What Is the Safe Speed of A Ship? COLREGs Rule 6 Explained!

Speed is an important characteristic of a ship. It impacts a ship’s economic performance as well as her ability to avoid danger. Because of the importance of speed to the safe operation of a ship and the fact that there is no numerical value prescribed for safe vessel operation, the International Regulations for Preventing Collisions at Sea, (COLREGS) Rule 6 stipulates the factors that should be taken into consideration when determining the safe speed of a vessel.

According to Rule 6 of the COLREGS, a safe speed should be the speed of a vessel that allows the maximum amount of time for proper action to be taken in order to avoid a collision and stop within an appropriate distance taking into account the external environmental and operational conditions.

With the increasing number of large ships and the speed at which they travel, it is more important than ever to ensure that they are operating at a safe speed. In this article, we will take a look at Rule 6 of the COLREGS, ad explain the factors that are given as guidelines when determining what a safe speed should be.

Safe Speed of A Ship COLREG Rule 6
Safe Speed of A Ship corresponds and is regulated by COLREG Rule 6

What is The COLREGS?

Before a standard set of international rules and practices for ships was made, different maritime nations in different parts of the world had their own rules, conventions, and unofficial ways of doing things.

Because of this, there were inconsistencies and even contradictions, which led to accidents. The International Regulations for Preventing Collisions at Sea (COLREGS) were established to govern the conduct of ships at sea to avoid collisions.

The International Regulations for Preventing Collisions at Sea (COLREGS) were established as an International Maritime Organization convention on October 20, 1972, and went into effect on July 15, 1977. They were intended to update and replace the 1960 Collision Regulations. The regulations have been amended several times since their first adoption.

The provisions contained in the COLREGS must be adopted by each member country that is a signatory to the convention and must also be domesticated in local legislation.  Each IMO member state is required to appoint an “administration” (national authority or agency) for administering the COLREG convention’s rules as they pertain to ships under their flag.  Rule 6 of the COLREGS is concerned with the safe speed of a ship.

What Is the Safe Speed of a Ship?

Determining the ideal speed for a ship is not always simple. At sea, speed limitations are less common than they are on land, and much depends on the surroundings as well as the weather and environmental circumstances at the time. Mariners are expected to make an accurate assessment of the current circumstances and adjust speed as necessary.

As a mariner, it’s important to know the safe speed of a ship. Depending on the vessel size and type, different speeds are required in order to maintain a safe distance from other vessels and objects in the water The answer to this question depends on many factors, including the type of ship, the weather conditions, and the traffic situation In general, however, there are some guidelines that can help you determine a safe speed for your ship.

To ensure the safety of everyone on board, it is crucial for mariners to adhere to certain rules. Limiting speed is one of the most crucial criteria. A ship moving at a safe speed has adequate time and space to precisely analyze and assess risky circumstances, allowing her to react to them carefully and effectively. The International Regulations for Preventing Collisions at Sea, (COLREGS) is an important regulation that dictates how fast a ship can safely travel.

Determining the Safe Speed of a Ship.

A watchkeeper is required under Rule 6 of the COLREGS  to determine what they consider as safe speed for their own vessel, taking into account the environment at that time and the situation they are expecting.

In several collisions, it has been found that excessive speed has been a contributing factor and that if either ship slows down, its closest point of approach (CPA) will increase, hence a slower speed is typically considered to be a “safe speed.” Slower speed, therefore, decreases the risk of a collision.

Reduced speed is considered a safe speed because it provides additional time for thought and action. Consequently, time to think and act, provided when speed is reduced, is of the utmost importance as excessive speed and insufficient time can adversely compromise any risk assessment methods. It also enables more effective stopping. Additionally, in the event of a collision, the consequent damage will be considerably less.

It should always be borne in mind that maintaining a safe speed is the officer on watch’s responsibility and they should always be at liberty to reduce speed when necessary.

ARPA and RADAR are not perfect. They could completely miss some targets or might portray huge targets as feeble echoes. GPS and ECDIS are two examples of navigational aids that can be questionable. And therefore, the practice of relying on only one tool should be avoided.

