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How to avoid too much or too little static stability in ship design
Some flowers have pollen stored in a closed chamber. This might sound counter-intuitive: how can they expect to grow new plants if their pollen is locked away? The thing is that it is not a perfect seal: there are tiny pores at the ends of the chamber. But they are so small the pollen won’t come out by itself or shake off from a passing breeze. However, some particular pollinating insects have a special trick to get the pollen out easily. It’s all done using sound waves. The process is known as Buzz Pollination. But how does it work exactly?
These bugs come right up to the plant and then change the rate at which they beat their wings. A specific sound frequency from their wings shakes the pollen right out of the pores. However, there’s a nuance to it. Only a particular sound note will unlock the pollen. It can’t be too low or too high; it needs to be balanced just right.
Likewise, in the world of design, there are many aspects that need a special balance. There’s a particular case for this when it comes to ship design. You might think static stability is good, and you need as much of it as possible so the vessel won’t sink. But it’s not that simple. Ship motion-affected comfort in waves is negatively affected by static stability. In this article, we’re going to cover how to avoid too much or too little static stability – in other words, find what’s just right – in ship design.
What is static ship stability?
Static stability has everything to do with how much force a particular ship will push to keep the hull upright in water. Almost every ship design, at some point, goes through a static stability analysis. In this case, dynamics are ignored, and the particulars of the ship design – hull shape, weight distribution, and so on – are used to directly calculate the amount the hull resists rolling. The stronger it resists this rolling action, the higher the static stability is.
Why is static stability important?
When in ocean waves, the ship will be moving around in different waves – linear motions of heave, surge, sway, and rotational motions, roll, pitch, and yaw. A real hazard is when the ship rolls so much that it capsizes and flips entirely over. In this case, human life is at extreme risk, and the vessel could also be lost. So static stability is crucial to avoid this scenario and is a gauge of how much the ship may be at risk of capsizing. Too little static stability could mean that, in severe seas, the ship roll might get so extreme that it can capsize. So, more static stability means more safety as you reduce the chances of the ship flipping entirely over. Does this mean you want to have as much static stability as possible? No way!
The problem with static stability is that it doesn’t account for the effect on people on board
A ship with too much static stability can cause big problems. In certain conditions, waves will move the hull no matter what. While a lot of static stability means the things won’t roll over very far, so you are safe from capsizing, there can be significant downsides. The downsides come in the form of harsh accelerations. So, what does that mean for people on board?
For people on board the ship, harsh accelerations have several downsides. Right off the bat, too much motion can mean a risk of falling and injury. But even if there aren’t any falls, it can cause regular interruptions – you can’t carry out work, and it can be jarring enough to interrupt your train of thought. But there are other effects, too, including the dreaded motion sickness. While everyone has a different threshold and susceptibility to motion sickness, there are ways of calculating it as a function of ship motion. Regardless, harsh accelerations directly affect motion-affected comfort on board vessels. This directly links static stability to comfort.
Too much static stability can be a bad thing for motion-affected comfort
To reduce these accelerations and negative effects on people, naval architects must reduce the strength the hull pushes to remain upright. However, ship designers must be careful not to diminish static stability so much that it compromises safety by risking capsizing. However, low acceleration – and a comfortable ride at sea – but with colossal roll motion results from swinging too far the other way in design, with too little static stability.
Good ship design doesn’t have too little or too much static stability – it needs to be just right. Computing static stability is a standard process in the early stages of ship design. But there isn’t any direct way to show when you have the right amount of static stability because this requires feedback on the dynamics of the vessel in waves. Calculating dynamic performance is a necessary step to understanding the potential impact on passengers and crew.
Seakeeping analysis provides feedback to counterbalancing static stability
While a seakeeping analysis is about resolving ship motion performance in a particular sea state, this might leave designers with questions on what sea states and operational limits to look for. The impact of people on board might vary depending on how the ship is used: to get a job done, to safely transport people or goods, or for the experience itself as a passenger on a yacht. So, an operability analysis that factors in seakeeping analysis across a range of conditions with limits on accelerations depending on the ship’s use is even better design feedback.
In the specific case of large yachts, a framework for computing motion-affected comfort is detailed in publicly available standards. This uses an operability approach that incorporates seakeeping analysis across a range of conditions.
