Month: January 2021

Why drag force is more than just resistance

It’s tough to beat Nature. You don’t have to look in a top secret lab to find one of the most miraculously slippery fluids on the planet. You can just look at your knee! Lining many of your joints is an egg-white coloured substance called Synovial Fluid. Among its many purposes is to keep your joints moving. But not just for a few days – for your entire life. And through all this time, it manages to control and minimize joint resistance.

Certainly, when resistance gets out of control, everything can come to a grinding halt. When it comes to hydrodynamics, when you think of resistance, drag forces may be the first thing that comes to mind. But there’s more to drag than just resistance, and this is what we’re going to cover in this article.

What is the drag force?

Forces appear when there are changes in momentum. Momentum means mass in motion, so for the specific case of fluids, this could mean water or air flowing in a current. The drag force arises when there is a change in momentum in fluids. More specifically, the drag force transfers momentum between a structure and a fluid it is immersed in. This might sound like an abstract idea, but you already have some experience with it every day.

You feel this effect when your hand is in water

A big part of this is the drag force. But if you are in a pool or bathtub, the water isn’t moving around – it’s your hand – and so the drag force in this case will feel like resistance to you. In this case, it’s your hand that has momentum: it is the mass in motion. The drag force is then transferring momentum from your hand into the water.

You can feel drag when moving your hand around in water

What does momentum look like in water?

Well, if it’s your bathtub, there will be swirling and churning water – turbulence, and probably some waves. All this moving water is mass in motion, too. But the drag force does work in reverse, too – it can transfer momentum from water into structures.

What happens when there are ocean waves or currents in the water?

Ocean waves and water currents are also examples of mass in motion, too. If a structure, like a mooring buoy or ship gets in the way of moving water, there’s going to be some drag forces. These forces will transfer momentum from the waves and currents into the structure. So in this way, drag force is really about the exchange and transfer of momentum.

Sometimes this means it creates resistance and slows down a structure. But it can create motion in floating structures, too. The key is that drag is proportional to the relative velocity between a structure and the fluid.

The drag force is one of the essential forces in hydrodynamics

It acts like a resisting effect on many structures. If you are trying to understand what will happen to an ocean robot driving around in the water, it will have a big impact on energy consumption as well as how it can maneuver.

But the drag force is also a key element in mooring designs. In oceanographic mooring designs, the aggregate drag on all the components and the mooring line causes the system to deflect in an ocean current. In reality, the water current loses some momentum from this drag force from the mooring – the wake from the float and mooring components reduces the ocean current flow speed by some small amount.

In an ocean current with speed U, the drag force from all mooring components causes deflection.

Drag is also a key element in the excitation forces when ocean waves are around. You need to know these excitation forces to properly design a system to perform the way you want in the ocean environment. But knowing drag forces is one thing. Knowing how big they are is really the million-dollar question.

How do we know what the actual drag force is?

The drag force is measured from an experiment. These experiments might be a physical test in a lab, or in more modern times, they might be virtual tests using fluid dynamics software. These experiments resolve what the drag forces are in certain specific conditions. There have been hundreds of thousands of tests completed in laboratories for many decades measuring the drag force on a vast array of shapes in different flow speeds and fluid mediums. But how do we take all this information on drag force and then use it in a specific application?

The key is the drag coefficient

Similar shapes produce similar and predictable drag forces. The concept of a drag coefficient works well and covers a wide range of fluid types and conditions. Ultimately, if you’re looking at something like a sphere, you can use a drag coefficient associated with a sphere and predict reasonably well what the drag forces will be for a good range of wind or water current speeds. These drag coefficients are often available in look up tables to help the design process.

But what about the drag on mooring lines?

A mooring line is indeed a much more complicated structure than a sphere. Depending on its orientation to the flow, the local drag can change drastically. The drag forces on mooring lines require a calculation that considers the local flow speed and tilt of the line of the whole system. Suffice to say, it’s not a back-of-the-envelope type calculation you can do like that of the drag on a sphere! But this is what we have programs like ProteusDS Oceanographic that help address these complexities.

It’s summary time

The drag force is an effect that arises when momentum transfers between a fluid and a structure. Yes, a structure will feel resistance when it’s moving in a fluid, but the reverse is true, too: a moving fluid, like ocean currents and waves, can also cause a structure to move around, too – and the drag force plays a big role in that. The drag force is a very important effect that affects a range of systems from vehicle dynamics to oceanographic mooring design.