Always keep an eye on your speed because conditions at sea are always changing and what is considered safe in one circumstance may not be so in another. At sea, things can change quickly and therefore commercial considerations should not qualify as a justification for continuing at a dangerous speed.

Factors To Be Considered When Determining Safe Speed

Similar to automobiles, ships are required to restrict their speed to prevent collisions. Rule 6 states that you must continuously analyze the current situations and circumstances. For instance, a safe speed outside of fog may exceed 10 knots. In or on approach to fog, a few knots may be a safe speed.

The Rules specify the variables that must be considered when determining what a safe speed should be in particular circumstances. Several of them expressly pertain to radar-equipped ships which emphasize the significance and importance of employing “all available means”. In determining a safe speed, the following factors shall be among those taken into account:

By all vessels with and without Radar.

  • Visibility conditions (fog, mist, rain, darkness, etc.)
  • Wind conditions
  • Tides and currents
  • Vessel maneuverability
  • The amount of traffic on the waterway
  • Navigational hazards
  • Water depth

Visibility and Weather Conditions

Visibility is traditionally the most essential factor in determining the safe speed of a vessel and must always be taken into account.  When evaluating a safe speed, the wind, sea, and current conditions, as well as the proximity of navigational hazards, must be noted and taken into account. 

The clutter must be adjusted appropriately so that targets may be spotted, sea clutter is utilized to reduce the interference caused by waves, while rain clutter is used with caution to reduce the interference caused by rain.

Background lighting can impair night vision, and backscatter can cause small or even large ship lights to be obscured by shore lighting. More look-outs could be needed, and usage of the radar and a reduction in speed might be explored.

The Amount of Traffic on the Waterway

Assessment of traffic density is essential since the chance of a collision increases with the density of traffic. In high traffic, it is important to remember that slowing down will give you more time to assess the situation.

Vessel Maneuverability

The maneuverability of the vessel, taking into account the vessel’s stopping distance and turning circle for its condition (loaded, lightship). Maneuverability impacts on vessel’s ability to get out of the way and hence affect the risk of collision

Water Depth

Draft requirements, as insufficient under-keel clearance increases the likelihood of grounding; slower speed provides more time to assess the situation; and if a grounding cannot be averted, the resulting damage will be reduced. If a vessel’s draft prevents it from navigating outside of a channel, its maneuvering choices are restricted to slowing down or stopping. A significant course change to avoid a collision may result in grounding. Lack of UKC and proximity to banks may also produce squat and banking, respectively.

Radar Usage

The navigator must be aware of any blind sectors the X-Band or S-Band radar system may have as one of the major limitations of marine radars. S-Band has superior long-range and weather-penetration vision. X-Band provides a superior vision of the surrounding environment but has a restricted range and is utilized for navigation.

 The selected range scale determines the nature of the available information to the navigator. The short range provides high resolution and permits the detection of tiny things. Long-distance scales compromise precision in favor of early detection. It is optimal for the navigator to switch scales frequently, or if they have multiple sets, to utilize a different range scale for each set.

The vessel’s location and possible activities in the region, such as fishing fleets. Season plays a significant role in determining whether undetected boats or ice may be present.

As the number of vessels increases, accurate radar plotting becomes more challenging. Automated radar plotting aids (ARPA) facilitate the undertaking. Visibility can be reliably measured by noting when a vessel is first spotted or, at night, when its lights are first observed.


In conclusion, it is important to adhere to Rule 6 of the COLREGs in order to ensure the safety of the vessel, the crew, and the environment. The safe speed of a ship is the speed at which the vessel can take proper action to avoid a collision and stop within a safe distance, taking into account all of the external conditions.

Rule 6 offers a list of factors to be aware of (see above), but this is not exhaustive because it is hard to forecast what other unusual circumstances may need to be considered. Every day at sea is unique.

External conditions that need to be taken into consideration include weather conditions, visibility, traffic situation, etc. By taking all of these factors into account, the safe speed of a ship can be determined. If a ship does not adhere to Rule 6, the consequences can be serious. Therefore, it is imperative that all ships adhere to Rule 6 of the COLREGs in order to ensure the safety of the vessel, the crew, and the environment.

Rule 6: Safe Speed | COLREGS In Depth