Tag: ShipMo3D

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

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 do these two have in common? The elephant’s trunk and manatee’s snout are both highly prehensile structures. Without any bones, tiny groups of muscles work together to have a large effect on motion and forces.

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.

You rarely see stabilizers as they are usually under the waterline. These active stabilizers clearly stand out on this 70m yacht while in drydock. Picture credit: Vumedia Group courtesy of Quantum Marine Stabilizers

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 ShipMo3D, 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 ShipMo3D. 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%.

The Generic Frigate with active stabilizers. They’re so tiny I added an arrow so you wouldn’t miss them!

The impact on roll RAO of the Generic Frigate with and without active stabilizers. The ship condition is 10 knots forward speed in beam regular waves.

You can see how to set up these active stabilizes 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.


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.


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

How antiroll tanks work to passively reduce ship roll

The Hippopotamus has four legs for walking on land, yet it is closely related to whales and dolphins. This shows particularly when it comes to sleep: Hippos can do so underwater. But they don’t have gills like fish – so how do they pull this off without drowning? It turns out they have a built-in reflex that helps them out. A Hippo will automatically gently rise, breathe, and submerge again without waking up. It is all done entirely passively.

When a system works passively, you don’t have to intervene, control, or monitor things closely. It frees you up to do something else (even if it’s sleep!). When it comes to controlling ship motion, a few systems work entirely passively. One example is antiroll tanks. So what are antiroll tanks, and how do they work?

Hippos can sleep underwater but don’t need a snorkel. A reflex lets them passively float to the surface and catch a breath of air without waking up.

What are antiroll tanks?

Antiroll tanks are large chambers partially filled with water on a ship. Typically, these structures take up space across the entire beam of the vessel. Channels may connect more than one chamber to facilitate air and water flow between the chambers. These tanks need to be significant to contain enough water mass that it can influence the motion of the entire ship.

Antiroll tanks can minimize the effects of roll motion on the ship

When designed properly, antiroll tanks can significantly reduce roll motion and acceleration. Reducing these severe motions and accelerations can substantially improve the vessel’s safe operation in unfavourable weather. It makes it easy to do work on the ship in all conditions, and minimizes sea sickness and the potential for damage to equipment, too.

The nature of these tanks is truly all about dynamics.

The significant mass of water in the antiroll tank can shift from one side of the ship to the other. But it takes a certain amount of time to move. As the ship rolls, this out-of-phase motion of water shifting from one side of the ship to the other creates resistance to roll motion.

When the antiroll tanks are designed well, there is both a beneficial static and dynamic effect. The static effect is from the weight of water on one side of the ship, which creates a counter-balancing roll moment that reduces roll motion. The dynamic effect is inertial: when the ship is accelerating in roll in one direction, the mass of water in the tanks is accelerating the other way, absorbing energy and resisting greater roll accelerations.

When do you use antiroll tanks?

It is not always obvious whether you should use antiroll tanks specifically or not. They are one of several options that a ship designer can consider to help improve the performance of a ship. There are always trade-offs in any design process. After all, these tanks use up a large volume on the vessel, so there is less space for cargo or other equipment. So the ship’s performance may be better, but it comes at a cost. There are additional costs as well, as these systems can create large forces within the ship and affect the structural design.

But despite these trade-offs, the safety and performance increase may be worth it. Often, they are well worth considering as an option for narrow ships with a relatively smooth and rounded hull that otherwise little roll damping. One common type of antiroll tank is the U-tube tank.

U-tube antiroll tanks typically have two significant chambers at either side of the ship

A sizable channel connects them at the bottom of the tanks to allow water to flow from one side to the other. An additional connection at the top also provides air pressure balance between the tanks. Without this air connection, the expansion and compression of air create pressures that resist the water flow and can prevent the tanks from working as expected.