Even super low friction fluids like those in your knee joints have some drag effects, too. While you don’t need to do much over your life to keep your knee joints going, you do need to be mindful of drag when designing structures in the ocean.

Next step

Figuring out a drag coefficient for a particular structure isn’t always obvious. While ProtesuDS Oceanographic includes drag coefficients for a variety of shapes, you can learn more about different ways to resolve drag coefficients here.


How to understand the nature of buoyancy

Sand is the most extracted resource on the planet. It makes sense, too, when you realize that it is a crucial ingredient in concrete, glass, and, of course, electronics – it shows up almost everywhere. But in recent years, the extraction of sand has increased dramatically. Growth in urban cities, and of course, smartphone use, has been a massive driver for this.

It’s a good thing we have huge deserts like the Sahara ready to go to help out with extra sand, right? Wrong! Unfortunately, the sand found in deserts is far too smooth to be adequately processed. The right kind of rough sand needed is most often found in certain rivers. In total, the correct type of sand is found only in a tiny fraction of the area of the planet. In terms of scale, the sand we need is a resource that is quite misunderstood.

These kinds of misunderstandings can lead to severe problems down the road. Sometimes, you can pinpoint them from a critical assumption. In hydrodynamics, the buoyancy force shows up in applications from marine vehicles to mooring design: it shows up almost everywhere. But it is a concept that is often misunderstood, too. In this article, we’re going to talk about the fundamental nature of buoyancy forces.

To make concrete, glass, electronic circuits, you need sand – but not the kind you can find in the desert

Buoyancy is one of the most essential forces

It is a key effect when designing almost any kind of equipment in a marine environment. The nature and behaviour of buoyancy is critical for understanding how marine vehicles and ships will move and float. For mooring systems, the buoyancy from floats keeps the mooring upright up in the water column and resists the effects of other forces like drag from ocean currents and wind.

You might know that buoyancy is the weight of displaced water

While this isn’t wrong, there’s more to it than that. Fixating on the volume and weight of the displaced water can sometimes cause confusion about what buoyancy is. But before we talk about buoyancy specifically and where it comes from, we first need to talk more about pressure in the ocean. Specifically, hydrostatic pressure. It’s this hydrostatic pressure that is the root of the effect of the buoyancy force.

Hydrostatic pressure is everywhere in the ocean

The ocean is a fluid: it doesn’t have any structure to support its own weight. This means that anything that goes into the ocean will feel the effect of the hydrostatic pressure field. You feel this when you jump into a lake or the ocean and feel the pressure on your ears once you’re in the water. There’s no escaping this pressure field – no matter where you go under the water – if you’re at the same depth, you’ll feel the same pressure.

But knowing there’s a pressure field is only the start of the story

The next key is understanding that the hydrostatic pressure field grows with depth. Why does it grow? Ultimately, you feel the pressure caused by the weight of all the water above you. The weight of all the water above you – and the corresponding hydrostatic pressure field – grows linearly with increasing depth. Again, this will be something you feel in your ears the deeper you swim down into the water – you can feel the squeeze increase as you go further down.

You will feel pressure in your ears the deeper you swim down into the water

This increase in hydrostatic pressure with depth causes buoyancy

When we are considering dynamics, we want to resolve forces acting on structures and this includes floats on moorings and vehicle hulls. Pressure is almost the same as a force, but not entirely: pressure is a force distributed over an area. Now that we know there is a linearly increasing hydrostatic pressure field, what happens when we put something in the water?

Consider a square float in the water. This is easier to draw pressure lines on than a circular float!

To understand this, you have to add up the cumulative effect of that pressure field around the entire shape in the water. The pressure field’s horizontal effects will cancel each other out. There’s no net horizontal motion from this force, but the hull will still feel a squeezing effect.

Now it’s the vertical effect where things get interesting. Since the hydrostatic pressure field grows with depth, the pressure on the top will be less than the pressure field pushing up on the bottom of the float. This difference is where the buoyancy force arises. It’s the difference between the vertical hydrostatic pressure pushing on these surfaces. This means there will be a net resulting force, but only vertically up toward the water surface: this is buoyancy.

A) Hydrodstatic pressure on all faces of the square float – but pressure increases with depth B) Horizontal pressure cancels out, and there is a net vertical pressure on the float: buoyancy!

So, where does the weight of the volume of water come into play?

It’s a mathematical shortcut that relates the effect of hydrostatic pressure on the surface of a shape to its displaced volume. This is also why a lot of the discussion about buoyancy focuses on the displaced water volume. It’s easy to work with volume formulas of simple shapes, after all. But at the end of the day, it’s essential to understand that buoyancy comes from the hydrostatic pressure field acting on a hull.

