Fog banks often roll in surprisingly fast, and heavy rain or snowfall can cut visibility drastically within minutes. For the German SBF exam (Sportbootführerschein, the boating license), the topic of restricted visibility is a fixed part of the syllabus, and in practice it matters even more: if you do not know how to behave once you can barely see other vessels, or cannot see them at all, you are taking on serious risk. This article explains what restricted visibility means in legal terms, which rules apply at sea and on inland waterways, and how to behave correctly as a recreational skipper in practice.

What counts as “restricted visibility”?

Restricted visibility exists whenever visibility is reduced by fog, rain, snowfall, haze, or similar causes. There is no fixed distance in meters that defines it. What matters is whether you can still keep a proper lookout with a genuinely clear field of view: if visibility no longer allows you to spot an unexpectedly appearing vessel early enough to react under the normal rules for vessels in sight of one another and avoid a collision, the special rules for restricted visibility apply. That threshold can already be reached in moderate fog, and it is not tied to one specific distance, since it also depends on your speed, traffic density, and the waters you are in.

Important: the rules for restricted visibility apply regardless of whether it is day or night, and regardless of whether you can currently see other vessels yourself. As long as you cannot be sure that other vessels see you in time and that you see them in time, you are operating in restricted visibility.

In practice, restricted visibility shows up in very different situations: the classic early-morning fog over a lake in early autumn that only lifts around midday, dense sea fog that rolls in along the coast within minutes, or a sudden heavy rain or snow shower in the middle of the day. What they all have in common is that they can worsen visibility abruptly while you are already underway. That is exactly why it is worth learning the following rules not just for the exam, but as a fixed mental checklist you can call up immediately when it actually happens.

Safe speed and lookout

The first and most important principle applies equally at sea and on inland waterways: you must adapt your speed to the visibility conditions. A “safe speed” in fog means you are able to react in time and, if necessary, come to a stop in time when another vessel or an obstruction suddenly appears. Rule of thumb: better to run noticeably slower than to hope for visibility that may no longer be there.

This also means keeping a heightened lookout, using every means available: sight and hearing, and radar or AIS if you have them on board. Engine noise makes it harder to hear sound signals from other vessels, or acoustic aids such as whistle buoys and bell buoys. It is therefore worth slowing down and throttling back the engine as much as possible so you do not miss acoustic signals.

COLREG Rule 19: behavior at sea in restricted visibility

On sea waterways, Rule 19 of the International Regulations for Preventing Collisions at Sea (COLREGs, known in Germany as the KVR) specifically governs conduct in restricted visibility. Its key requirements:

  • Every vessel must proceed at a safe speed adapted to the circumstances of restricted visibility.
  • A power-driven vessel must have her engines ready for immediate maneuver.
  • When complying with the basic rules on lookout, safe speed, and assessing risk of collision, the special circumstances of restricted visibility must be duly taken into account. The steering and sailing rules for vessels in sight of one another (such as stand-on and give-way vessel), by contrast, do not apply as long as the vessels are not in sight of each other.
  • A vessel that detects another vessel solely by radar must determine whether a close-quarters situation is developing or risk of collision exists. If so, it must take avoiding action in ample time, and if altering course for this purpose, must as far as possible avoid: an alteration of course to port for a vessel forward of the beam, other than a vessel being overtaken, and an alteration of course towards a vessel abeam or abaft the beam.

This radar-specific rule mainly concerns larger vessels equipped with radar. For recreational boats without radar, the practical takeaway is the same: if you become aware of a possible danger only acoustically or via AIS, reduce speed and adjust course as cautiously as possible, and avoid sudden, unpredictable maneuvers that would surprise other vessels.

Sound signals in restricted visibility

So that other vessels can detect you even without visual contact, sound signals are mandatory in restricted visibility. Vessels underway (a power-driven vessel making way or stopped and making no way, or a sailing vessel) sound their signal at sea at intervals of not more than two minutes. A vessel at anchor, by contrast, draws attention to herself by ringing her bell, at shorter intervals of not more than one minute. The exact tone sequences for sea and inland waters, including the most common mix-ups between the two, are covered in detail in our article Sound signals for the SBF exam.

Practically speaking: even small recreational boats should carry a working horn or fog horn loud enough to be heard, kept within easy reach so it is ready to use the moment fog rolls in unexpectedly.

One detail that is often overlooked in practice: in restricted visibility you must show your navigation lights, even during daylight hours. Navigation lights are not only meant for darkness; they are mandatory whenever visibility is reduced, whether that means dense fog at ten in the morning or a snow squall in the afternoon. Which lights apply to which type of vessel is explained in detail in our article Navigation lights explained.

Restricted visibility on inland waterways

Inland waterways have their own regulations for navigating in poor visibility. Larger vessels there are generally only permitted to continue if they are equipped with functioning radar approved for inland navigation and matching radio equipment. Most recreational boats and small craft, however, do not carry this equipment.

For you as a recreational skipper, that means in plain terms: if you cannot use radar, you should, as soon as visibility becomes significantly restricted, head for a safe mooring or anchorage at the earliest opportunity and make fast there, rather than continuing your passage. When doing so, adapt your speed on the approach to the visibility conditions, other traffic, and local circumstances, and once stopped, avoid blocking the fairway as much as possible, so that other vessels navigating by radar can still pass safely. If you regularly cruise busy inland waterways, this principle will already be familiar from dealing with commercial shipping traffic: it is better to give way clearly and early than to hope for a close pass.

In practice this also means: if you are planning a trip and the forecast suggests fog, build enough slack into your schedule that you can wait it out if needed without being under time pressure. Fog on rivers and lakes forms especially often in the early morning and during transitional seasons, but it frequently burns off again over the course of the morning.

A typical example from practice: you are cruising a canal when a fog bank suddenly settles over the water ahead of you and visibility drops to just a few boat lengths. Without radar on board, the right response is not to feel your way forward cautiously, but to reduce speed immediately and significantly, sound the required signals, and head promptly but under full control for the nearest suitable spot to make fast or anchor, rather than trying to push through the fog bank on your own.

Practical tips for your own safety

Beyond the formal rules, a few simple habits help make sure a trip in restricted visibility does not turn into a real risk:

  • Reconsider the trip early: if the forecast calls for fog, plan your route so you can wait it out in a harbor or a sheltered mooring if needed.
  • Log your position: regularly record your position, course, and speed, whether on a chartplotter or through dead reckoning, so you always know where you are even without visibility. Our article on modern navigation explains how GPS, chartplotters, and VHF radios work together.
  • Throttle back to listen: sound signals and the wash of other vessels are hard to hear over a wide-open throttle.
  • Do not push on hoping for the best: if visibility gets so poor that you can no longer navigate safely, anchoring or mooring is the better choice than continuing an uncertain passage.
  • Use known landmarks and buoyage: buoys, beacons, and distinctive shoreline features help you orient yourself if you already know the waters. Understanding the lateral buoyage system is useful background knowledge here.

These principles align with the general right-of-way rules that apply in good visibility, but in fog they are supplemented by the extra caution that follows from reduced perception.

How this comes up in the exam

The theory exam usually tests the underlying logic: what speed is required in restricted visibility, which sound signals must be given, and how a vessel that detects another only by radar or by ear should behave. Once you have internalized the core principles, safe speed, a heightened lookout, correct sound signals, and navigation lights even by day, you can work out most exam questions on the topic logically instead of memorizing isolated facts. Restricted visibility is part of the official ELWIS catalog for both the SBF Binnen (inland) and the SBF See (coastal) exam. With the Boatpass app you can train exactly these questions and immediately see which connections still need more practice.

Conclusion

Restricted visibility from fog, rain, or snow demands one thing above all from you as a recreational skipper: adapted, cautious behavior instead of routine. Reduce your speed, keep a heightened lookout, sound the required signals, and show your navigation lights even during the day. If you lack the necessary equipment, such as radar on inland waterways, heading for a mooring or anchorage in good time is the safer choice over pressing on uncertainly. Once you have internalized these principles, you will get through even an unexpectedly rolling-in fog bank safely, and you will keep a clear head both in the exam and out on the water.