These are the most simple antiroll tank design. U-tube tanks can be cost-effective as they are the most straightforward design. However, their antiroll capability depends on the tanks’ size and spacing, which means there is little room for any adjustment once installed. These may work best on ships that keep a relatively consistent displacement through their operation – such as military, coast guard, and pleasure craft. But there are other types of antiroll tanks, too.

The RV Investigator has a U-tube tank to control roll motions in heavy seas. Picture credit: Mike Watson

A free surface tank is like one large container

Unlike a U-tube tank, a free surface tank is like one large tank. It might sound simple, but often there can be a bit of complexity. Often, there are baffles throughout the chamber to help adjust how much time it takes for water to slosh from one end of the tank to the other. This may be important to ensure the tank works as intended but also to help decrease the chance of sizeable internal slamming loads from the sloshing water. The dynamic response can also be adjusted based on the water level in the tank. This makes it more useful for ship designs that may have a broader range of displacement throughout their operational life. But this comes at a cost – more support and control systems are needed to add, remove, and measure the tank’s water level.

How can you properly size an antiroll tank?

Sizing an antiroll tank is not always obvious. Experience with existing vessels is always helpful, but not all designs are identical. Each new ship design comes with specific requirements. One possibility to size an antiroll tank is with seakeeping analysis software. Many seakeeping analysis software tools can compute ship motions in a seaway with and without antiroll tanks. This gives a way to establish the required size of the tank that will have a meaningful impact on ship motion. ShipMo3D is an example of seakeeping analysis software with antiroll tank capabilities like U-tube and slosh tanks.

Let’s look at an example

We can add a U-tube tank to the Generic Frigate model in ShipMo3D to see the effect on motion response. A U-tube tank configuration can be seen in the ShipMo3D validation report in section 10 available on the ShipMo3D website here. Once the U-tube tank is configured in ShipMo3D, it’s a relatively rapid calculation to evaluate the resulting effect on ship motion.

Representative picture of a U-tube tank set up in the Generic Frigate in ShipMo3D

When the right dimensions are set up for the U-tube tank, the resulting impact on ship motion can be dramatic. The plot shows the change in peak roll response for the Generic Frigate with and without the U-tube tank. There’s almost a 50% reduction in roll motion at the roll natural period.

The impact on roll RAO of the Generic Frigate with and without a U-tube tank. Ship condition is 10 knots forward speed in beam regular waves.

You can see how to set up this U-tube tank with the Generic Frigate in this video tutorial from our YouTube channel. The U-tube tank is based on the same parameters as in the ShipMo3D validation report.

It’s time to summarize

Roll motion in heavy seas can often hobble a ship. The result can be Interrupted work, seasickness, and even damaged equipment. Antiroll tanks are one option for ship designers to control roll motion and increase the ship’s safety and performance. The movement of water across the tanks creates resistive weight and inertia that reduces the overall roll of the ship. But they come at a cost – they take up valuable space on the ship and have their structural design requirements, too. Ship designers must carefully evaluate antiroll tanks to ensure they work as expected at sea. Fortunately, many seakeeping analysis programs, such as ShipMo3D, have these capabilities, making evaluating and designing antiroll tanks cost-effective. Well-designed systems that work passively allow you to sleep easily – just like a Hippo who can rest easily under water!

Next step

This particular U-tube tank configuration is detailed in the ShipMo3D validation report. Read more about verifications of functionality like the U-tube tank and comparisons of ship motion predictions to tank scale and full scale ship motion data here.


ShipMo3D Update

After extensive beta testing in partnership with Defense Research and Development Canada, ShipMo3D version 4 is now available. So what are some of the new capabilities in version 4?

Reduce noisy hydrodynamic results with automatic mesh refinement at the static waterline

Hydrodynamic loads can be sensitive to details resolved at the waterline of the ship model. The result can be noisy loads or motions at some frequencies that aren’t likely in reality. Automatic mesh refinement at the waterline increases the accuracy of resolving hull pressures across a range of wave conditions and cuts back on noisy results. Automatic mesh refinement applies to hulls created with hull lines or with a custom mesh directly imported to the software.

