Hull speed is a hydrodynamic concept that deals with the complex physics of a vessel's interplay with the water around it. It is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As a boat's speed increases, the wavelength of the bow wave increases, and so does its crest-to-trough dimension (height). When hull speed is exceeded, a vessel will appear to be climbing up its own bow wave.

## What You'll Learn

- Hull speed is the speed at which a boat's bow wave is equal to the waterline length of the vessel
- The speed of a boat is limited by its hull speed
- Hull speed is calculated by the formula: hull speed in knots = 1.34 x square root of the waterline length in feet
- A boat's effective waterline length increases as it goes faster
- A boat's hull speed depends on its length and shape

**Hull speed is the speed at which a boat's bow wave is equal to the waterline length of the vessel**

Hull speed, or displacement speed, is a complex hydrodynamic concept that deals with the interplay of a vessel and the surrounding water. It is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel.

As a boat moves through the water, it creates two series of waves – one at the bow and one at the stern. The speed of these waves is determined by the law of natural physics, which states that the speed of a series of waves in knots equals 1.34 times the square root of their wavelength (the distance between wave crests). As the boat's speed increases, so does the wavelength of the waves, and therefore the height of the waves.

At hull speed, the bow wave cycle and the stern-wave cycle merge, creating one long trough that runs the length of the hull. The boat's bow and stern are supported by their respective waves, and it can continue moving forward efficiently. However, if the boat tries to go faster, the back of the boat will fall into the trough of the wave, and the boat will have to climb up its own bow wave, which is now relatively large.

**Hull speed can be calculated using the formula:**

> Hull speed in knots = 1.34 x square root of waterline length in feet (or hull speed = 1.34 x √LWL)

For example, a boat with a waterline length of 28 feet will have a hull speed of a little over 7 knots (1.34 x √28 = 7.09).

It's important to note that hull speed is not a defining factor in maximum boat speed, and many boats can easily exceed their nominal hull speeds. Additionally, hull speed is not used in modern naval architecture, where considerations of speed/length ratio or Froude number are considered more helpful.

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**The speed of a boat is limited by its hull speed**

Hull speed, or displacement speed, is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As a boat increases in speed, the wavelength of the bow wave also increases, usually leading to an increase in its crest-to-trough dimension (height). When hull speed is exceeded, the vessel will appear to be climbing up its own bow wave.

The concept of hull speed is based on the assumption of displacement-type hulls, which are the simplest and most common type of vessel. These hulls rely on the basic Archimedes' principle of displacement and buoyancy. As a displacement hull moves through the water, it displaces significant amounts of water, creating two series of waves - one at the bow and another at the stern.

The speed of these waves is governed by a law of natural physics, which states that the speed of a series of waves in knots is equal to 1.34 times the square root of their wavelength, or the distance between the wave crests. As the boat's speed increases, so does the wavelength of the waves, and therefore their size. At hull speed, the bow and stern waves coincide to create one long trough running the length of the hull.

Once hull speed is reached, a boat can maintain its speed and continue moving forward efficiently. However, if it tries to go faster, it will fall into the trough of the waves, and the boat will be left trying to climb up its own bow wave, which is now relatively large. From this point on, much larger increases in power are needed to achieve even small increases in speed, requiring a disproportionate amount of power.

The speed of a boat is therefore limited by its hull speed, as it becomes increasingly difficult and expensive to achieve further acceleration once this speed has been reached.

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**Hull speed is calculated by the formula: hull speed in knots = 1.34 x square root of the waterline length in feet**

Hull speed, or displacement speed, is a hydrodynamic concept that deals with the complex physics of a vessel's interplay with the surrounding water. As a ship moves through the water, it creates waves that oppose its movement. The wavelength of these waves increases as the boat's speed increases, and the crest-to-trough dimension (height) usually increases as well.

The hull speed is the speed at which the wavelength of the vessel's bow wave is equal to the waterline length of the vessel. At this speed, the vessel will appear to be climbing up its own bow wave. From a technical perspective, the bow and stern waves interfere constructively, creating relatively large waves and, thus, a relatively large amount of wave drag.

The hull speed can be calculated using the formula: hull speed in knots = 1.34 x square root of the waterline length in feet. Here, the constant 1.34 is the speed/length ratio, which quantifies the relationship between a boat's speed and its waterline length.