Beware of confusing buoyancy with net buoyancy

All things have weight, regardless if they are in the water or not. This weight is the force from the gravitational pull of the Earth. Sometimes this weight is referred to as the dry weight of oceanographic equipment – it’s the weight measured when the equipment is not in the water. A lot of dense equipment only has a small amount of buoyancy force. But this small amount of buoyancy force opposes the gravitational weight. This is often referred to as the wet weight of the equipment, which is less than the dry weight.

Oceanographic moorings have a series of floats and equipment, each with their own net buoyancy or, equivalently, wet weight.

Likewise, the net buoyancy is the buoyancy force less the gravitational weight. The net buoyancy is what a real float will do when it’s in the water because, in actuality, those two simultaneous forces are present at the same time: the hydrostatic pressure field from the ocean, as well as the Earth’s gravitational pull. There are two kinds of mistakes that may happen when working with buoyancy in mooring design.

The floats have much more buoyancy than weight, B1 and B2 and help lift the mooring up in the water column. The instruments and other components typically have only a little buoyancy that reduces their weight in the water. This is the wet weight W1 through W4.

One mistake is forgetting to account for weight

This mistake uses buoyancy alone and forgetting to include the weight of the component itself. If a designer makes this mistake with an oceanographic mooring, there may not be enough flotation. After deployment, the resulting mooring tensions would be lower than expected, and knockdown in a current would be larger than expected.

Another mistake is confusing net buoyancy with buoyancy

Remember, in net buoyancy, the weight is already deducted from the net buoyancy value. If buoyancy is confused with net buoyancy, the weight is deducted twice. If a designer makes this mistake with an oceanographic mooring, there may be too much flotation. After deployment, the resulting mooring tensions would be higher than expected in reality, and the knockdown would be less than expected. Higher mooring tension than expected can reduce the anchor holding capacity and cause the mooring to drift.

Often, supplier spec sheets will report the net buoyancy for flotation

Net buoyancy is useful for direct mooring design as it is the resulting lifting force float and buoys will provide. But the terminology is inconsistent in spec sheets, and you may see terms like net uplift, uplift, net buoyancy, buoyancy, flotation, etc. When in doubt, it’s essential to confirm the terms mean with the supplier.

Our civilization fundamentally needs sand for buildings and electronics. But assuming the deserts are an almost endless supply for us to use is a big misunderstanding. Similarly, buoyancy is the foundation for nearly everything we do and design for in the marine environment. Hopefully, this article helps clarify a few misconceptions about where it comes from and how it works, and where you may go wrong.

Next step

While buoyancy is one of the essential forces to understand, so is drag force. Read more on the nature of drag forces here.

Why free body diagrams eliminate confusion in engineering design

There is a small town on the border of the Netherlands and Belgium that has an identity crisis. That’s right – it’s not only people that can have existential angst but a physical town, too! This town is split between the Netherlands and Belgium – literally. The problem stems from hundreds of years of historical treaties that left land traded back and forth between the countries, resulting in a completely bizarre border.

You would think it would be bad enough that the border zig-zags through the town. But worse, it actually makes a series of pockets of one country or the other sprinkled throughout the region. It’s so intricate that even houses are split in two, straddling the nations. You can imagine not knowing which country you are in just by crossing the street. Without a map, you’re left not really knowing where you are, and it can leave you dizzy with confusion.

The border between the Netherlands and Belgium at the town of Baarle Nassau / Baarle Hertog cuts through town in very strange ways

Likewise, many hydrodynamics problems can leave you feeling dizzy with confusion at first glance. There can be many complex effects involved and it is easy to lose track of what’s going on. This article will discuss how free body diagrams act like a map to keep things organized and help eliminate that confusion. We’ll also use free body diagrams to introduce how buoyancy and drag are key forces in oceanographic mooring design.

A free body diagram comes in to play when we want to understand forces acting on a structure

A ship moored in a storm presents a complex scenario. There may be one or more hulls – ships or buoys – or both – and interconnected mooring lines. There are complicated forces involved caused by wind, current, and waves all happening at once. We use a free body diagram to isolate one entity and one entity alone in the scenario. Isolating this one body, we lay out all the forces acting on it from the environment onto the body itself.

Laying out the forces in a free body diagram is the first step in understanding the problem

Once you understand the problem, you can start solving it – that is – understanding the motions of the system and the structural forces involved. This will help you know if your ship is likely to capsize or if your mooring lines are likely to break or not in these harsh ocean conditions.