ShipMo3D now automatically refines the numerical mesh at the waterline between the dry hull (green) and wet hull (yellow) regions for increased accuracy and numerical stability

Compute the effect of active stabilizers

Active stabilizers are small but can have a significant impact on vessel motion. But stabilizer form and control coefficients need to be specified to see how they work. Use a simple interface to set exact stabilizer parameters and rapidly calculate the resulting influence on ship response.

Course and speed control in maneuvering simulation

In any maneuvering simulation, ship heading, speed, and course can drift without some active intervention from the rudder and propeller. Drift in course and speed can make it difficult to compare with other seakeeping load cases or follow a specific track. In previous versions of ShipMo3D, only heading control was possible with rudder control. Now both course and speed control are possible in maneuvering simulation.

Lid method to eliminate irregular frequencies

Irregular frequencies can unexpectedly produce wave effects inside the ship during the numerical solution process. These can create unrealistic ship motions at particular frequencies. In previous versions of ShipMo3D, manual inspection or detection to filter out specific wave frequencies was one way to deal with the problem. But this could lead to losses of hydrodynamic information. Lid methods suppress all wave effects inside the ship hull and eliminate irregular frequencies automatically while maintaining more hydrodynamic information to drive ship motion.

Rapidly review version 4 workflows with updated video tutorials

Learning new functionality can take time to figure out on your own. Speed up the process by checking out a series of new video tutorials showing specific software workflows for completing the most common tasks.

New sample project files to help explore the software

Learning any new software tool can be daunting and take a lot of time. Sample projects accelerate the learning process significantly and give you a good way to experiment with features and functionality you’re unfamiliar with. We’ve added new sample project files that include the KRISO Container Ship (KCS) and two projects from the ShipMo3D validation report: the Generic Frigate active stabilizers and test tank barge with slosh tanks.

New virtual training resources

It can be overwhelming to learn software that has many inputs and functions like ShipMo3D. For those who are newer to seakeeping and maneuvering analysis, there are also training modules to help provide context for working with any ship motion analysis software. Contact support@dsaocean.com if you’re interested in commercial virtual training options with DSA Ocean engineers.


ShipMo3D v4 is now available for download. Contact support@dsaocean.com if you need your download credentials.

Button to software download page


Click below for a detailed technical list of additions, changes, and resolved issues for ShipMo3D:


Why the sensitivity of ship roll damping makes naval architects sweat

Dolphins might act like they have x-ray vision. But it’s not their eyes that help them see through the ground. Their fine-tuned sonar capabilities have made them a valuable part of maritime security and the protection of coasts and harbours for decades. They’re used to living and working in all ranges of conditions, including murky water and acoustically noisy environments – where there are many other animals and lots of marine traffic. Trained Dolphins can easily find intruders or even mines in places and conditions where our own sonar technology completely falls flat. In fact, Dolphins can differentiate between entirely distinct types of metal, like brass, aluminum, and steel, buried almost a meter under the seabed because their natural sonar is so sensitive.

But a high level of sensitivity can be both a blessing and a curse. It can be a curse because seemingly minor changes can cause a completely different response. When it comes to ship motions, roll is extremely sensitive. Small changes in the ship design can result in significant changes in motion characteristics. In this article, we’re going to talk about why naval architects sweat the details to really understand just what will happen to a ship in roll. So why are ship roll motion so sensitive?

Dolphins have sonar so sensitive it can differentiate different metals buried in the soil.

Not all ships have sensitive roll

Barges and multi-hull ships like catamarans and trimarans don’t tend to have sensitive roll. The kind of ships we are talking about are smooth monohull systems. The key is that there is a single primary hull that is relatively slender in terms of length to beam ratio. Often, monohull ships are quite smooth with a round form, which is one crucial reason roll motion is so sensitive. The rounded shape means there is very little damping resistance when it rolls.

