You have probably noticed a V-pattern-like series of waves diverging behind a vessel and even surfers taking advantage of them. Ship-generated wakes have a science of their own. Here’s what sets them apart from regular ocean waves.

“Waves and Beaches” (Patagonia, 2020) by Willard Bascom and Kim McCoy is a book every ocean enthusiast should read.

It teaches us, without getting too technical or scientific, the fundamental dynamics of the sea and coastlines, and how they all interact constantly and rather harmoniously.

One of the book’s subchapters is about waves produced by ships.

Surfers have been increasingly drawn to riding these unusual waves or wakes. They search for them, identify the best opportunities, study the timings, and then go for them.

But what’s so special about the waves generated by a ship that slices through the sea? What’s behind and beneath that foamy trail?

You could tell that these are human-made waves, born from engines and hulls rather than wind and weather, and they unveil fascinating secrets about how energy moves through water.

Pressure and Lost Energy

As surprising as it might be, every ship, be it a tugboat or a massive tanker, spends part of its fuel budget not on forward motion but on wave-making.

In fact, a surprising chunk of propulsion energy is spent stirring up waves that spread uselessly away.

This, as the dear reader might imagine, is a performance issue from several angles, but mainly in terms of economic costs and speed/time.

Hydrodynamic pioneers like Daniel Bernoulli, Lord Kelvin, and William Froude devoted most of their lifetimes to understanding this inefficiency.

The thing is that speeding water lowers pressure.

You can see it, for instance, when a moving ship carries two gentle mounds of water – one at the bow, one at the stern – with a dip in the middle.

On a calm day, you’ll spot a faint ridge moving with the vessel, not ahead or behind it; it’s a wave that never breaks, a traveling bulge created by pressure imbalance.

If you’ve never witnessed it, you can try simulating it with a remote control speedboat on a fountain or lake.

But this visual phenomenon is, again, an inefficiency.

Modern engineers still chase the same dream as their 19th-century predecessors.

“How can we cut fuel waste and noise by taming these waves?” they ask themselves.

The answer is more and more in computational fluid dynamics (CFD), which now plays the role of the towing tank, and tests how hull shapes and propellers can keep more energy in motion and less in the wake.

CFD is actually also used in surfboard design to optimize variables like pressure, velocity, and turbulence.

Let’s take a closer look at these vessel-generated ripples.

The Kelvin Wake: Nature’s Signature Pattern

Lord Kelvin (1824-1907) was a precursor of understanding these near-magical wake patterns.

The British mathematician, physicist, and engineer revealed that every moving object on water creates a unique wave system.

Drop a stick or drive a ship – it doesn’t matter. The pattern that fans out behind is universal. Even ducks produce it.

The so-called Kelvin wake is made of two families of waves. 

One set diverges outward, forming a V-shaped wake. The other, the transverse waves, march perpendicularly to the ship’s path.

The two systems intersect at a constant angle – about 39 degrees- no matter how fast the ship moves.

For large ships, however, the pattern becomes more complicated. Why? Because the hull itself acts as a chain of wave sources.

Bow, shoulders, and stern, each creating ripples that mix and sometimes cancel each other out.

Change the speed, and the wave systems shift, lengthen, and interfere in new ways.

Sometimes the crests line up to cancel one another; other times they amplify, creating the big rolling wakes surfers love to chase.

Actually, wakeboarding/wakesurfing is nothing less than taking advantage of an inefficiency and accentuating it as much as possible.

The Bulbous Bow: When Two Waves Cancel Each Other Out

You might not have ever known, but a ship’s bow wave is its biggest and most “wasteful” (least efficient) feature.

Early naval architects tried to fight it by brute force, but in 1907, American naval architect and engineer David Watson Taylor discovered a smarter trick.

If you give the ship a rounded underwater nose – the so-called “bulb” – the wave it makes can cancel the main bow wave above it.

The groundbreaking bulbous bow, later refined by W. C. S. Wigley in the 1930s, works like noise-canceling headphones for water.

What happens is that the crest from the bulb meets the trough from the bow, and the two mostly flatten out.

Wigley found that a good bulb should project forward, stay deep, and be wide and low.

At high speeds, the impact is absolutely relevant.

Just to put things into perspective, a 900-foot cargo ship cruising at 30 feet per second can cut its resistance by up to 15 percent and boost propeller efficiency by 5 percent.

That’s roughly a 20 percent reduction in power needed, which any accountant will see as an enormous saving at sea.

We also know that even the ancient Greeks may have stumbled onto the same design tweak with their triremes’ “rostrums,” the bronze rams beneath their bows.

As far as we know, their ships ran at just the right speed range for the effect to work.

Obsession with lean performances has been, it turns out, an ever-present constant in human development.

The Invisible Waves Below

Now, here’s a bit of a next-level complexity to what we’ve learned so far: ship wakes don’t always stay on the surface.

Confused? Let’s see.

When fresh water floats above salt water – common near melting ice or river mouths – a moving vessel can create huge, unseen internal waves at the boundary between the two layers.

These slow, ghostly waves can “capture” a boat, making it feel as if it’s stuck in syrup or any muddy body of water.

Norwegian fishermen once called this eerie phenomenon “dead water.” However, in 1904, scientist V. W. Eckman explained it.

The boat’s bow pushes down the interface, starting waves that move far slower than surface waves.

The boat rides those hidden swells, dragging itself along and losing speed until it can break free by accelerating beyond about five knots.

Dolphins, Surfers, and Horses That Ride Wakes

So far, we’ve seen that ship waves are an inefficiency that should be minimized.

Nevertheless, these highly visual Vs of energy can also be rideable. And this is their fun, non-economic side.

Let’s start with dolphins, an aquatic mammal that has perfected the art of surfing waves by tilting their streamlined bodies just right to ride the pressure surfaces beneath a yacht’s bow wave.

And so they glide effortlessly for miles.

Humans, too, have found joy in ship wakes.

“As young boys on the Hudson River in New York, we used to paddle frantically to get a canoe into the proper position behind a ferryboat as it pulled away from the pier so we could get a free ride across the river, merely steering to hold position on the steep slope of the first transverse wave in its wake,” Willard Bascom recalled of his youth.

This practice has been pushed to the limits in the 21st century with the development of tanker and ferry surfing. 

Even canal horses in the 18th century got in on the act.

Believe it or not, they learned to speed up before a narrow section so their barge could ride its own bow wave – an early form of tow surfing – centuries before anyone coined the term.

Words by Luís MP | Founder of SurferToday.com