There’s more than one way to use a free body diagram

But ultimately, the first step is isolating the body you want to look at in more detail. Once you have that body isolated, you then need to show all the forces acting on it from the environment.

Once you have all the forces laid out acting on the body, you need to resolve the forces from each source. The balance of the forces acting on the system will tell you more information on the structural loads, the accelerations involved, or both.

When do we use free body diagrams?

When you are looking at a new system for the first time, it’s a good idea to draw one out to get an idea of what’s happening. If you don’t have a specialized software tool that solves these problems, it’s a free body diagram is an essential way to get an initial understanding of the problem.

Free body diagrams help understand what’s happening in problems like subsurface mooring design in an ocean current

But what if I miss a force in my diagram?

Free body diagrams can also be handy to validate your assumptions. If you measure something in reality – like a mooring tension from a system at sea – writing down the balance of forces you know about in a free body diagram is essential to help you understand if you’re missing something or not. This is especially important if there are significant differences from the software tool you’re using to design a system when compared to what you see in reality.

It’s time for an example

Free body diagrams can be an abstract concept without grounding them with a specific example. In this example, we are going to look specifically at the main float of a subsurface mooring. We’ll make some assumptions on this problem to keep it very simple. It is in a steady state, so there’s no motion, only a deflection of the taut mooring line in the current. Let’s draw a free body diagram of the subsurface buoy.

Figure 3: The forces acting on the top float of a subsurface mooring are buoyancy, drag, weight, and mooring line tension

There are a lot of complicated effects happening in what may look like a simple scenario. There is a hydrodynamic drag force acting on the float from the current. There is buoyancy pushing up on the float and gravitational weight pulling down. The mooring reaction load comes off at some angle to restrain the whole system.

We can characterize the environmental forces – weight, buoyancy, and drag. Once these are known quantities, computing the balance of forces acting on the float reveals the only remaining unknown – the line tension. Since this tension comes from the mooring line, it will show us what angle the mooring line is at and the magnitude of load. Knowing the magnitude of the tension means we can make sure we have the correctly sized rope for the problem.

This is a very simple example to show what it might look like when laying out the forces. But the goal isn’t to overwhelm with details here. This kind of evaluation is done behind the scenes in software tools like ProteusDS Oceanographic to help you resolve the correct size of mooring components to use.

It’s time to review

Free body diagrams help you focus on one specific element in a problem. You pick one body and draw out all the forces acting externally on it. Primarily, these will be environmental forces like those from wind, waves, and currents. This is the first step in breaking down the problem and resolving structural loads and motions – and it is a concept that is at work behind the scenes in software like ProteusDS Oceanographic. A free body diagram will also help you find out if you’re missing something when you validate your understanding with measurements from real systems. You can even directly solve some simple systems – like our example of a subsurface mooring.

In some small towns straddling Belgium and The Netherlands, the border is so convoluted you may lose track of which country you are in as you cross the street. A map is a must to help reduce confusion. In hydrodynamics problems, a free body diagram is very much like a map that helps reduce confusion.

Next step

Buoyancy is one of the most important forces in any kind of design for marine structures. Read more about where it comes from and common mistakes when designing oceanographic moorings here.


Read more on the complex border between the Netherlands and Belgium in the town of Baarle Nassau / Baarle Hertog here.



When profilers are optimized for control

It’s impossible to see a Frogfish. It’s not because of their camouflage, even though it is incredibly intricate. It’s not because they swim quickly, either: they are not very streamlined, and many Frogfish will lumber around slowly and mainly try to stay in one spot. What I mean is that you literally won’t be able to see them. When it comes time for them to strike their prey, they can do so in as little as 6 milliseconds. How fast is this? It is well below normal human reaction time – and most other animals’ – of 200 milliseconds. But while the speed is impressive, it means nothing if they miss their target.

So how do Frogfish control their attack to make sure they don’t miss? Their jaws are extremely flexible: when they expand outward, they increase their mouth volume by a factor of 12. This helps them contain their prey more easily. In addition to this, they suck in a massive amount of water around their target to draw them in. The excess water is simply filtered out their gills while their meal gets sucked right into their stomachs. Enveloping their prey and sucking them in makes sure their attacks are well controlled.

Control is key to getting consistent and specific results. In the case of oceanography, there’s more than one way to measure profiles in the water column. Wave powered profilers are impressive, but they have limitations. Making consistent and specific profiles may require a powered profiler, and in this article, we’re going to talk about how these technologies work.

