Observing the wake behind a duck, a kayak, or a ship, you may notice two peculiarities. Firstly, the wake isn’t simple, like the perfectly straight shock wake of a supersonic projectile. Instead, it presents a fascinating, feathery, ripple-like pattern. Secondly, this feathery pattern appears the same, regardless of whether it’s made by a duck, a kayak, or a ship, even though they’re all moving at different speeds and the waves are of different sizes.
The reason water wakes always have this particular shape and pattern is due to the surprising physics of water waves. Unlike light waves and sound waves, which each have a single speed, water waves don’t have a single speed. In water, longer waves travel faster, while shorter waves travel slower. This phenomenon, where different wavelength waves travel at different speeds, is known as dispersion, and it makes water waves both interesting and complicated.
To understand the shape of a boat wake, we can start by looking at water waves of just a single wavelength (and speed). A boat traveling across this simpler water creates a series of circular waves. If the water waves are faster than the boat, the waves encircle it, but you don’t get a wake. If the waves are slower than the boat, the boat outruns them and the circles all add together to create a V-shaped wake. Slower waves make narrower wakes, while faster waves make wider wakes.
However, water waves are repetitive, meaning every circular wave is really the first of a series of circular waves. This means that instead of creating just one v-shaped wake, a moving boat creates a train of v-shaped wakes that are each exactly one wavelength apart.
The reason a real boat doesn’t make straight v-shaped wakes is that a real boat makes waves of many different wavelengths. Due to dispersion, different wavelengths travel at different speeds: the longer ones travel faster and shorter ones travel slower. Faster waves create wider wakes, and because faster waves also have longer wavelengths, that means that wider wakes are further apart. Similarly, slower waves create narrower wakes, and because slower waves also have shorter wavelengths, narrower wakes are closer together.
When you add together narrow, closely spaced wakes, with wider, more widely spaced wakes, with even wider, even more widely separated wakes, and so on, you get the shape of a boat wake. This beautiful repeating feathery ripple pattern on the edge, and the wider repeating arcs inside the wake itself, can be replicated with smoother waves at the appropriate angles and spacings to get an even more convincing boat wake. This can also be done in 3D to get a really realistic-looking boat wake.
In summary, wakes have the shape they do because water waves travel at different speeds. Slower water waves create narrow, closely spaced V-shaped wakes, and faster water waves create wider V-shaped wakes that are further apart. When you add all these different v-shaped patterns together, at the correct angles and spacings determined by water’s dispersion relation, you end up with the unique shape of a water wake.
Take a trip to a nearby pond, lake, or river. Observe the wakes created by different objects such as ducks, boats, or even thrown stones. Try to identify the feathery ripple pattern and the V-shaped wakes discussed in the article. Take notes of your observations.
Using a large container filled with water, create your own wakes. You can use different objects and move them at different speeds. Observe how the shape and pattern of the wake changes with different speeds and objects. Try to relate your observations with the concepts discussed in the article.
Using a wave generator (you can easily find instructions to build one online), create water waves of different wavelengths. Observe how the speed of the waves changes with the wavelength. This will help you understand the concept of dispersion in water waves.
Based on your observations and experiments, draw the wakes created by different objects moving at different speeds. Try to draw the feathery ripple pattern and the V-shaped wakes. Analyze your drawings and discuss how they relate to the concepts discussed in the article.
Research more about the physics of water waves and wakes. Look for additional resources that explain these concepts in more detail. You can also look for simulations and animations that can help you visualize and understand these concepts better. Share your findings with your classmates.
Physics – The branch of science concerned with the nature and properties of matter and energy. – Physics deals with the study of motion, energy, and the behavior of matter.
Water wakes – The disturbance created on the surface of water due to the movement of an object through it. – The boat created large water wakes as it sped across the lake.
Wake – The track left by a moving object, especially on the surface of water. – The wake of the ship extended for miles behind it.
Feathery – Having the characteristics or appearance of feathers. – The bird had beautiful, feathery wings that allowed it to glide through the air.
Ripple-like – Resembling or having the appearance of ripples, small waves, or undulations. – The wind created a ripple-like effect on the surface of the lake.
Pattern – A repeated decorative design or arrangement. – The rug had an intricate pattern of flowers and leaves.
Dispersion – The process of scattering or spreading out over a wide area. – The dispersion of light through a prism creates a rainbow of colors.
Boat wake – The disturbance or waves created by a boat moving through the water. – The boat’s wake rocked the smaller boats nearby.
Wavelengths – The distance between successive crests of a wave, especially electromagnetic waves. – Different colors of light have different wavelengths.
Speeds – The rate at which someone or something moves or operates. – The car reached high speeds on the highway.
