Handbook of the Nautical Rules of the Road by Llana & Wisneskey

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Rule 6 -- Safe Speed


INTERNATIONAL

INLAND

Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions.

Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions.

 

Like Rule 5, Rule 6 begins with the words "Every vessel shall at all times," indicating its universal application, in good visibility as well as poor, and like Rule 5, it places a great deal of responsibility on the good judgment of the mariner.

How much does speed affect safety? Even if excessive speed is not the most glaring cause of maritime accidents, there have been very few collisions between stopped vessels; vessels involved in a collision are apt to have been moving too fast.

Some mariners are reluctant to change speeds and so pay more attention to factors that support their cruising speed and less attention to factors that indicate a need to slow down. It is very important that you give due consideration to any factor suggesting a change in speed. Because a closing situation may develop rapidly, the person in charge should feel free to call for a reduction in speed without having first to notify another person (for example, the master or engineer). Some powerplants are capable of quicker speed changes if certain preparatory steps are taken. If a speed change becomes likely (or even a real possibility), the person in charge should provide timely notification to the engineers so that they can prepare the engines. Rule 19(b) specifically requires that power-driven vessels have their engines ready for immediate maneuver when in an area of restricted visibility. (The engines on smaller power-driven vessels, and on larger automated vessels, are normally controlled from the helm position and respond immediately.)

While not directly relevant to collisions between vessels, a vessel's speed also is roughly proportional to its wake. The vessel operator will be liable for damages caused by a wake that is excessively high (for the circumstances).

Safe Speed versus Moderate Speed

The safe speed rule first came into effect in 1977 for the International Rules and was adopted for the Inland Rules in 1980. Older repealed rules called for "moderate speed," but only in restricted visibility. Rule 6 uses the term "safe speed" and applies in all conditions of visibility. The term "moderate speed" was replaced by "safe speed" because for many conditions the term "moderate" was too restrictive.

The Rules now recognize speed as an important factor in preventing collisions in good visibility as well as poor. Newer vessels are bigger and faster and may take longer to stop and maneuver. Smaller vessels are also much faster, some capable of speeds greater than 50 knots. A vessel's performance limit is often no longer the controlling factor in good visibility; other conditions must be considered.

Proper and Effective Action

The first objective of maintaining a safe speed is to permit the vessel "to take proper and effective action to avoid collision." To be able to maneuver as prescribed by the Rules, the vessel must be moving slowly enough to control its forward motion. In some cases, it must also be moving fast enough for the rudder to effect a turn promptly.

A vessel passing close to a bank (as in a channel) or close to another vessel generates hydrodynamic forces that can pull the vessel off its course. If the speed is great enough, these hydrodynamic forces can overpower the correcting forces of the rudder. Vessel operators are expected to be familiar with these effects and to reduce their speed sufficiently to maintain positive rudder control.

Even vessels to which the Rules assign a right-of-way must proceed at a safe speed, which sometimes involves planning for the unexpected. Because Rule 2 sometimes makes a departure from the Rules madatory, and Rule 17(b) requires action by the stand-on vessel when the risk of collision becomes extreme, a fast-moving stand-on vessel may find the action is expected under the Rules not to be the "proper and effective" action needed to avoid a collision. Rule 17 requires a stand-on vessel to maintain its course and speed after risk of collision has been established. A too-high initial speed will therefore place the stand-on vessel in a dangerously awkward position.

Stopping Distance

The second objective of requirng a safe speed is to enable the vessel to be stopped "within a distance appropriate to the prevailing circumstances and conditions." In most cases where the risk of collision exists, a course change will be the most common action. However, if maneuvering room is limited or if visibility is poor, stopping the vessel (perhaps in conjunction with a turn) could be the best way to avoid or minimize damage.

Before radar was common, an old rule of thumb was that a vessel should be able to stop within half the range of visibility. Thus, two vessels on opposite courses would be able to stop before colliding. This rule of thumb was not widely accepted by the courts, which wisely decided that the many factors involved warranted a case-by-case consideration.

Older rules concerning moderate speed (applied only in restricted visibility) included a statement about stopping or maintaining bare steerageway. Although Rule 6 does not explicitly contain the same provision, Rule 8 requires vessels to slow or stop to avoid collision or to give more time to assess the situation. Rule 19 requires that vessels in areas of restricted visibility encountering vessels forward slow to the bare minimum needed for steering, or stop altogether.