Motion-affected comfort can be evaluated through seakeeping analysis
Modern commercial software tools like ProteusDS can calculate ship motion from a variety of basic parameters of a ship – the hull form, vertical CG, displacement, and so forth. ProteusDS has the functionality to evaluate a motion-affected comfort rating for large yachts, so it can be straightforward to incorporate in early-stage design. This provides ship designers with feedback to strike a balance in a standard static stability analysis.
Let’s look at an example
We’ll use the 50m Generic Yacht with no stabilizers. The nominal vertical CG location is 3.0 m from baseline. For reference, in this configuration, the metacentric height is 0.9 m, and natural roll period is 7.7 s. Generally speaking, if the vertical CG location is lowered, we expect a stiffer, less comfortable ship but with greater static stability. And if the vertical CG is raised, we expect a more comfortable ship, but at the cost of reduced static stability. Now, let’s look at what the motion-affected comfort rating tells us for more quantifiable feedback.
We used ProteusDS to compute the motion-affected comfort rating for the 50m Generic Yacht and a variation with vertical CG 10% lower, or 2.7 m from baseline. This variation had a metacentric height of 1.2 m, and natural roll period of 6.7 s. The comfort ratings were calculated using a reference 1.5 m significant wave height.
The comfort rating provides a basis for quantified feedback on the effect on comfort. The reduction in vertical CG does indeed produce a stiffer, less comfortable ride – and the comfort rating has dropped by a star rating, corresponding to 43% reduced operability.
| Vessel | VCG (m) | GM (m) | Roll period (s) | ISO 22834 Comfort Rating (%) |
| 50m Generic Yacht | 3.0 | 0.9 | 7.7 | ★★★ (51%) |
| 50m Generic Yacht Variant | 2.7 | 1.2 | 6.7 | ★★ (29%) |
Looking closer at the specific comfort details behind the comfort rating, the stiffer vessel has many more challenges meeting the tipping threshold, or Effective Gravity Angle (EGA) limit of 2 degrees, compared to the nominal Generic Yacht design.
The ProteusDS results of the ISO comfort rating help illustrate which areas of the ship are struggling to pass the comfort criteria. This gives quantifiable feedback on just how much comfort might be affected with an adjusted target vertical CG, whether up or down, in tandem with a static stability study. This then could be used to guide design decisions or reallocation of space in the early design stage to ensure the design has a good balance of safety and comfort.
Can anti-roll technologies help improve comfort without reducing static stability?
A wide range of commercial technologies like gyros, fin stabilizers, and anti-roll tanks can help improve motion-affected comfort. These technologies create forces that can change the resulting ship motion and acceleration in certain conditions. While they can often help in many circumstances, each technology has advantages and disadvantages.
Naval architects need to carefully understand the implications of how each technology works and how it interacts with the overall dynamic behaviour of the vessel. In any case, a naval architect who factors in a motion-affected comfort analysis in the early stages of design has a better chance of optimizing ship performance and meeting client requirements.
Summary
We covered a few facets of static stability and how it relates to motion-affected comfort, and now it’s time to review. Ship stability is all about safety. It’s about ensuring a ship stays upright in the water in various conditions and is vital to protecting human life. Generally, the more static stability, the greater the force exerted to maintain an upright position. But too much of this force can create harsh accelerations and motion when in ocean waves, making more than just headaches for people on board. It can cause constant interruptions, people can fall over and get hurt, and it can create big problems with motion sickness. Good ship design is a balance of not too little and not too much static stability – and the feedback mechanism for this ship motion prediction calculation that incorporates a motion-affected comfort analysis. Commercial software, like ProteusDS, is available, which makes it easy to incorporate at the early design stage.
Much like buzz pollination with a sound frequency that is not too low and not too high – it’s just right – a successful ship design often needs static stability that is not too low and not too high – something just right.
Next step
We used the 50m Generic Yacht in this example. It’s one of several publicly available sample projects anyone can explore in our ship motion and operability analysis tools. Take a closer look at what’s involved in evaluating motion-affected comfort with variation in vertical CG from this video tutorial on our YouTube channel here. Click the image to watch the video:
