1. Dock Fenders: Large Rubber Cushions Between a Ship and the Wall
If you look at the wall of a dock where ships arrive, you might see large blocks of rubber lined up along the surface. These are fenders, placed between the ship and the wall. Kawasaki City materials describe fenders as equipment designed to prevent damage to the ship’s body and the dock wall when a ship docks [1]. They are also called fenders in English. Yokohama Rubber introduces them as shock-absorbing materials that protect against impacts during docking or when ships are moored side-by-side [2]. This means they do not only protect dock walls; they can be used between ships as well.
Fenders protect both the ship and the wall. So, what exactly are they absorbing? The clue lies in the term "kinetic energy." Kinetic energy is the energy an object has because it is moving. In the next section, we will see how this energy is determined.
2. Why the Shock a Fender Must Absorb Grows with the Square of Speed
A slowly moving object is easier to stop than one moving quickly. Moving objects require more force to stop the faster they are. The National Institute for Land, Infrastructure, Management and Technology (NILIM) states that docking energy is the kinetic energy of the ship at the moment of docking, and it is heavily influenced by the docking speed [3]. The force as the ship slows is transmitted to the wall through rubber fenders and similar materials.
Kinetic energy is proportional to the square of the speed. For the same ship, if the speed doubles, the energy becomes four times greater. If the speed is halved, the energy becomes one-quarter. For example, if you can reduce the docking speed from 10 cm per second to 5 cm per second, the amount the fender must absorb is only one-quarter (these numbers are hypothetical examples for explanation). If a heavy ship approaches at the same speed, the amount to absorb increases further. Therefore, before thinking about "making the fender stronger," one option is to "dock slowly." What is actually happening in real ports?
3. Why Larger Ships Dock More Slowly, and How Docking Speed Is Measured
NILIM materials show a trend where the larger the ship type, the smaller the docking speed tends to be [3]. The reason for this is not written in the materials, so we only accept the trend here. This data collects actual measurements of docking speeds using ultrasonic and laser distance meters. For example, there are 36 data points for large tankers (approximately 126,000 to 292,000 deadweight tonnes (DWT)). DWT, or deadweight tonnage, describes how much weight a ship can carry.
Differences also appear in docking methods. In measurements of container ship docks, ports that use tugboat assistance, side thrusters, or a method where the ship stops parallel to the wall before slowly moving in, showed smaller average speeds and smaller standard deviations (a measure of variation) [3]. The materials view this as a result of more cautious docking practices. In fender design, these measurements are used as data. According to the materials, it is desirable to determine the docking speed used in design based on actual measurements from the specific port or past measurements [3]. On the other hand, a lack of data for large ships is listed as a challenge. So, we have methods to dock slowly and tools to absorb impact. What kinds of tools are there?
4. Solid Rubber Fenders and Pneumatic Fenders: Two Ways to Absorb Shock
Fenders are mainly divided into two types based on what absorbs the shock: solid type, which uses the elasticity of rubber, and pneumatic type, which uses the elasticity of air inside. Yokohama Rubber explains that solid type fenders account for about 80% of total fender demand [2]. Note that this is one company’s explanation, not industry-wide statistics.
Another way to classify them is fixed type and floating type. According to a February 2026 announcement by Sumitomo Rubber, rubber type fenders are fixed to the wall and are characterized by high energy absorption capability. Floating type fenders (pneumatic or foam type) float in the water, so they follow changes in tide levels, and their initial reaction force upon impact is softer [4]. Pneumatic fenders require management of internal pressure, while foam type fenders do not. The foam type is an introduction of a new product and is not necessarily a replacement for pneumatic fenders.
Regarding the history of pneumatic fenders, Yokohama Rubber states that they developed the world’s first pneumatic fender for use in connecting two ships at sea in 1958, according to their own explanation [6]. They say the rubber technology used in car tires and aircraft fuel tanks was the foundation. The claim of "world first" is from the company’s explanation and is not corroborated by other sources.
5. How to Spot Fenders at a Port and Check the Idea at Home
If there is a port nearby, look at the wall surface of the dock. If you see rubber in V-shapes or round tube shapes lined up, these may be fixed type fenders. It is also said that old tires are sometimes hung on tugboats and other vessels [5]. The edges of docks are dangerous, so please follow fences and "No Entry" signs and observe from a distance.
At home, with an adult nearby, you can do a small experiment on the floor with a towel. Push a toy car across the floor toward a wall and observe how it stops when moving slowly versus quickly. Next, place a folded towel between the wall and the car and repeat the experiment. How does the presence or absence of a cushion and the difference in speed affect the sound and the way it bounces back? If you want to know more, the NILIM materials (Analysis of Docking Speed Characteristics) are a good starting point. The materials include charts of actual measurements.
Safety Note: The edges of docks are dangerous. Do not enter restricted areas. Always observe from a safe distance.