WAVES
So why a post about waves?
As I am a live-aboard, each and every day I feel the boat moving. (It’s moving a lot as I write this from the pilothouse.) The surrounding current causes some of this movement. Wind too. But mostly, it’s the waves.
I decided to research waves to understand them better. The more I researched, the more I thought other people, especially boaters, might find this cool.
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So What Is A Wave?
A wave is a kind of disturbance that travels through space and matter. Wave motions transfer energy, not matter, from one place to another. This applies to all waves -- longitudinal and transverse. Sound waves are longitudinal: they compress and decompress. Water waves are transverse: the surface goes up and down. Since my blog relates to my experiences as a live-aboard, for the rest of this post, I'm going to focus on water waves (even though our boat relies on other kinds of waves for our radios, satellite phone, GPS, radar, AIS, and remote controls).
Here's the most amazing thing I learned about water waves. Though boaters see waves moving along in a certain direction, the vasty majority of a wave doesn't move forward or backward at all. The water molecules are actually moving in circles! The closer to the surface, the bigger the circles. The closer to the bottom, the smaller and flatter the circles (they are more like ellipses).
It’s kind of hard to accept this because our eyes tell us otherwise. These illustrations helped me understand better.
Image source: Satellitesportsimages.comYou can see how there's very little wave movement below the wave base. In fact, at a depth of 1/2 the horizontal wavelength (the measurement from one wave's peak to the next), the circular motion all but ceases.
But, while waves don't actually move water forward, they can move objects floating in the water. Most of that movement, however, is up and down. While floating objects look like waves are pushing them forward, it's actually the wind (which is making the waves themselves) pushing the objects in that direction.
The Three Main Causes of Water Waves
Wind
Most waves are caused by wind. And what causes wind? Uneven heating of the earth by our sun. In other words, the sun's energy is the root cause of our waves! And do you know what that means? It means the root, ROOT cause of waves is how the sun gets its energy... nuclear fusion. I suppose you could even say the root, ROOT, ROOT cause of waves is The Big Bang, because that's what eventually led to our sun (and our water). But that might be carrying it a bit far.
😁
Wind-driven waves are surface waves, created by the friction of wind blowing over the water. The more wind, the greater the friction disturbance and the greater the wave. At their worst, waves caused by the winds and pressure of a hurricane can come in the form of a storm surge, which is a series of long waves that form far from shore and become stronger as they approach land.
Astrophysics
Remember I mentioned the sun is the main cause of waves? Well, our sun and moon are the main cause of another type of wave -- tides: 25% and 75%, respectively. (Although tsunamis are sometimes called "tidal waves," that is incorrect. Tidal waves are, in fact, just the tides.) I always thought the rising tide was a series of waves, because that's what it looks like on the beach. But that's not true. The tide (a tidal wave) is a massive single wave that travels across an entire ocean basin.
Here's something else I'll bet you didn't know about tidal waves. They are shallow water waves. "No way! Shallow water? The ocean is so deep!" However, it's true. This is because the massive wavelengths of tidal waves vastly exceed the depth of even the deepest oceans.
Another factor that creates tidal waves is centrifugal force. This arises as the earth and moon revolve around each other and the ocean shifts away from the center of our planet's rotation.
There is so much more amazing stuff I could write about tides, but I'm going to save that for a future post.
Geological events
Another cause of waves is underwater earthquakes. Tsunamis are caused by the sudden lurching upward or downward of the sea floor at tectonic plates, abruptly lifting or dropping massive volumes of water. Other tsunamis are created by volcanic eruptions, when parts of the volcano blast or collapse into a body of water. Still other tsunamis are caused by another type of event: landslides. When millions of tons of rock and earth suddenly fall into the ocean, water is displaced all at once. That displacement forces the water upward into sometimes enormous waves. The most famous of these landslides was that in Lituya Bay in 1958. It has been scientifically proven that water from this wave climbed the opposing shore to a height of 524 meters. That's 1,719 feet tall!
Though I mentioned Lituya Bay, there are dozens of other known tsunamis dating back thousands of years. Most recently the 2004 Indonesian and 2011 Japanese tsunamis shocked the world and claimed many lives. In prehistoric times, the Chicxulub wave from the 10 km wide asteroid which wiped out our dinosaurs was also a big one. Though that wave was an estimated 325 feet tall, it was only that "small" because the asteroid struck in relatively shallow water. Had the asteroid struck in deep sea water, the calculated wave height is 2.9 miles tall.
Whatever the cause, I learned that waves can actually travel across an entire ocean basin if nothing obstructs them!
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There are seven key properties of a wave.
1. Crest - The wave's highest point (peak).
2. Trough - The wave's lowest point (base).
3. Frequency - The number of waves that pass in a given time.
4. Amplitude - The distance from the middle of a wave to its peak or trough.
5. Wavelength - The distance from the peak of one wave to the peak of the next.
6. Speed - The distance a wave travels in a given time.
7. Oscillation - The movement back and forth of a wave at a regular speed.
In deep water, waves travel extremely quickly. Tsunami waves far at sea travel at around 500 mph. But as waves near the shore (including tsunami waves), the circular orbits of water molecules begin to hit the bottom, slowing the waves down. This slowing down is why waves break on the shore, their tops often "spilling over."
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Breaking Waves
I explained why waves break when they reach the shore. The reason they sometimes break in open water is much the same: the top of those waves -- pushed by wind -- moves faster than the bottom.
According to NOAA (National Oceanic and Atmospheric Administration), a wave will break when the ratio of steepness to wavelength exceeds 1:7
Here are the most common types of breaking waves:
Spilling breakers - Spilling breakers occur as waves travel across a gently sloping bottom. The waves break longer and slower, losing their energy as white water spills from the crest down in front of the wave.
Plunging breakers - Plunging breakers occur as waves approach moderate to steep bottoms. These are the famous Pacific waves that surfers worship. (Incidentally, the center of those plunging breakers, which surfers love to ride in, is called the "barrel.")
Surging breakers - Surging breakers occur when long wave period (or low amplitude) waves approach moderately steep shores. These waves don't spill or curl. They build up and then slide rapidly up the beach with less foam or spray than the other two types of breakers.
So what is "surf?" Surf is not a wave type. Rather, it is all the wave activity between the outermost breaking waves and the shoreline.
Sea State
Sea state is the condition of the surface of the water with regard to wind and wave action at a given place and time.
There are so many different names and measures for different sea states. The most famous of these is the Beaufort scale, created in 1905 by a famous Irish hydrographer (a person who makes charts for navigation) of the same name. He did it to standardize weather conditions. Before then, one seaman's "gale" could be another's "stiff breeze."
While the scale originally was based on how sails responded to winds, and then based on cup rotations of an anemometer (a device which spins to measure wind speed), it eventually became based on sea behavior. The scale originally ran from 1-12, but then 13-17 were added to describe sea states in large storms, such as hurricanes.
This chart emphasizes the importance of studying weather forecasts before getting underway. The orange and red stuff is what we live-aboards hope never to experience. My dad and I know that, although our 47' Nordhavn is an incredibly seaworthy boat, when waves are on our nose, even the green Beaufort 5 conditions are unpleasant. (Far less so when we quarter them and our stabilizers can dampen them.)
Another widely used scale for sea state is the Douglas Sea Scale. It is used by the World Meteorological Organization.
The Dynamic Force Of A Breaking Wave
Remember Force = Mass x Acceleration? Well, consider this. One cubic meter of water weighs 2,205 lbs. Take that cubic meter, and accelerate it. That's what waves are. Already heavy, yet made much more forceful by motion.
Physicists say that a large wave that breaks on the shore can have a force of 250 to 6,000 pounds per square foot, depending on size. Dynamic force is the boat's movement (kinetic energy) toward water as well as the water's movement (kinetic energy) toward the boat. When these energies collide -- such as when our boat hits a big wave -- the forces on the hull can be tremendous.
The kinetic energy of a wave can be calculated with a formula KE = 1/2wg, where w is weight and g is the force of gravity. Hold on, you say. How does gravity come into play if the boat is moving horizontally. Well, remember that the boat rises and falls as it goes over waves and waves rise and fall against the boat. Some of gravity's force is at work each time. That's why some collisions with waves can be violent, especially if the boat rises high on a wave, then descends and slams into the next one. With all that pressure, you can imagine how seriously the nautical engineers who design oceangoing ships and yachts must take wave forces into consideration.
My dad and I have learned from experience that, beyond the mere size of waves, their direction relative to our boat, and their frequency -- the frequency with which we hit them -- matters enormously to our comfort. As a result, we sometimes speed up or slow down, or change course, to hit them with less force.
Freshwater Waves vs. Saltwater Ones
I've heard that many ship captains say they'd rather weather a bad storm at sea than on The Great Lakes. That seems counterintuitive, given that there is far more fetch (open water for wind or waves) on a bigger body of water, along with a longer duration of steady wind. And given that storms at sea can produce far larger waves than on The Great Lakes. So why is this? It's because of the differences in density between fresh and salt water. Fresh water is much less dense than salt water. Additionally, though plenty deep in some areas, The Great Lakes are comparatively shallower than the ocean, which has the effect of compressing wave energy. As a result, Great Lakes waves are notorious for being much sharper, and for having a shorter frequency, leading to choppier sea states, and for whipping up into a frenzied state very rapidly.
[Incidentally, the captains of ships that move between fresh water and salt water must take the differences in water densities into consideration when they load and ballast. Because their buoyancy is less in fresh water, their draft can increase by a foot or more.]
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The Study Of Waves
Man has been navigating by boat for thousands of years. Does this mean we know everything about wave behavior by now? No. People who study wave action are called Physical Oceanographers. Beyond studying how waves behave, they study their interaction with natural objects, such as beaches, and man-made objects, such a piers, oil rigs, and boats. Rising sea levels makes their expertise in demand for protecting coastlines.
Here's why it's important to test designs in wave tanks. Better to prove the design of a rig or ship in a tank than to "hope" it can withstand conditions like these.
Scientists study this wave activity in large wave tanks, typically using a hydraulic device called a wave generator to create whatever size and frequency waves they want. Here are two completely different uses of wave tanks.
And finally, here's a very special wave tank designed to study ship hydrodynamics, which is "the study of forces acting on fluids."
Wave tanks allow nautical engineers to design ships and oceangoing yachts robust enough to circumnavigate the globe. (All Nordhavn's, except their 35 ft and 59 ft coastal boats, have this capability.)
When crossing an ocean, one is bound to encounter some pretty unpleasant weather. This video of a Nordhavn 63 shows how capable the boat is in rough conditions off the Pacific coast, and how the owners, who are enduring those seas, might be uncomfortable, but are not worried. In times like that, that's priceless!
Container Ships
An estimated 90% of goods sold worldwide travel by container ship. There are an estimated 5,400 of them in operation worldwide, and the number keeps growing. It's amazing to look at Marine Traffic and see where they all are.
Screenshot from MarineTraffic.com. As many as you see here, this is a small fraction of the AIS (Automatic Identification System) -equipped commercial and pleasure vessels out there. That's because, zoomed out this far, the icons overlap -- sometimes many times.
Even more amazing was watching so many container ships backed up on either side of the Suez Canal when the Ever Given* blocked it, while watching others change course and go around the Cape of Good Hope (where some of the world's worst seas are).
* My dad says the Ever Given will almost certainly soon be sold, re-named, and possibly re-flagged, and then disappear back into service under that new disguise. This is what happens to ships that have a "bad experience."
Why am I singling out container ships in a post about waves? Well, it's because there are so many container ships out there, and because each of them carries thousands (sometimes over 10,000) containers. AND, because wave action sometimes leads to containers falling overboard. The worse case was the MOL Comfort -- a loss of all 4,293 containers, and the ship itself.
So how many containers are lost overboard each year? That's the problem. While the World Shipping Council reports around 1,382 annually for each of the past 12 years, nobody really knows, because shipping companies worldwide are not required to report the losses. There's no global "authority."
So what causes these ships to sometimes lose containers? Improper lashing of the containers is one possible cause. But not the root one. Maximizing the capacity of the ships is another cause. But still not the root one. The number one cause: waves. But not just waves, or even storm waves. Container ships lose containers mainly due to a combined effect of wave size, direction, and frequency, in a condition called "parametric rolling." Parametric rolling is when a ship rolls and pitches simultaneously, resulting in a twisting and dipping motion. Here's a video showing a container ship experiencing bad parametric rolling.
And here's a short video showing how such parametric rolling begins. (While the video is useful, you might want to turn down the overly-dramatic background music.)
Why does parametric rolling significantly raise the risk of losing containers overboard? It's basic physics. (In fact, it's the stuff I'm learning right now in my boat-schooling High School Physics class!) It's a combination of three forces on the containers: the path of inertia, reactive centrifugal force, and gravity.
Parametric rolling causes containers, mainly those piled high (furthest from the hold of the ship), to be thrown from side to side. The force with which they are being thrown means they want to continue to move in that direction unless that force is overcome by one equal or greater to it. (That's Newton's 1st Law.)
Additionally, as the containers swing further and further from a 90 degree angle to the ship's deck, the vertical force of gravity increases (meaning there's less and less force pushing up from beneath them stopping them from falling). The lashings can only take so much of these increased loads. And once one snaps, a domino effect can happen.
When I researched why container ships are so susceptible to parametric rolling, I learned it's largely a result of their design. The big flare of their bows and long overhang of their sterns mean the profiles of their waterlines change dramatically in head seas. The fine underwater hull form (fairly flat bottom) also makes them susceptible. It's easy to see how these factors combine to make the ship more sensitive in how it reacts to a big sea state. Look at the difference in the waterlines below depending on where the boat is in the wave.
Image source: lshipdesign.com
So if we know ship design contributes significantly to parametric rolling, what's the fix? They could design ships less susceptible. But these ships are designed with one purpose: to carry as much as possible as cheaply as possible. So design changes that add cost or reduce capacity are unlikely. In fact, vastly bigger ships are in the works.
Given the ships will stay the same basic design, that leaves controlling parametric rolling up to the captain. A seasoned captain can actually avoid it completely by planning well and paying close attention to sea state, course and vessel speed in heavy weather.
Even if the ship begins parametric rolling, which can happen rather quickly, the captain can change speed and/or course in order to change the relationship of the ship to the sea state. In certain conditions this might not be entirely possible, and might even be dangerous (i.e. if it poses a risk of broaching -- capsizing due to waves approaching from the stern), but in many cases it is possible to make a significant reduction in roll with good decisions from the bridge. Why don't captains automatically do this when there's a risk of parametric rolling? I think it's because a change in course or reduction in speed increases the time to port. And so captains endure roll, hope their containers are sufficiently lashed down, and hope the roll won't escalate to a critical point.
So how many containers are there afloat at sea? No one knows that either. While most sink pretty quickly, others do not. They stay hunkered low in the water and lurk as a threat to smaller boats because they are hard, if not impossible, to spot with the naked eye or radar. In theory, some containers which are filled with buoyant contents can float indefinitely.
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Rogue Waves ("Uh oh" waves)
I briefly mentioned rogue waves above. Rogue waves are also called "freak waves." By definition, rogue waves are unusually large, unpredictable, and suddenly appearing. Their size and power and unpredictability (they can come from different angles than the normal waves) makes them extremely dangerous to even the largest ships. For years, they were thought to be myths, because they reportedly towered far above the calculated highest-possible sea states.
This is a fascinating video about the effect rogue waves can have on ships, and the moment when scientists discovered rogue waves were as big as mariners had been claiming. This triggered widespread worldwide research into rogue waves because navies, shipping companies, and ship designers realized the conditions ships operate in can be far more dangerous than ever believed.
Since the famous Draupner Wave, all sorts of rogue waves have been measured around the world by buoys and by satellites. Others have been reported by captains at sea. In 1995, the Queen Elizabeth II, a massive 963 ft ocean liner, was forced to surf a 95' wave. The captain said the wave "looked like the white cliffs of Dover." In 2004 alone, 10 waves higher than 82 feet were recorded. And in 2007, it was determined that waves exceed 98 feet from trough to crest more frequently than thought.
Though studies of rogue waves continue, it has been determined that it's the types of waves and their onset (where they come from) that create rogue waves. And that while waves meeting at angles less than 60 degrees break downwards, waves that meet at 60 degrees or greater actually break upwards and more steeply. This is called a "vertical jet."
And if this isn't scary enough for you, consider this. Scientists have also replicated a kind of wave also long-claimed by mariners but never conformed: rogue wave holes. Rogue wave holes are extra deep troughs that can appear between waves. How deep? Well, as deep down as rogue waves can reach up. Sometimes the holes are adjacent to the rogue wave itself. But rogue wave holes can also appear in the middle of ordinary wave heights. I can't imagine what it must feel like to have a ship fall that far into a wave trough!
A different kind of rogue wave can also occur on freshwater bodies. Lake Superior is known for having a sequence of waves called the "Three Sisters." This is when a series of three waves strike in less time than the normal sea state, each one piling more water on the ship's deck before it can clear. This is one of the theories for what dealt the final blow to the already-flooding Edmund Fitzgerald.
What types of damage can occur to ships from rogue waves?
Bow slam damage - This damage can happen when the tip of the boat descends abruptly into the trough of a wave or the wave immediately afterward.
Cracking - This can happen to the hull toward the bow section.
Buckling of plates - This can happen from the bow to about 25% aft of the bow.
Failure - This can happen if stresses are repeated time and again and if ships are not regularly inspected for structural fatigue.
Another, especially dangerous, type of damage from rogue waves is loss of power. The height of rogue waves means they can reach (or exceed) the height of the bridge. Though a ship's glass is engineered to be very strong, it is no match when tons of water slam into it and wash into the bridge. This floods the electronics causing systems to short and shut down. When a ship loses power in heavy seas (which is called a "blackout"), it can no longer face those seas safely and will typically turn sideways and roll heavily. The ultimate risk of this is capsizing.
Below are a couple examples of damage to ships from rogue waves. These are ships that survived. One that didn't was the 857 ft MS Munchen (Munich), only 5 years old and of modern design and safety. She was lost with all hands, likely to a rogue wave. It is believed the wave caused a blackout.
So how often does a rogue wave appear? Well, you can see in this graph that the probability of the highest waves deceases with greater wave height. That said, scientists now believe there are an estimated 10 large rogue waves in the world's oceans daily.
I want to end this post with images of some famous waves in art. Here are a few I selected.
The most famous wave in he world is "The Great Wave Of Kanagawa."
It's also the only wave with its own emoji: 🌊!
Interestingly, I learned Kanagawa is a woodblock from a series of 36 views of Mount Fuji by Japanese artist Katsushika Hokusai. So in theory, I suppose you could say the great work is about the mountain in the background, not the wave. But it's the beauty and power of the wave that attracts viewers. The others in that series of 36 are far lesser known, but are very beautiful as well. Here's a link to them. https://japanobjects.com/features/hokusai-fuji
Kanagawa was first published between 1829 and 1833. There's no definitive answer to why this wave become the most famous in the world and an icon of Japan and the seas. It's popularity in Japan however is somewhat because the artist chose a color imported from Europe, never before used in Japanese art: Prussian Blue. The work was apparently admired by Van Gogh, Monet and Whistler and inspired Debussy's "La Mer."
(By the way, my dad told me he was always so focused on the wave itself, it took him years to see there are boats facing it.)
The next picture I chose is called "Neptune's Horses," painted in 1910 by Walter Crane. I like this one because the artist saw horses in the breaking waves, and because the sound of breaking waves can be imagined to sound like the thundering of many galloping horses.
In researching this painting, I learned Crane wasn't the first to link a god to horses and waves.
Down Poseidon dove and yoked his bronze-hoofed horses
onto his battle-car, his pair that raced the wind
with their golden manes streaming on behind them,
and strapping the golden armor round his body,
seized his whip that coils lithe and gold
and boarded his chariot launching up and out,
skimming the waves, and over the swells they came,
dolphins leaving their lairs to sport across his wake,
leaping left and right—well they knew their lord.
And the sea heaved in joy, cleaving a path for him
and the team flew on in a blurring burst of speed,
the bronze axle under the war-car never flecked with foam,
the stallions vaulting, speeding Poseidon toward Achaea’s fleet.
– Homer, The Iliad, Book XIII
The final painting I chose for this post is by Monet. It's called "Stormy Sea." The name made me realize most of the wave paintings and pictures I looked at online show waves that are angry and intense, and focus a lot on the light off the waves.
Monet's lighting may be subtle at first glance but the more I looked at it the more beautiful it became.
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Yes, there was plenty of pop culture in the wave images I saw, but I chose to ignore that.
Well, most of it.
🙂
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I hope you found this post as interesting to read as I found researching it. As with my other posts, I'm coming to see that the more I understand my live-aboard surroundings and experiences, the more I appreciate them and connect with them.
Great stuff and I always learn something. Here’s a tip. Don’t jump cruiser wakes or play in the prop wash of freighters in a 10’ Glen L Squirt. More comments later.
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