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Why keep the same design DNA when exploring ship hull design variations

If it looks like a snake, slithers like a snake, is it a snake? Sometimes, not at all! There are animals called legless lizards, which might take offence at being confused with snakes. There are some ways a trained eye can tell the differences between a legless lizard and a snake, but most people won’t be able to understand what’s right in front of them. While both snakes and legless lizards might seem like a minor variation on a theme, they are very different creatures. So how can this be?

At first glance, these species might seem almost identical: long, slender bodies, no arms or legs, and slither to get around. They do have some commonalities in that they’re both reptiles. There are tiny details you can sometimes find to tell them apart – like how snakes can’t blink, but legless lizards can. However, the differences become much more stark when you look more closely. This is because they’re an example of convergent evolution.

They split from a common ancestor millions of years ago. Over time, they lost their arms and legs and developed similar features and body forms independently from each other. So they look the same, but they’re fundamentally separate. More specifically, among all snake species, there is around 95+% common DNA. But there’s nowhere near this level of common DNA when you compare legless lizards to snakes – the differences show they are a completely different animal.

In the world of design, we continually introduce changes and explore comparisons. It’s through this process of evolution that we move toward performance we want. But you can get confused if you make changes so big that you have an entirely different animal on your hands. In the more specific case of ship design, it’s common to explore how changes in the hull form might affect motion performance in waves. Ideally, you make a small change to the hull shape – a variation on a theme – and that gives you an indication of how performance might improve or deteriorate. Though two ship hulls might look similar, they could have entirely different motion performance at sea. Without controlling and checking several fundamental parameters – the design DNA – you may not truly understand which is a more beneficial hull form. We’re going to cover these fundamental ship design parameters and how they are crucial to keeping consistent across hull design variations:

1. Length
   
2. Metacentric height
   
3. Displacement

These three factors govern the fundamentals of how things move at sea. Ultimately, these parameters are linked to more general concepts in dynamics: geometry, stiffness, and inertia. By keeping these factors consistent, you can gauge whether the hull form changes you are making are truly helping either dampen motions more or avoiding stronger excitation forces, potentially improving ship motion performance in certain conditions. Among these factors, we’ll start with keeping the ship length consistent.

Ships with different lengths can have drastically different characteristics

There’s a strong link between ship length and other characteristics. For example, it’s common to see displacement grow substantially with even small length changes. The beam might be much larger as well. However, this perspective considers ships from the standpoint of an existing fleet. This kind of thinking is about making comparisons between completed designs. Right now, we need a perspective that considers ships from the standpoint of making comparisons between hulls for the same concept vessel. So why does length matter so much in this context?

The hydrodynamics and interaction with waves are strongly influenced by length

The length is the foundational measure of a ship. The smaller a craft is, the more likely ocean wave forces will push it up and down to follow the slope of the wave surface. So conversely, the longer the hull is, the more likely it can spread that excitation pressure over a wider area. This gives longer ships an inherent advantage, all else being equal. This alone can be a way to improve motion-affected comfort at sea, as highlighted by the history of the long-ship and axe-bow design developed at TU Delft and championed by Damen Shipyards. In particular, the overall length can have a significant impact on how the ship interacts with waves, especially in head sea or bow quartering conditions. In any case, to make sure you have a fair comparison, it’s vital to maintain a consistent length between concepts. While length is a measure of the fundamental geometry of the hull, there’s another parameter that fundamentally governs motion at sea, which is its restoring stiffness. This brings us to the second point: maintaining a consistent metacentric height.

A consistent metacentric height indicates consistent restoring stiffness

The metacentric height is a function of both the hull shape and also the location of the center of gravity of the ship. These factors together indicate the amount of rotational force the hull pushes back to keep upright in the water. The metacentric height gives a direct measure of how stiff this rotational force is. For example, a high metacentric height ship will be very snappy and try to stick upright even in a range of wave conditions. However, a lower metacentric height ship may have a more gentle response. However, the metacentric height doesn’t provide a complete picture of static stability because restoring forces can change as the hull rolls through larger angles. Yet it does indicate the onset or initial static stability. So how does hull form and center of gravity change the metacentric height?

Generally speaking, for the same hull form, the lower the center of gravity is, the higher the metacentric height is. This also means a higher center of gravity reduces metacentric height. On the other hand, for a given vertical center of gravity, a narrower hull form tends to have a lower metacentric height, and wider hull forms have a higher metacentric height. Gaining an intuitive understanding of the metacentric height is not possible just by looking at a hull because you don’t know the center of gravity. Nevertheless, metacentric height is usually easily calculated by ship design software throughout the design process. Because it strongly affects roll motion, this influences ship motion performance in beam sea conditions, which can be the most sensitive of all in terms of affecting people on board. Now, so far, we’ve covered why hull length and metacentric height are crucial parameters in making fair comparisons between hull variations. However, there is another key element that can significantly impact dynamic motion: mass. This brings us to the third and final point in making fair comparisons between hull variations: maintaining a consistent displacement.

