1. How a Ship's Rudder Deflects Water to Create Turning Force
A rudder is not a flat board that blocks water coming from behind. Instead, it is a slanted surface that deflects the flow of water to one side. By changing the direction of the water, the rudder pushes back against the water. This reaction force pushes the stern (the back) of the ship sideways, causing the ship to turn [1].
This turning force gets much stronger as the ship moves faster through the water. If the speed doubles, the effectiveness of the rudder does not just double; it increases by about four times [1]. This means that when a ship is moving slowly, the rudder is much less reliable than you might think. The force comes from the propeller pushing water backward. If the engine stops and the propeller stops spinning, the water flow over the rudder weakens, making it difficult to steer [1].
There is a limit to how much you can turn the rudder. As the angle increases, the force grows, but past a certain point, the water flow separates from the rudder surface, and the force actually decreases [1]. Most ships are built so the rudder can only turn about 35 degrees to one side [1][2]. This limit is required by the International Convention for the Safety of Life at Sea (SOLAS) [2]. The SOLAS convention requires that even when navigating at maximum draft and full speed, the steering gear must be able to move the rudder from 35 degrees on one side to 35 degrees on the other [2].
2. Why a Ship Turns Around a Pivot Point and Gives an Initial Kick
When a ship turns, it does not rotate around its center of gravity. Instead, it rotates around a point called the "pivot point," which is determined by water resistance [3]. When moving forward, this point is located about one-third to one-sixth of the ship's total length from the bow (front) [3]. When moving backward, the pivot point shifts to a similar distance from the stern [3].
Because the pivot point is closer to the front than the center, something interesting happens. Immediately after turning the rudder, the stern, which is behind the pivot point, swings briefly in the opposite direction of the intended turn [3]. This is called the "kick." Only after this small backward swing does the ship begin to turn in the desired direction [3].
In Japanese maritime terminology, turning the rudder to the right is called "omokaji," and turning it to the left is called "torikaji" [4]. You might hear the command "omokaji ippai" (hard to starboard) in old ship movies or stories [4].
3. How Turning Circles Differ Between Types of Ships
If you keep the rudder hard over while moving, the ship will trace a circle. This path is called the "turning circle," and it is the track made by the pivot point [5]. The turn happens in stages: first, the turning force is created; then, the heading changes by 90 degrees; finally, the forces balance, and the ship continues in a steady circle [5].
The size of this circle depends heavily on the ship. Generally, larger ships have larger turning circles, so small boats can avoid obstacles faster than large bulk carriers [5]. This is similar to the difference in maneuverability between a bicycle and a large truck. However, size is not the only factor. Long, narrow hulls, like container ships or frigates, tend to have larger turning circles than bulk carriers of the same length and speed [5].
Deeper drafts (how deep the ship sits in the water) also lead to larger turning circles. If the stern is heavy and tilted down (stern trim), the turning diameter increases significantly. If the bow is heavy (bow trim), the circle tends to be smaller [5]. In shallow water, the space between the ship and the seabed changes how water flows, requiring more force to turn [5].
Because of these factors, the International Maritime Organization (IMO) requires ocean-going ships over 100 meters (about 328 feet) long to undergo turning performance tests. Gas and chemical tankers must be tested after launch and before delivery, regardless of size [5]. These tests are done in wide, deep waters, in calm weather, with the ship fully loaded and level, at about 90% of its design speed [5].
4. How International Collision Rules Developed Since the 19th Century
Before modern international rules, the seas in the 19th century were much less regulated. Navigation customs varied by country and region. Many sailing ships did not show navigation lights at night, which contributed to collisions [6].
In 1863, Britain and France created new rules. These rules were popular, and by 1864, more than 30 countries had adopted them [6]. However, details still differed from country to country. In 1972, the International Maritime Organization (IMO) adopted a set of universal rules to prevent collisions. These are known as the COLREGs (International Regulations for Preventing Collisions at Sea), which came into force on July 15, 1977 [6]. One reason for this adoption was the need to manage the world's first "traffic separation schemes," introduced in the Dover Strait in 1967, which divided shipping lanes into one-way routes [6].
5. What the COLREGs Require Ships to Do to Avoid Collisions
The COLREGs include a rule called "Action to Avoid Collision" (Rule 8). It states that if danger approaches, a ship must change its course or speed early enough and significantly enough to be obvious to the other ship [6]. A small turn of the rudder is not enough; the movement must be large enough to be clearly seen [6].
Normally, one ship maintains its course and speed (the stand-on vessel), while the other avoids it (the give-way vessel). However, if the stand-on vessel realizes the other ship is not taking action, it is allowed to take avoiding action itself (Rule 17) [6]. Being the stand-on vessel does not mean you can never move.
Japan has its own law based on these international rules, called the "Maritime Collision Prevention Act." It was promulgated on June 1, 1977, and enforced on July 15, 1977 [7]. The basic principle of this law is that "the more maneuverable ship should avoid the less maneuverable one" [7]. This applies to small dinghies and personal watercraft, but not to windsurfers [7]. This law works alongside the Maritime Traffic Safety Act and the Port Regulations Act to form Japan's three maritime traffic laws. In busy waters, the Maritime Traffic Safety Act takes priority [7].
6. What the Exxon Valdez Accident Shows About Collisions at Sea
Sometimes, even with rules, accidents happen. In 1989, the large tanker Exxon Valdez left its designated route at 11:47 PM to avoid icebergs [8]. The officer on watch noticed the danger and ordered a course change. But at 0:04 AM, before the order was completed, the ship ran aground on a reef [8]. Only a dozen or so minutes passed between noticing the danger and the grounding, showing how quickly things can go wrong [8].
Rudder failures are not just history. In July 2025, the Australian cargo ship Matthew Flinders III lost control for 8 hours [9]. According to the Australian Transport Safety Bureau (ATSB), which investigated the incident, the cause was a loose component connecting the rudder and autopilot [9]. The ATSB also notes that common causes of steering failure include oil leaks, valve malfunctions, and poor adjustment of the levers connecting parts [2]. Other causes can include air in the system, rising oil temperatures, and faulty limit switches [2].
7. A Simple Experiment to Test How a Rudder Works
If you want to explore this further, try a simple experiment. Fill a basin or bathtub with water. Take a plastic ruler or a cutting board and tilt it at an angle in the water, moving it sideways. Feel the force pushing your hand. Compare the force when you tilt it slightly versus when you tilt it sharply. If you tilt it too much, does the force feel weaker? This is a clue for thinking about how the rudder's force changes. Note that real ship rudders stay mostly still in the water flow, so this experiment is slightly different. If you spill water, clean it up.
For more reading, look up the Wikipedia entry for "Rudder" to understand the force and stall angles [1]. You can also read the Wikipedia entry for Japan's "Maritime Collision Prevention Act" to see the specific rules for give-way and stand-on vessels [7]. A question to think about: Next time you see a large ship turning, observe which part of the ship seems to be the center of the rotation.