 

INTERNATIONAL

INLAND

In determining a safe speed the following factors shall be among those taken into account.

In determining a safe speed the following factors shall be among those taken into account.

 

Most of Rule 6 presents factors that must be considered in determing safe speed. These factors are not necessarily listed in order of importance, and the list is not exhaustive. Paragraph (a) contains factors to be considered by all vessels; paragraph (b) contains factors that are to be considered by vessels with operational radar.

 

INTERNATIONAL

INLAND

(a) By all vessels:

(i) the state of visibility;

(a) By all vessels:

(i) the state of visibility;

 

Visibility has traditionally been the most important consideration in setting the speed. Rule 19 (Conduct of Vessels in Restricted Visibility) restates the necessity for limiting speed and adds that power-driven vessls shall have their engines ready to maneuver. That Rules also mandates further precautions when another vessel is detected ahead.

 

INTERNATIONAL

INLAND

(ii) the traffic density including concentrations of fishing vessels or any other vessels;

(ii) the traffic density including concentrations of fishing vessels or any other vessels;

 

Traffic density is important because the probability of a collision increases with the density and because the probability that three or more vessels will share risk of collision also increases. In this latter special circumstance (see Rule 2), some departure from the Rules may be required, leading to unusual and perhaps unexpected action. Areas containing many small vessels require extra caution since those vessels are often difficult to detect either by radar or by sight. In either case, slowing will give extra time to assess the situation. Rule 8(e), on slowing or stopping to avoid a collision or to assess the situation, will probably come into play in these situations.

 

INTERNATIONAL

INLAND

(iii) the maneuverability of the vessel with special reference to stopping distance and turning ability in the prevailing conditions;

(iii) the maneuverability of the vessel with special reference to stopping distance and turning ability in the prevailing conditions;

 

The vessel's operator cannot establish a safe speed without knowing how far the vessel will travel before stopping, for any normal loading condition or speed. Stopping distances will vary substantially depending on whether the vessel is turning or proceeding in a straight line. Many vessels will stop most quickly when put into a sharp turn. Large tankers are a good example; because their great bulk dwarfs the propeller, turning their broadsides against the line of travel stops them more efficiently than would running their engines astern alone (doesn't work in a narrow channel, of course).

Tug and towboat operators should be aware of their vessels' stopping characteristics both without barges and with different numbers of barges.

The maneuvering characteristics of most larger vessels are required to be posted on the bridge. Operators should learn the characteristics before the information is needed.

 

INTERNATIONAL

INLAND

(iv) at night the presence of background light such as from shore lights or from backscatter of her own lights;

(iv) at night the presence of background light such as from shore lights or from backscatter of her own lights;

 

Background lights and backscatter decrease the effectiveness of a lookout by sight and therefore require a proportional decrease in speed. A small vessel has a particular problem because the vessel's own lights are close to the operator. Careful design of the navigation light arrangement will minimize backscatter and reflection from the vessel itself.

 

INTERNATIONAL

INLAND

(v) the state of wind, sea and current, and the proximity of navigational hazards;

(v) the state of wind, sea and current, and the proximity of navigational hazards;

 

The need to reduce speed in the face of mounting adversity is obvious (we hope).

 

INTERNATIONAL

INLAND

(vi) the draft in relation to the available depth of water.

(vi) the draft in relation to the available depth of water.

 

Draft restrictions relate to speed in several ways. If there is little underkeel clearance, it is likely that shallower water is nearby. It is easier to avoid running aground from a low speed, and if a grounding cannot be avoided, the damage will be less.

If a vessel's draft exceeds the depth outside a channel, the vessel will be limited to straight-line stopping, which is less effective than a combination of slowing or reversing engines and turning away. Hence a lower speed is usually required. Rule 9 gives further direction for vessels operating in narrow channels.

In shallower water, a vessel's speed introduces hydrodynamic forces that are not present in deeper water. As a vessel moves forward, the water in front moves away and then closes in after the stern passes. In shallow water, especially in channels, the water ahead of the vessel is squeezed quickly through the relatively small space around the hull to the stern, moving fastest where it is squeezed the most. This happens under the bottom of the vessel in shallow water or, if the vessel is near a bank, then along that side. The fast flow of water creates lift, in the same manner that lift is created by a wing or a sail. On an airplane, the lift is directed up; on a sailboat, to the side; and on a vessel moving through shallow water, the lift that is produced is directed down or toward the bank. The force on the moving vessel pushes it closer to whatever it is close to.