Displacement is the ultimate measure of the mass of a ship

The amount of mass in the vessel is a primary indicator of how it will move in the ocean. After all, the forces acting on the hull are going to result in accelerations – and it’s these accelerations that evolve into motion through time. So, to make a fair comparison between two different hull designs for the same concept, we need to make sure we have similar displacements. The good news is that it’s relatively easy to evaluate the displacement of a hull shape.

The displacement is a function of the static pressure over the hull

For a given draft and specific hull shape, adding up the force from the static water pressure over the hull produces the total buoyancy. In static conditions, this is equal and opposite to the total weight of the vessel. So, with a consistent draft, as the hull shape changes, it’s a good idea to keep an eye on displacement to make sure it remains consistent between variations. The hazard is that a heavier ship may be overall less sensitive to motions and mislead you into thinking it’s only because of a superior hull shape. Displacement, and therefore mass, of the ship governs resistance to the primary linear motions – heave, surge, and sway. But rotational motions – roll, pitch, and yaw – are another story.

These rotational motions encounter a different type of resistance to motion, known as rotational inertia, which is affected by both the shape of the ship and its mass. For monohulls, typically, the roll inertia is a function of the beam size in combination with displacement. This reinforces why maintaining consistent displacement is crucial due to the direct link to roll inertia. On the other hand, it also highlights that the roll inertia could change as the hull form shifts. Unfortunately, it’s generally not practical to maintain a consistent roll inertia due to the more nuanced link to the ship’s geometry. Yet, keeping other parameters of length, metacentric height, and displacement consistent, you could exploit the fact that a wider ship with more roll inertia might be less sensitive to wave motion for more comfort.

But what about keeping the ship’s natural periods of motion in heave, pitch, and roll consistent?

The natural periods of ship motion in heave, pitch, and roll are helpful to know because these are major indicators of expected motion in different sea states. The problem is that there is a more nuanced relationship between fundamental ship design parameters that you can experiment with in early-stage design. Roll natural period might be the most significant in assessing overall motion performance. However, this is a function of multiple ship parameters you can control in the design process, making it harder to control between design variations. For example, for a monohull, the natural roll period is a combination of displacement, beam size, hull form, and VCG. That’s a lot of variables to adjust to get a consistent roll period. Meanwhile, the parameters of length, metacentric height, and displacement are more directly linked to the hull shape that you have more direct control over. That’s not to say you shouldn’t pay attention to the natural roll periods, or even use them as objectives to explore new designs. But that is a different scope of evaluating concept designs – here we are merely trying to tease out what variations in hull shape might be beneficial to motion-affected comfort at sea.

Let’s look at an example

In this example, we compare two hull variations to check the ship motion performance. The two vessels are both variations from the 50m Generic Yacht design you can find in the ProteusDS ShipMo3D toolset sample projects. Each variants have the same design DNA as the 50m Generic Yacht – namely, 50m length, 550 tonne displacement, and 0.9m metacentric height. We also kept the draft consistent at 2.4m, maximum beam of 9m, and assumed the rotational inertia was the same for each variation. The hulls were generated using the Orca3D Hull Assistant, with one form factor using a conventional bow and the other using a vertical bow.

The hydrodynamics and motion RAOs were computed ProteusDS. We then compared the ship motion performance by evaluating the ISO motion-affected comfort rating for each variation using a reference 1.5m significant wave height. The results from the motion-affected comfort rating show for these particular configurations, the conventional bow has about 7% improvement in motion affected comfort rating when compared to the vertical bow.

This doesn’t mean a vertical bow will always be less comfortable at sea. The details of each design matter. What the example does highlight is that following a systematic process, and keeping the fundamental ship parameters consistent across hull variations, means you have the best chance of isolating how the hull form factor is contributing to changes in ship motion performance.

Summary

We covered several elements of comparing ship motion performance of hull designs, and now it’s time to review. In early-stage design, it can be helpful to explore how different hull shapes could improve ship motion performance. To verify that it’s mainly the hull shape that helps improve motion performance, you have to keep certain factors, the design DNA, consistent across these hull variations. These factors are first overall length – the ultimate geometric factor that drives ship interaction with waves. Changes in length alone can make substantial differences in motion performance because of how the hull can interact with waves – especially in a head sea or even bow quartering conditions. The second factor is metacentric height. This correlates with a roll stiffness effect strongly influences performance in beam seas. The final factor is displacement, which is a primary measure of the ship’s overall mass. The amount of displacement directly influences heave, surge, and sway motions. It also influences rotational motion – roll, pitch, and yaw – performance as well – but this is in combination with the shape of the hull.

Keeping ship design DNA – length, metacentric height, and displacement – consistent is one way to help ensure you have similar dynamic characteristics between design variations, and you are more likely to identify beneficial hull shapes. Much like legless lizards and snakes – who both figured out separately over millions of years what was the best beneficial shape for them.

Next Step

There are a few steps involved in generating hull variants using Orca3D and Rhino, and then comparing motion-affected comfort ratings with ProteusDS. Click the image to follow along and use the workflow for your own designs using this video tutorial on the DSA Ocean YouTube channel: