A moving ship pushes water out of its way, creating waves that spread across the surface. The wave at the front of the hull is called the bow wave, and the pattern trailing behind the ship is called the wake. These wave patterns matter because they affect fuel use, shoreline erosion, navigation safety, and how ships are detected.
A ship's wake is also a visible record of how energy is transferred from the vessel to the water.
The familiar V-shaped wake behind many ships is called a Kelvin wake, and it forms because surface waves travel away from the ship while the ship keeps moving forward. The wake includes diverging waves that spread outward and transverse waves that trail behind the ship. In deep water, the outer edges of the Kelvin wake make an angle of about 19.5 degrees from the ship's path on each side.
Submarines can also create wakes near the surface, especially when they move shallowly or disturb the water above them.
Key Facts
- A bow wave forms where the ship's hull first pushes water aside.
- A wake is the disturbed water and wave pattern left behind a moving vessel.
- The Kelvin wake has a V shape with an approximate half-angle of 19.5 degrees in deep water.
- Wave speed in deep water depends on wavelength: v = sqrt(gλ / 2π).
- Ship speed can be found from distance and time: v = d / t.
- A higher ship speed generally produces a stronger bow wave and a more energetic wake.
Vocabulary
- Bow wave
- A bow wave is the wave that forms at the front of a moving ship as the hull pushes water outward.
- Wake
- A wake is the trail of disturbed water and waves left behind a moving ship or submarine.
- Kelvin wake
- A Kelvin wake is the V-shaped wave pattern made by a moving object on the water surface.
- Diverging waves
- Diverging waves are wake waves that spread outward from the ship's path at an angle.
- Transverse waves
- Transverse waves are wake waves that form behind the ship and are roughly across the direction of travel.
Common Mistakes to Avoid
- Calling every wave behind the ship the bow wave is wrong because the bow wave forms at the front, while the wake trails behind the vessel.
- Assuming the wake angle always gets wider when the ship goes faster is wrong because the classic deep-water Kelvin wake angle stays about 19.5 degrees on each side, although the waves can become stronger.
- Ignoring water depth is wrong because shallow water can change wave speed, wake shape, and the amount of disturbance near shore.
- Thinking a submarine makes no wake is wrong because a shallow submarine can disturb the surface through pressure changes, turbulence, and waves.
Practice Questions
- 1 A ship travels 600 m in 40 s. What is its speed in m/s?
- 2 In a top-down diagram, the outer edge of a ship's Kelvin wake is 19.5 degrees from the path on each side. What is the total angle between the two outer wake edges?
- 3 Two ships have the same hull shape, but one moves slowly through deep water and the other moves faster. Explain how their bow waves and wakes would differ, and why the faster ship transfers more energy to the water.