Frogfish envelop their prey and suck in water to control their attacks.
Picture credit: Betty Wills (Atsme), Wikimedia Commons, License CC-BY-SA 4.0

A powered profiler has an onboard energy supply

These powered profilers carry their own battery and electric motors and use them to move on the mooring. This is in contrast to wave powered profilers that ratchet along a mooring that moves from ocean waves.

The powered profiler drive mechanism is straightforward: it uses a traction wheel to crawl in either direction along the mooring line. The onboard batteries and electric motor ultimately drive this traction wheel.

Powered profilers may seem a bit overkill

After all, there’s extra complexity with carrying their own batteries and motors – particularly in contrast to a simpler alternative like wave powered profilers. However, wave powered profilers just won’t work if there are no waves or no surface buoy on the mooring. But there is another advantage to powered profiles, and that is in their control capabilities.

Recovery of a McLane Moored Profiler. Picture credit: Scripps MOD

Powered profilers have a lot of ways to make controlled profiles

There are lots of options to control when profiling. Powered profilers can make profiles on a regular schedule. This schedule can be adjusted over time to sample seasonal variations in the environment, too. Either way, regularly scheduled profiles will generate a consistent dataset with evenly spaced samples in time, reducing data post-processing later on.

When powered profilers are used in a real-time mooring with satellite communication, they can even profile on-demand. But when to profile is only half the story. The other half is where to profile.

There’s also a lot of control over the profile span

A basic approach is to use mechanical stops on a mooring line. But powered profilers have many programming options, too. They can run between specified depths. Different depth ranges can even be used in separate profiles, as well.

All these time and spatial measurement capabilities give a lot of control over profiling. While this is a significant advantage, there are also a few instances when they are the only choice for making profiles.

Mooring motion is vital for wave powered profilers

Wave powered profilers need the mooring motion from surface waves to work correctly. Subsurface moorings, by their nature, have no surface buoy. So if you want to make profilers on subsurface moorings, your only option will be a powered profiler.

But the longer moorings are, the less the lower portion of the line moves when there is wave action. If profiling is required in the lower part of the mooring that is isolated from surface waves, or even in particularly deep water, powered profilers may be the only option.

Like everything in life, there are advantages and limitations

There is only so much energy in the batteries. The traction motors can only exert so much force, too. If there are high current regimes or excessive wave action, it can slow down profiling or use more energy than expected. The drag forces can also exceed the traction motor’s ability to move on the mooring. So there are some limits for use in extreme environments. While wave powered profilers thrive on ocean waves and excel in profiling near the surface, powered profilers typically operate in deeper than 20m water depth to avoid disruption from wave forces.

Another limitation is that the mooring deployment length may be governed by how much energy the batteries can hold for profiling. So how easily you can get out to the mooring to service it is a factor for how they can be used effectively.

Let’s look at an example

Scripps Institute of Oceanography used powered profilers on subsurface moorings to measure internal ocean waves in the Tasman Sea. You can see a schematic that highlights the use of two McLane Moored Profilers below. Note the mooring schematic is not to scale: each powered profiler traversed over a kilometre of line independently. The profilers needed to cover this distance because the internal waves Scripps Oceanographers were looking for were hundreds of meters in size, though also slow-moving, as they propagated through the ocean.

Two McLane Moored Profilers were used on this subsurface mooring to track internal ocean waves propagating in the Tasman Sea. Picture credit: Gunnar Voet from Scripps MOD

It’s summary time

Powered profilers carry their batteries and an electric traction motor on board to work. This means they can also control the depth and rate of profiling. These parameters can vary specifically over time – such as to account for seasonal changes in the ocean. But their onboard energy supply may be a limiting factor for the mission duration. They also have a limit to how much force they can use to crawl along the mooring line – so be wary of extreme environments with large surface waves or currents.

Even though a Frogfish can strike in the blink of an eye, it still needs a way to control its attack so they don’t miss. Similarly, control may be a significant factor when you are planning profiles for your oceanographic project – and a powered profiler may be just what your project needs.

Next step

McLane Research Laboratories makes both powered and wave powered profilers. Read more on their powered profiler MMP here and the wave powered profiler Prawler here.


Thanks to Tom Fougere at McLane Research Laboratories for the discussion and information on powered profilers. The helpful mooring example was also provided by Gunnar Voet at Scripps MOD.


You can see a video of how fast a Frogfish can strike below. Don’t blink!