What do you mean by resistance?

By resistance, I mean either ocean wave radiation or viscous damping. Barges are large and flat – almost like floating paddles – and their particular shape is ideal for generating waves when rolling. These radiated ocean waves absorb massive energy, making an enormous amount of resistance that slows down barge motion in roll. A multi-hull vessel like a catamaran also does an excellent job of radiating waves when rolling. This is because of the way the flotation hulls are offset from the centerline. But a rounded monohull makes almost no waves when it rolls and, as a result, relies heavily on viscous drag for resistance.

But viscous drag damping doesn’t work very well in oscillation

Even if the ship roll velocities are high, because of the back-and-forth motion, drag loads tend to be weak. Because of the low resistance, even relatively mild ocean wave conditions can lead to significant roll motions and accelerations. This can limit ship operations or, worse, cause injuries. Equipment, cargo, or the ship itself can be damaged, too.

The result is that naval architects spend a lot of time and energy thinking about ship roll

A lot of time and energy goes into the design details of the shape of the hull and appendages that help amplify viscous drag resistance. Features like bilge keels are often added specifically to create viscous resistance to help reduce roll motion. Other technologies can help, including passive and active systems like antiroll tanks and stabilizers, but simple appendages are the most straightforward place to start. But with hull appendages, it’s vital to understand the viscous drag effects reasonably well because of how they can substantially impact roll motion performance. So how is viscous roll damping evaluated?

Slender monohull ships like the Uchuck III can have very round and smooth hulls which can make controlling roll motion a design priority


Hull appendages like rudders and skegs can help with roll damping, but they can only do so much

One way to evaluate roll motion is from physical scale model testing

But this approach is by far the most costly and time-consuming. A scale model of a ship in a wave basin will verify the ship’s expected roll response based on the design details used. But because of the costs involved, it’s not an economically viable approach to iterate on the design of the hull and appendages – it serves more as a confirmation of the expected roll performance. Yet there are alternative numerical methods that work well in evaluating roll.

Roll damping can be computed with Computational Fluid Dynamics tools

These software tools resolve the physics and details of the water flow interacting with the hull and appendages. While they provide a great deal of useful information and are helpful for a degree of design iteration, it can be a computationally costly and time-consuming approach. A tight project timeline won’t leave room for investigating many different loading conditions or design iterations on hull and appendage configurations. But there is more than one numerical approach to resolve ship motion.

Some numerical models use empirical relationships to resolve the viscous forces

Empirical equations describing viscous roll resistance are helpful but have more significant uncertainty than the other approaches. Yet ship designers can use them across various conditions and appendage configurations. They are also very computationally efficient and can help screen out different designs quickly. Dynamic analysis tools like ShipMo3D are designed to set up typical hull design, including appendages and resolve ship motion in a broad range of sea and ship conditions much faster than CFD tools.


The RV Investigator hull is also very smooth, but has multiple tricks to help with stability in roll like bilge keels and an antiroll U-tube tank. Picture credit: Mike Watson

It’s example time

So how much difference can something like bilge keels make to damp roll motion on a smooth bottomed monohull vessel? Here is a comparison of ship roll response of the Generic Frigate computed by ShipMo3D with and without bilge keels.

The ShipMo3D Generic Frigate model with bilge keels on the left and without on the right

A roll decay comparison starting the vessels at 10 degrees shows a significant difference. While bilge keels don’t eliminate roll oscillations entirely, they are substantially better than nothing at reducing roll motion. But this is only the roll decay response. What happens to the ship motion response across a range of wave frequencies? The RAO can give some insight.

In a roll decay test computed by ShipMo3D, bilge keels are significantly better than nothing at reducing roll motions

The RAO gives a broad spectrum idea of how the ship will respond, including at resonance. Often, for slender monohull ships, one of the worst-case loading conditions is in a beam sea. The roll RAO of the Generic Frigate with and without bilge keels in a beam sea condition is illustrated below. The RAO shows that maximum roll motion is reduced by a factor of two! There is also much lower roll motion in the region around the peak as well. This implies that the bilge keels help significantly control roll motion, especially around the natural frequency in roll.

