COLREGS Rule 19: Navigating in Restricted Visibility with Radar

How to Apply the Collision Regulations When You Cannot See Other Vessels

Fog rolls in unexpectedly. Rain reduces visibility to a few hundred metres. Darkness falls and you can no longer see approaching vessels until they are dangerously close. In these conditions, the normal rules of collision avoidance, where you can see another vessel and determine who gives way, no longer apply. Instead, you enter the domain of Rule 19 of the International Regulations for Preventing Collisions at Sea (COLREGS).

Rule 19 governs the conduct of vessels in restricted visibility, and understanding this rule is essential for anyone operating a radar-equipped boat. Unlike the visual rules in Section II of COLREGS, where vessels can be classified as “give way” or “stand on”, Rule 19 creates a different framework based on radar detection and systematic avoidance.

What is Restricted Visibility?

COLREGS defines restricted visibility as any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms, or any other similar causes. The key point is that you cannot see other vessels at a sufficient distance to apply the normal visual rules of collision avoidance.

This does not mean zero visibility. Restricted visibility exists whenever visibility is significantly reduced from normal. If you would normally expect to see another vessel at two miles but can only see one mile due to haze, you are operating in restricted visibility for practical purposes.

Night-time presents an interesting case. Technically, you can see vessel navigation lights at night, so pure darkness is not restricted visibility in the COLREGS sense. However, the combination of darkness with any reduction in visibility, perhaps light rain or sea mist, quickly creates restricted visibility conditions.

The Rule 19 Framework

Rule 19 applies to vessels not in sight of one another when navigating in or near an area of restricted visibility. The rule establishes several key requirements.

Safe speed: Every vessel must proceed at a safe speed adapted to the prevailing circumstances and conditions of restricted visibility. If you cannot see far enough to stop within that distance, you are going too fast.

Ready to manoeuvre: A vessel with engines must have them ready for immediate manoeuvre. This is not the time to be running on autopilot with the engine at idle.

Consider circumstances: The rules in Section I (Rules 4-10), including safe speed and risk of collision determination, continue to apply. Rule 19 adds specific guidance for radar-detected vessels, but does not replace the fundamental principles.

Detecting Vessels by Radar Alone

Rule 19(d) addresses the critical situation where you detect another vessel by radar alone, meaning you cannot see them visually. In this case, you must determine if a close-quarters situation is developing and/or risk of collision exists.

This determination requires systematic observation of the radar target. You need to track its bearing and distance over time to establish whether it is approaching, and if so, what its closest point of approach (CPA) will be. Modern radar systems with ARPA or MARPA can calculate this automatically, but you need to understand what the displayed information means.

If you determine that a close-quarters situation is developing, you must take avoiding action in ample time. The word “ample” is important; last-minute manoeuvres in restricted visibility are dangerous because the other vessel cannot see what you are doing.

How to Alter Course Under Rule 19

Rule 19(d) provides specific guidance on how to alter course when avoiding a vessel detected by radar. These are not arbitrary rules but are designed to reduce the risk of both vessels turning towards each other.

For a vessel forward of the beam (ahead or on your bow), avoid altering course to port, except when overtaking. The logic is that if both vessels turn to starboard (their natural avoidance direction under visual rules), you will pass safely. If you turn to port while they turn to starboard, you could turn into their path.

For a vessel abeam or abaft the beam (alongside or behind you), avoid altering course towards them. If a vessel is to your port side, do not turn to port. This prevents you from turning into a vessel that may itself be altering course.

Notice that the rule says “avoid” these alterations, not “never make” them. In some circumstances, such as when constrained by other traffic or shallow water, you may have no choice but to make an otherwise prohibited alteration. But you should only do so having considered the implications and having no better option.

Speed Reduction

Rule 19(e) addresses what to do if you hear a fog signal apparently forward of your beam, or cannot avoid a close-quarters situation with a vessel forward of your beam. In either case, you must reduce speed to the minimum at which you can be kept on course, except where you have determined that risk of collision does not exist.

Speed reduction makes sense for several reasons. It gives you more time to assess the situation. It reduces the severity of any collision that does occur. It improves your radar’s ability to detect and track targets by reducing sea clutter from your own wash.

Many operators are reluctant to slow down in poor visibility, feeling that maintaining speed will get them through the area faster. This is dangerous thinking. Safe speed in restricted visibility is often considerably slower than your normal cruising speed.

Practical Application with Radar

Applying Rule 19 effectively requires competent use of your radar equipment. You need to be able to detect targets at long range, track them systematically, determine CPA and time to CPA (TCPA), and make timely decisions about avoiding action.

The RYA RADAR Course teaches these skills through practical simulator exercises. You will practise tracking multiple targets, calculating collision risk, and deciding when and how to take avoiding action. These are skills that are difficult to develop while simultaneously skippering a vessel in real restricted visibility.

Understanding radar limitations is equally important. Small vessels may not return strong echoes. Targets can be lost in sea clutter or rain clutter if controls are not adjusted correctly. Shadow sectors behind your own vessel’s structure can hide approaching traffic. The RADAR course covers all of these limitations so you know when to trust your radar and when to be cautious.

The Yachtmaster Connection

Rule 19 scenarios are a standard part of RYA Yachtmaster examinations. Examiners may present you with radar-based restricted visibility situations and expect you to demonstrate competent decision-making.

Candidates who have completed the RYA RADAR Course are significantly better prepared for these scenarios. The systematic approach to target tracking and the practice with prohibited alterations provides exactly the knowledge examiners are testing.

Even if you are not pursuing Yachtmaster certification, the standard represents what a competent skipper should know. If you operate in waters where restricted visibility occurs, and that includes most UK coastal waters, you owe it to yourself and other water users to understand Rule 19.

Combining with Other Sensors

While radar is your primary tool in restricted visibility, it should be used in conjunction with other available information. AIS (Automatic Identification System) can provide identity and course information for vessels that may be difficult to track on radar. VHF radio allows you to contact other vessels to confirm intentions.

However, rely primarily on radar. Not all vessels carry AIS, and even those that do may have it switched off or malfunctioning. VHF conversations take time and can be confusing in multi-vessel situations. Your radar display shows you what is actually happening in real time.

Building Your Skills

Rule 19 competence comes from understanding the rule, knowing your radar equipment, and practising the decision-making process. The one-day RYA RADAR Course provides all three elements.

Based at Saxon Wharf Marina in Southampton, Ocean Sports Tuition offers the course with modern radar simulators and instructors experienced in Solent navigation, where restricted visibility is a regular occurrence. To book your place, call us on 02381 242159 or visit our website.

You may also benefit from RYA Yachtmaster Theory, which covers COLREGS comprehensively, and the RYA Advanced Powerboat Course, which includes practical night navigation where radar skills are applied in real conditions.

For the official rule text and RYA guidance, see the RYA RADAR Course page.