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Why undersized vessel anti-roll fins backfire (and how to avoid this problem)

The Pistol Shrimp has a superpower: it can make deadly shockwaves in ocean water. It uses this blast wave to stun and even kill prey when hunting for food. But how does it do this?

It has some serious equipment to make this happen: an oversized claw that snaps shut incredibly fast. It goes so fast it makes a cavitation bubble that collapses, creating a sonic wave that can pulverize small animals near it. But these blast waves travel in a spherical pattern. So, the pistol shrimp also gets hit by this impact pressure. But how does it remain unfazed?

The trick is with a reinforced shell around its head. Much like a helmet, it provides protection and insulation. Making shockwaves works very well for the pistol shrimp: it’s a deadly hunter with a niche ability that makes life easier for them. But they can’t use that ability without their built-in helmet and the protection it provides. They’d quickly stun themselves and end up with no dinner on top of it all. Without the right-sized equipment, their approach would completely backfire.

Similarly, we are always looking for ways to get the right-sized equipment for a particular project. Nobody truly wants an undersized system that isn’t built to work for the stress it’s put under. Yet, in some situations, an undersized system can backfire and leave you much worse off than you might expect. This is the case for ships with anti-roll fin stabilizers. If the fins are undersized, performance can degrade very quickly. In this article, we’ll talk about why undersized anti-roll fins can backfire:

1. the risk of stall at an excessive fin angle
2. the risk of jerking at high velocity and acceleration
3. the risk of sound, vibration, and wear

First, we’re going to cover the risk of stall at excessive fin angles.

At forward speed, fin stabilizers act just like airplane wings

These fins rely on the effects of lift to create forces that counteract ship motion. Most of the time, active fin stabilizers are used to reduce rolling motion. Timing is everything. The fins must actively change their angle at the right time so the lift force pushes against the ship as it rolls back and forth. But timing is not really the issue with fin stabilizers. It’s how much force they can make to resist ship motion. And the amount of angle of attack comes into play that creates these forces.

The amount of force a fin can make depends on a few things, and the angle of attack is one of them

The other factors are fin area and forward speed. But fin area and speed have well-understood and well-behaved effects on the system’s performance. Generally, the larger the velocity or fin area is, the more force you get out of the system, and it’s as simple as that. Similarly, the fin lift force grows with the angle of attack of the fin. However, unlike the other factors, there is a significant limit on performance when the lift completely disappears.

The angle of attack is different because of the threat of stall

The effects of lift on a fin will grow as the angle of attack increases. But this doesn’t continue forever. At a certain point, there’s a complete change in the hydrodynamic flow. The flow detaches, or separates, from the fin, and the lift force disappears. This is sometimes referred to as a stall condition. If the lift force abruptly disappears, so will the system’s ability to resist ship motion entirely. So, in terms of anti-roll performance, the fins should avoid angles of attack that are too large because stall will decimate anti-roll performance. However, a stall condition can be avoided with built-in safety features. Some equipment suppliers will specifically have angle limits on their fins to prevent a stall condition. But when fins hit this angle limit, it still limits, or clips, the amount of roll resistance from the system. You certainly can’t get more roll reduction out of the system at this point, but at least there isn’t a sudden loss of anti-roll performance. But how does all of this relate to an undersized system?

A sign that fins are too small for a ship is that the angles of attack they are reaching for adequate anti-roll performance are approaching a stall or clipping condition. Frequently reaching this limit is a sign of an undersized system. So if fin angles are large, approaching stall often clipping, the next step is to look at a bigger fin – in other words, sizing up.

Larger fins mean greater area. Since a larger fin area will produce more substantial lift forces, these fins don’t need to rely on such large angles of attack to get the same anti-roll performance. The amount the fins move also relates to other ways that performance can be affected, which brings us to the second point on actuation performance during high accelerations and velocity.

Larger fin angles can require larger loads to actuate

Regardless of the fin actuation system – hydraulic or electric – the larger the fin angles required, the larger the actuation loads will be. It might seem counterintuitive, but smaller waves at a shorter period waves can cause a strain on the anti-roll system. It has to do with the speed the system needs to respond. The ship rolls faster in more common shorter period waves. And the more quickly the ship rolls, the quicker the actuation system needs to move the fins. The quicker the actuation system moves the fins, the greater the actuation loads required to accelerate the system as the fins change direction. When these actuators are pushed to the limit of their load capacity, these accelerations can lead to a jerking motion at the fins. And this jerking can disrupt the timing and direction of the hydrodynamic lift forces. In turn, this disrupts the system’s ability to smoothly resist ship motion and the resulting comfort levels.

So, what’s the sign of an undersized system in this case? It might look like excessive fin angles at mild to moderate ocean wave periods – perhaps 3-6 seconds. Of course, you need the right-sized actuator power for the system, too, so you aren’t pushing the hydraulic or electrical system performance to the limit. This highlights that hydrodynamics are not the only aspect of what drives anti-roll system performance. This brings us to the last point, on pushing the actuation system to the limit.

The more often you are at maximum actuator performance, the harder it is on the overall system

The higher the actuator forces and accelerations are, the more wear and tear there will be on the system. You will end up with a higher wear rate on components and seals. This means the risk of degrading performance may creep up and up with time. But more importantly, noise and vibration are more likely at maximum actuator performance. This vibration can also disrupt fin actuation.

If fin actuation is vibrating, it’s going to amplify into vibrations in the hydrodynamic lift forces, which can affect the anti-roll performance. The system vibration and noise might be another way passenger comfort is affected, too. The other downside is the increased risks of maintenance costs as well, with a shorter interval between refits and servicing because of the stresses on the system.

So, how do you avoid all these symptoms of an undersized system?

An adequate fin area is necessary to reduce the need for larger angles of attack. This reduces the burden on the actuation system to reach higher velocities and accelerations. It also means there’s less risk of hydrodynamic stall or hitting the clipping limit in the forward speed condition. And there is less chance for vibration and noise in smaller period waves with faster ship motion. There’s less wear and tear on equipment, increasing the chances for smooth and reliable performance.

Ship motion prediction software can help establish initial fin areas that may work well

Naval architects use software like ProteusDS to predict how a particular design will respond with anti-roll fin systems. A given fin area will reduce ship roll a certain amount and give feedback on expected fin actuation ranges in different sea states. There are also tools in ProteusDS that provide an ISO-compliant comfort rating that shows how much difference anti-roll fin stabilizer systems can make.

Let’s look at an example

We used the 50m Generic Yacht with a range of fin sizes to highlight performance in a specific sea state. The conditions were a JONSWAP 1.5m significant wave height and 7 second sea, with the yacht in beam condition and cruising at 12 kn. Each fin had the same control gain and relatively forward position on the hull, so the only variation was in fin area. The results of the roll motion and fin actuation are listed in the table below. The roll reduction performance is not necessarily optimized, so you might be able to squeeze more roll reduction from smaller fins. But the main trend to highlight was how with smaller fins, they need to work harder and go through a much larger dynamic sweep angle, or the fin root mean square or RMS, to stabilize the ship in roll. The smallest fin at 1m² area has a fin actuation of 11 degrees RMS – and maximums would be even higher than this – and it seems likely it would be subject to a lot of clipping, or even stall.

Fin area (m2)	Yacht roll motion (degrees, RMS)	Fin angle (degrees, RMS)
4	1.9	4.9
3	2.4	6.1
2	3.1	8.0
1	4.6	11

While we focused on ship motion prediction in the example, hydrodynamics are not the only consideration. Ensuring you have the right sized power capacity and actuation equipment is also vital. This is where consultation with equipment suppliers like Quantum Stabilizers, Naiad Dynamics, or CMC Marine can give insight into what’s needed for the specific vessel you’re looking at and the fin area range.

It’s summary time

We covered a few aspects of fin stabilizer performance and now it’s time to summarize. Anti-roll fins can reduce ship motion and increase comfort, but you want to avoid undersizing the system. An undersized system runs the risk of poor performance in several ways. The anti-roll forces involved increase with stabilizer fin area and angle of attack. But excessive fin angles can lead to a stall condition, when the anti-roll forces disappear. Or, at best, they may reach a clipping limit, where the anti-roll capacity saturates. In addition, relying on excessive fin angles in shorter period waves can require a lot of acceleration from the fin actuation system. There’s a risk of jerking and vibration in these conditions, which can significantly disrupt the system’s anti-roll capabilities and passenger comfort, not to mention wear and tear. A right-sized stabilizer system with an appropriate fin area will go a long way to avoiding these risks to comfort.

With these ideas in mind, you can make sure your fin stabilizer system won’t backfire, and leave you with a headaches. Like you’ve been hit by a pistol shrimp.

Next Step

Finding a starting point for fin area can be a challenge if you don’t have a reference point. Check out this video tutorial from our YouTube channel on finding a starting point for fin stabilizer area based on the particulars of the ship you are working with. Click the image to see the video:

Thanks

Thanks to Francisco Miguel Montero from Quantum Stabilizers for the discussion on performance of fin stabilizer systems and collaborating on this article.