The bilge keels have a substantial effect at reducing the roll RAO peak by a factor of two. This implies significantly lower accelerations and motions especially around the natural roll frequency.

It’s time to summarize

We covered a few details on ship roll response, and now it’s time to review. Roll response is something to get right because the stakes are high – large roll motions can make the ship inoperable, cause injuries, or even damage. Not all ships have sensitive roll response, and usually, it’s an essential issue for monohull vessels. The slender form of the vessel with a rounded bottom means there is little resistance in roll.

These vessels don’t radiate ocean waves very well compared to other vessel forms like barges or multi-hulls like catamarans and trimarans. So monohulls rely almost totally on viscous damping to control roll motions. Viscous forces are weak in the back-and-forth oscillatory motion in roll. So ship designers and naval architects spend a lot of time characterizing the viscous roll effects however they can. Like dolphins measuring different materials through seabed soil and mud layers, ship designers will use whatever tools they can to measure ship roll response.

Next step

Bilge keels are one way to help control roll motion, but there are other systems that help. Anti-roll tanks are another example that can make a big difference on roll activity. Read more on what anti-roll tanks are, how they work, and how to evaluate them in the next article here.

Why drag loads are underwhelming at damping floating system motion

Snow is rare in my Canadian city. This might surprise you, given Canada’s reputation as a northern and wintry country, but I am lucky to live near the coast. The marine climate helps keep the weather surprisingly mild. Despite this, I do own a snow shovel. It’s a big one, too. It has a wide scoop, all the better to clear a large swath along the path on our driveway. I expected it to do well and help quickly clean up any snow that came along.

But that expectation threw me off on our last rare snowfall. The problem with snow in a marine climate is that it often comes down super wet and thick. Because wet snow is so dense, a massive scoopful in my large shovel was too heavy to lift. Piling up a giant heavy glob of dripping snow almost broke my shovel and my back at the same time! I had big expectations for my large shovel, but in wet, heavy snow, it was underwhelming.

An underwhelming response can throw anyone off. It can cause nasty surprises, especially if there are big expectations. In the world of hydrodynamics, there are often big expectations with drag loads. Any analyst should be wary of drag forces – after all, they are very sensitive to current speed and can become a dominating effect. But there are some circumstances when they fall flat and have little impact on ships and buoys. But first, it helps to understand why drag loads can be so powerful in the first place.

In wet, heavy snow, larger shovels are underwhelming

Why can drag loads be overwhelming?

Viscous drag forces appear when there is relative flow past a structure. Whether the structure is moving, the flow is moving, or a combination of both, you’ll see drag loads. The key is that there needs to be some relative motion involved. Generally, the greater this relative motion, the greater the magnitude of forces involved.

One portion of drag on a hull is a skin friction effect, and while important in some ways, it tends to be a much smaller effect in general. The other critical portion of drag is a pressure differential caused by eddies and wake structures that form in the lee of the relative flow behind the hull. Eddies and wakes are regions of low pressure in the flow field, and the resulting imbalance in pressure on the hull causes the drag force. But when are these effects overwhelming?

Drag forces from current can be overwhelming

The drag on the side of a ship hull or buoy in a current can be so significant as to dominate all other effects, overpowering a ship or destroying a mooring system. When a hull and flow have a large relative velocity, the pressure differential from these wake structures grows larger.

These forces increase rapidly with the square of relative flow speed. But it doesn’t mean they are always significant just because they are so sensitive. Viscous forces can also be weak in certain circumstances, and in particular, this can happen in oscillation.

Drag forces are underwhelming at damping oscillation

Oscillation, or any cyclical back-and-forth motion, is common for any hull in the marine environment. For example, a ship or a buoy will bob up and down vertically in heave. You see this in the tilt degrees of freedom of pitch and roll, too. So why does oscillation mean underwhelming drag forces?