The effect on the vessel is called "squat," and it increases as the underkeel clearance decreases and as the vessel's speed increases. Thus, a vessel that has ample clearance when moving slowly through shallow water may at high speed scrape the bottom. The hydrodynamic effect of high speed through a channel may cause a vessel to be pulled toward or into the bank or may pull two vessels passing close together off course.

 

INTERNATIONAL

INLAND

(b) Additionally, by vessels with operational radar:

(b) Additionally, by vessels with operational radar:

 

Radar-equiped vessels are obligated to use their radar in restricted visibility unless there is a compelling reason not to. Rules 5, 6, 7, and 19 together place great emphasis on the effective use of radar.

Vessels using radar in restricted visibility are justified in going somewhat faster than vessels without radar, but not as fast as they would go in good visibility. In open waters a ship using radar may proceed at a relatively high speed, providing the speed is adjusted appropriately upon detection of another vessel.

 

INTERNATIONAL

INLAND

(i) the characteristics, efficiency and limitations of the radar equipment;

(i) the characteristics, efficiency and limitations of the radar equipment;

 

Radar equipment varies greatly in power, sophistication, antenna installation, and so forth. The mariner needs to understand these qualities and limitations thoroughly. For instance, a vessel's course might be changed regularly to ensure that any vessel in a blind arc, which may be caused by a vessel's masts or other structures, could be detected early.

There are two basic types of marine radar--navigation and search. Navigation radars transmit short-wavelength radio frequencies, and search radars use long-wavelength transmissions.

Navigation radars send out short high-frequency pulses. These rapid and sharply defined pulses bounce back from surfaces facing the transmitter, yielding a very accurate and detailed image of the surrounding area. Because of their lower power and higher pulse repetition rate, these navigation radars--also called three centimeter (3 cm), X-band, and high frequency radars--have a limited range.

Search radars, on the other hand, pack a lot of power into their low-frequency, long wavelength signals and consequently are able to look into and beyond weather. When they reflect off a target, the signal returns to the receiver with more power, and they can detect objects at further ranges. These radars go by various names--search, ten-centimeter (10 cm), S-band, or low-frequency radars. The name used is a matter or personal preference and does not distinguish variations.

 

INTERNATIONAL

INLAND

(ii) any constraints imposed by the radar range scale in use;

(ii) any constraints imposed by the radar range scale in use;

 

No matter how good a radar set might be, the range scale selected determines the nature of the information available to the operator. Short range scales give good resolution and enable the detection of small targets; long range scales sacrifice detail to gain early detection. Radar equipment is most effective if the operator switches scales regularly, or if the operator has two or more sets and uses a different range scale on each. To the extent that different range scales are not available, speed should be reduced.

 

INTERNATIONAL

INLAND

(iii) the effect on radar detection of the sea state, weather, and other sources of interference;

(iii) the effect on radar detection of the sea state, weather, and other sources of interference;

 

Vessel speed should be reduced when interference (caused by large waves, heavy rain or snow, or the like) impairs the performance of the radar.

 

INTERNATIONAL

INLAND

(iv) the possibility that small vessels, ice and other floating objects may not be detected by radar at an adequate range;

(iv) the possibility that small vessels, ice and other floating objects may not be detected by radar at an adequate range;

 

The location of the vessel and the season of the year are important in judging whether undetected vessels or ice may be present.

 

INTERNATIONAL

INLAND

(v) the number, location and movement of vessels detected by radar;

(v) the number, location and movement of vessels detected by radar;

 

Accurate radar plotting becomes more difficult as the number of vessels increases. Automated radar plotting aids make the task easier.

 

INTERNATIONAL

INLAND

(vi) the more exact assessment of the visibility that may be possible when radar is used to determine the range of vessels or other objects in the vicinity.

(vi) the more exact assessment of the visibility that may be possible when radar is used to determine the range of vessels or other objects in the vicinity.

 

The observed radar range of a vessel can be correlated to visibility by noting when the vessel can first be sighted. At night, when the vessel's lights can first be seen, the radar range of the vessel equates the visibility (assuming that the visibility is not so good that masthead light intensity becomes the controlling factor.

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