t intuitive; gyro stabilization invokes some weird physics. So, let’s explain gyro stabilization. How is the Seakeeper so effective at roll mitigation?
Ships don’t stop on a dime. At some intuitive level, we all understand momentum. Get a big heavy thing moving forward, and it wants to keep moving forward. More mass or more speed both mean more momentum. Push a heavy boulder forward, and it takes a lot of force to stop that boulder. We call this conservation of momentum.
The same concept applies to spinning motion. A boulder traveling in a line has linear momentum, and a spinning boulder has angular momentum. Spin the boulder, and it takes a lot of torque to stop that boulder. To increase the angular momentum, you can either increase the mass, or spin faster. (Technically mass gets replaced with inertia when we talk about angular momentum. Slightly different math that also considers the distribution of that mass.)
Angular momentum gets really interesting when we consider three different dimensions. We live in a three dimensional world, with three directions: X, Y, and Z. In this world, we get three sets of angular momentum, one for each direction. The interesting question: how do these three sets interact with each other? Hint: weird things can happen. (Figure 2‑1 shows a weird example.)
Figure 2‑1: Spinning Gets Weird in 3D Dimensions [2]
Gyros take the basics of angular momentum and start exploiting them. We take a small disc and spin it really, really fast. Now, a small object contains lots of angular momentum. The intuitive example of this shows how an ice skater tucks in their arms to spin faster. We can see that they contain a lot of angular momentum. Gyros do the same thing, spinning faster than humans, so they get even more compact than a figure skater.
But remember, the really interesting part happens in 3 dimensions. Think about that spinning ice skater. She’s spinning vertically, rotating about the Z-axis only. But what if we could tilt while spinning? What happens then? This is the fun part behind gyros.
If we take a spinning gyro and force it to tilt the axis of rotation, the gyro starts to rotate about the third axis. We call this gyroscopic precession. (Figure 3‑2)
Take a gyro that’s spinning about the Z-axis. Then apply a torque, tilting the gyro around the X-axis. And the gyro behaves weird. Instead of following the direction you pushed, it takes this new motion and redirects it. A torque to rotate about the X-axis instead creates motion about the Y-axis.
Figure 3‑1: Ice Skater Example [3]
Figure 3‑2: Gyroscopic Precession Demonstration [4] [5]
Why does this matter for seakeeping? We can use gyros to stabilize ship motions. Take a gyro, spinning vertically, and mount it on the ship. So, the ship and the gyro roll as one unit. Now apply a torque about the X-axis. This happens when waves try to roll the ship from the side to side. But the gyro doesn’t like that. Gyroscopic Precession kicks in, which reduces the roll motion. (Figure 4‑1)
Figure 4‑1: Example of Gyro Stabilization [6]
To understand how this works, it’s best to think of the ship and the gyro and two separate items, and these two trade forces between each other, following their own rules. (Figure 4‑2)
Figure 4‑2: Model for Gyro Stabilization
If our Gyro were just free to pivot around the Y-axis, it would eventually rotate a full 90 deg, with its spin axis in-line with the X-axis. At that point the gyro doesn’t notice the ship roll and it stops working. So, we need something to counteract this pitch motion in the gyro and restore it to a vertical orientation.
Seakeeper counteracts the pitch through a hydraulic brake (the two large hydraulic cylinders on the side). Going back to our model of the ship and gyro, let’s examine what that hydraulic brake does
The gyro takes that torque from rolling the ship and converts some of it into rotation about the Y-axis. Pitch motion. So the gyrostabilizer converts torque from roll and turns it into pitch motion. Why does this matter? Because most monohull ships are long and skinny. Their shape means they resist pitch motions far better than roll motion. By converting roll into pitch, we use the strength of pitch motion to reduce our roll motions.
Looking at the animations of a Seakeeper, you might think those hydraulics are responsible for moving the gryo. Those giant hydraulic cylinders the side appear to start rocking the gyro. But no. Those cylinders actually stop the gyro from pitching back and forth. The cylinders are just a brake. When you release a Seakeeper, those cylinders release. The roll motion from the boat and gyroscopic precession cause the gyro to pitch back and forth. They are the invisible forces driving everything.
At first, I didn’t understand this, because when the gyro stays locked down, there is no roll mitigation. But when you run through the math, it turns out that gyro stabilization depends on both the speed of the gyro rotation and gyroscopic precession. Stop either one, and the gyro forces all go to zero. But ease off that hydraulic brake, and all the physics kick in. I think this hydraulic break is the real innovation behind the Seakeeper design.
Figure 5‑1: Seakeeper Animation
That hydraulic brake is the key to making a Seakeeper so effective. When we dig into the physics, the torque provided by a gyrostabilizer depends on the speed of roll (your angular velocity). In mild seas, we get slow roll speeds. For effective stabilization, we need to compensate with a bigger gyro and faster gyro rotation speed. But the situation reverses in large seas. Then, you want to temper the stabilization, prevent the boat from creating a jerky response. Except now, those larger waves drive big roll speeds, meaning the physics want to create a massive mitigating torque. So with large waves, we need to compensate with smaller gyros and slower rotation speeds. Except you can’t have it both ways. The gyro is a fixed size. And it takes a lot of energy to alter the rotation speed of that gyro. You don’t change these things in a second. But a second may be all that you have going from a small wave to a big one. How to create an adaptable gyro, without changing the physical parameters?
If you can’t change the physical parameters, change the timing. With the hydraulic brake, Seakeeper can rapidly switch the state of gyro stabilization. Constantly flip between on / off, without altering the gyro itself. This is because that hydraulic brake controls gyroscopic precession. No precession means no roll mitigation. They can even use the hydraulic brake for partial roll mitigation, allowing just a little bit of precession. Now, they can build the gyro oversized to handle light wave conditions. And in heavy conditions, the hydraulic brake controls the effect, only allowing partial precession. By controlling the hydraulic brake, they take a single gyro and make it adaptable to a wide range of wave conditions.
The second innovation for Seakeeper lies in the computer that controls the hydraulic brake. The computer constantly senses the ship roll. Not just roll angle, but also the angular velocity and angular acceleration. With all that monitoring, we get a bit of fortune telling. On a small time scale, ocean waves become somewhat predictable. Looking forward only 5-10 minutes, we normally see the same ocean waves. Same period, same amplitude. Sure, we still get some random variation. But there’s a dominant pattern. The computer detects that pattern, usually sensing subtleties that you and I may miss. Once it knows the pattern, it predicts the future. The computer predicts how your boat will roll 1 second into the future. This allows it to adjust the hydraulic brake and start the gyro responding before the wave even hits.
This hydraulic brake also allows mass production of Seakeeper units. Normally, we need to custom build a gyro, sizing it to each individual ship. But with that hydraulic brake, we don’t need an exact fit. We just need to get in the general ballpark of the best size, and the hydraulic brake then adjusts the response of the gyro, sizing those forces to perfectly match your ship. So Seakeeper doesn’t need to build custom units. They can standardize everything into few different sizes. Standardization allows mass production, delivering a lower cost per unit built.
Gyro stabilizers should not become a universal solution. For example, they depend on ships having much more stiffness in pitch motion than roll motion. For monohulls, this makes sense. But when we talk about multi-hulls, catamaran and trimaran, that isn’t true. Gyro stabilization is much less effective on multi-hulls.
They also suffer from a size limit. As ships get larger, we need more and more torque to counter the roll motions. Bigger and bigger gyros. As size increases, practical problems creep in. It becomes difficult to structurally handle the forces from a supersized gyro. Remember, this gyro puts out enough torque to roll the entire ship. Imagine that for something the size of a cruise ship. It becomes a nightmare to distribute that roll torque into the ship structure. This is why we generally don’t use gyro stabilizers on ships over 100 ft (30 m) long.
Finally, people need to remember that gyro stabilizers are not the only solution for motion control. Other options include:
Even Seakeeper makes a line of interceptors that actively adjust for ride control. The best solution depends on many factors like the size of the ship, speed, mission, and your budget for motion control. This is where the naval architect steps in to design the best combination.
Rather than immediately going to Seakeeper, I prefer to use gyro stabilization as one component in a whole system of motion control. We start with the cheap solutions, like bilge keels, and then supplement them with more advanced solutions like a Seakeeper. With some careful planning, we achieve the same goal, using smaller units and less expense. Especially on commercial ships, ride control shouldn’t break the bank.
No doubt. The Seakeeper works. They took classic gryo stabilization and added in active control for a truly effective device. But the core principle still depends on gyroscopic precession. That weird bit of physics which couples roll motion with pitch motion. Apply this on monohulls, gyro stabilizers provide major advantages. No magic. It’s a machine like any other, with limits and best case applications. But even I will admit, it’s a pretty impressive machine.
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