Buoys or ship hulls, like the HMCS Halifax, have lots of drag. But drag from steady effects like currents or forward speed is different from viscous drag loads in oscillation.

A fully developed wake structure doesn’t appear instantly: it takes time for the low-pressure region to form behind the hull. Only once it is fully formed is the pressure differential greatest. This means there will be a time lag for the drag forces to come up to full strength.

But when a hull oscillates back and forth, it’s also constantly changing the flow structure back and forth. There’s almost no chance for a wake to form on either side of the system as the flow regularly reverses one way and the other. In the short term, fluid flows over the contours of the hull, causing some skin friction drag but without forming these larger low-pressure regions, preventing significant drag forces from forming.

The key is that in any kind of back-and-forth motion, the relative flow speed hovers around zero, as you would expect when you see a buoy or ship roll back and forth. The flow speeds can be high at a moment in time if the motion is violent, but the resulting drag loads are underwhelming because the flow structures haven’t had time to form and create large pressure differentials.

The Generic Frigate hull head on. The yellow portion is the wetted hull in this displacement. In a lateral ocean current, a steady wake forms in the lee of the hull and drag would be enormous. But drag during side-to-side sway oscillations, it is less likely these pressure regions can form and the effects of drag can be underwhelming.

But aren’t there other more significant hull damping forces?

Drag and viscous effects are not the only sources of damping in motion oscillations. Wave radiation can be substantial in certain conditions. Ship motion in pitch is usually controlled with damping by a totally different force: wave radiation. Smaller structures like buoys can some wave radiation effects, but it’s not as significant. Regardless, buoys often use a mooring that helps to control motion to some degree.

Sometimes drag is the only source of motion damping

This is a particular issue in specific buoy systems in pitch and roll and spar hulls in heave. It’s also a common problem in ship roll. These are only a few particular cases. Generally, it’s best if you are careful to understand both how your real system will work and how that reflects in your analysis of the floating hull dynamics.

Let’s look at an example

Here are two motion decay tests of the Generic Frigate computed by ShipMo3D. These are time-domain simulations in calm conditions with an initial deflection. The first is a decay test with the Generic Frigate starting at 10 degrees pitch. In this case, wave radiation dominates the damped system response, and after about 20 seconds, the motion is entirely damped.

Generic Frigate pitch decay test. Wave radiation effects are substantial and create a strong damping effect that stops pitch motion after about 20 seconds.

The second scenario is a decay test with the Generic Frigate starting at 10 degrees roll. The big difference from pitch motion is that there’s no significant wave radiation in roll because of the slender hull, and this means the damping effects are essentially completely from viscous drag loads. The decay response is far different than in pitch: after 100 seconds, it’s still going!

Generic Frigate roll decay test in calm seas. In roll, motion damping is almost completely from viscous drag effects. The system is still oscillating after 100 seconds and will likely keep going for a while!

In summary

Drag loads can be devastating when there are high relative flow speeds. Low-pressure wake structures cause a large differential pressure on the hull that we think of as the drag force. But when a floating hull oscillates, these wake structures don’t often have time to develop fully. Even if the oscillation velocity can be high, it’s only for a moment in time before the system reverses backward. It can mean drag loads become vanishingly small and ineffective at damping oscillatory motion. The result can be underwhelming – much like my disappointing giant snow shovel. By the way, my son’s toy snow shovel ended up working much better at clearing my driveway of heavy snow. Because it had a small scoop, it was far easier to quickly scoop and heave loads of snow out of the way, and it took far less time to get the job done!

Next step

In this article, we saw how viscous drag forces fizzle in oscillation motion – particularly in rotation.  In the next article, we focus on roll motion and see why it’s such an important topic in ship design. Read more on why the sensitivity of roll damping makes naval architects sweat here.