It's important to note that the concept of hull speed is not used in modern naval architecture, where considerations of speed/length ratio or Froude number are considered more helpful. Additionally, the hull speed formula does not apply to needle-like hulls such as racing shells.

The speed/length ratio can be calculated using the formula: speed/length ratio = boat's speed in knots / square root of the boat's load waterline length. This formula assumes that 1.34 is the maximum speed/length ratio that can be achieved due to the characteristics of waves.

It's worth mentioning that hull speed is not the maximum rated speed of a vessel, nor is it the service or design speed. It is simply a measure of the vessel's performance in relation to its waterline length and the waves it creates.

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**A boat's effective waterline length increases as it goes faster**

Hull speed, or displacement speed, is the speed at which a vessel in displacement mode will appear to be climbing up the back of its bow wave. At this point, the vessel's speed is equal to the speed of the bow wave.

**The hull speed of a vessel can be calculated using the formula:**

> Vmax (in knots) = square root of LWL (in feet) x 1.34

Where LWL is the load waterline length, or the length of the hull at the water's surface when the boat is carrying a normal load.

As a boat's speed increases, its effective waterline length increases, which means its speed potential under the formula also increases. This is why a boat's hull speed is not necessarily its actual maximum potential speed, but rather a minimum maximum figure.

For example, let's consider a 35-foot boat with a waterline length of 28 feet. Its hull speed works out to a little over 7 knots (1.34 x √28 = 7.09). However, if the boat has long overhangs, its effective waterline length will increase as it goes faster, and its speed potential will also increase.

Additionally, the design of a boat's stern can also impact its speed potential. A boat with a stern that suppresses its stern wave can achieve higher speeds as it helps to make the hole created by the wave trough smaller, and also increases buoyancy aft, preventing the stern from falling into the hole.

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**A boat's hull speed depends on its length and shape**

The speed of a boat is influenced by several factors, including its hull shape and length. The concept of hull speed is based on the complex physics of how a vessel interacts with the water around it. As a boat moves through the water, it creates waves that cause resistance, which can be divided into frictional resistance and wave resistance. The interplay of these waves with the boat's hull determines its speed.

The hull speed of a boat is the maximum speed at which it can travel without experiencing a significant loss in speed or a surge in power requirements. It is influenced by the length of the boat and is calculated using the formula: Hull Speed (in knots) = 1.34 x square root of the waterline length in feet. This formula assumes a displacement hull, which is the most common type, and refers to a hull that travels through the water rather than on top of it. The multiplier of 1.34 in the formula comes from the natural physics law that the speed of a series of waves in knots is equal to 1.34 times the square root of their wavelength.

The shape of a boat's hull also plays a crucial role in its speed. Modern hull forms use chines to create volume forward while maintaining a narrow entrance at the waterline. The angle of the waterlines to the centreline of the ship, known as the half-angle of entrance, is critical in determining wave resistance. A smaller angle results in smaller waves and lower wave-making drag. Additionally, the stern design can suppress the stern wave, reducing the size of the hole created by the wave trough and increasing buoyancy to prevent the boat from falling into the trough.

While hull speed provides a rough estimate of a boat's potential speed, it is not always a limiting factor. Many boats, even those with displacement hulls, can exceed their nominal hull speeds. This can be achieved through various design features, such as long overhangs that increase the effective waterline length at higher speeds. Additionally, flat-bottomed boats, regardless of their stern configuration, may be able to get on top of the water and plane, surpassing their hull speed.

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**Frequently asked questions**

Hull speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel.

Hull speed can be calculated using the formula: Hull speed in knots = 1.34 x square root of the waterline length in feet (or Hull speed = 1.34 x √LWL).

When a boat reaches hull speed, its bow and stern waves coincide to create one long wave system. The boat is effectively trapped in a hole and must use a disproportionate amount of power to achieve further increases in speed.

Hull speed is only a practical limitation for full displacement hulls. Lighter boats with well-designed semi-displacement hulls can ride up on the bow wave with less power, and some boats may even be able to plane.

Hull speed is important in small boat racing because it helps sailors understand key principles for sailing fast. For example, sailors can maximise waterline length and hull speed by sailing their boat "on its lines", i.e. sailing flat to get the longest waterline.