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

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Rule 7 -- Risk of Collision


INTERNATIONAL

INLAND

(a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt, such risk shall be deemed to exist.

(a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt, such risk shall be deemed to exist.

 

Nothing grips a mariner's attention so fast as a late determination of risk of collision. The principles commended by this Rule can be taken lightly only at the risk of some very unwanted exciting moments on the water. The determination of risk of collision, timely or otherwise, triggers a number of other Rules on which the mariner must then act.

Rule 12, for instance, requires action by one or two sailing vessels approaching each other on a collision course. Rule 14 requires action in the case of power-driven vessels approaching each other on reciprocal or near-reciprocal course so as to involve the risk of collision. Rule 15 applies to power-driven vessels crossing so as to involve risk of collision. Rule 19 prescribes action by vessels in restricted visibility when risk of collision exists and in certain cases when the degree of risk is unknown. The existence of risk of collision is implicit to the operation of other Rules: Rule 13 (Overtaking), Rule 16 (Action by Give-way Vessel), Rule 17 (Action by Stand-in Vessel), and Rule 18 (Responsibilities between Vessels).

It is not surprising then, that Rule 7, like Rules 5 and 6, begins with the words "Every vessel shall." These three Rules set up the mariner to take the proper and effective action required by the remaining Steering and Sailing Rules. The full responsibility for Rules 5, 6, and 7 is not excused or lessened for any vessel.

Like Rule 5 (Lookout), Rule 7 is an information-collection and analysis rule, although the tracking function of Rule 7 may involve more analysis than does the detection function of Rule 5. Besides the beginning mandate "Every vessel shall," Rule 7 also shares with Rule 5 the phrase "all available means appropriate to the prevailing circumstances and conditions." Rule 5 requires the proper lookout "to make a full appraisal of the situation and of the risk of collision." The transition from Rule 5 detection to Rule 7 tracking is not a sharp one.

The taking of compass bearings is one of the most important means of determining risk of collision. The technique depends on good visibility, on the vessel being tracked maintaining a constant course, and on several observations. Observations may be taken with a simple hand-bearing compass, a pair of binoculars incorporating a magnetic compass in its optics, or on larger vessels, a bearing or azimuth circle or an alidade on a fixed gyro-compass repeater. Compass bearings will be discussed more fully below.

In restricted visibility, the primary tracking instrument is radar, if fitted and operational. Radar should also be used to track a vessel in good visibility in open areas after the vessel has been sighted visually. Paragraph (b) of Rule 7 gives specific guidance on the use of radar.

Having sighted a vessel, you may contact it by radiotelephone to confirm its intentions. The radiotelephone is especially valuable on U.S. inland waters where several vessels may be involved, maneuvering room is limited, and courses are frequently changed. In the United States, the Vessel Bridge-to-Bridge Radiotelephone Act and implementing regulations require larger vessels to monitor channel 13 (VHF-FM), which is used for broadcasting and exchanging navigation information.

The International Telecommunications Union's Radio Regulations, Appendix 18 (q), now designates channel 13 for use on a worldwide basis as a navigation safety communications channel, primarily for intership communications. The International Maritime Organization's Global Maritime Distress and Safety System (1988 amendments to SOLAS Convention) includes a requirement for all passenger vessels and for cargo vessels 300 gross tons and above to be capable of transmitting bridge-to-bridge (channel 13) communications. No international requirement to guard the channel has been established. Channel 16 can be used if nobody answers on channel 13.

Once you have detected and tracked another vessel in your vicinity, how do you judge whether risk of collision exists? What, in fact, is "risk of collision"? The Rules do not say. Risk of collision certainly exists for two vessels whose paths would take them to the same spot in two minutes. On the other hand, risk of collision would not exist for two slow-moving vessels 20 miles apart heading for the same spot of water, nor would it practically exist for two vessels passing a half mile apart in a busy harbor. What might be the risk of collision for two large ships would probably not be risk of collision for two small vessels in the same situation.

A number of factors are involved in such an assessment:

  • Closest distance of approach
  • Type of waterway
  • Vessel size and maneuverability
  • Speed
  • Distance out from closest point of approach
  • Relative bearings

The closest distance of approach is perhaps the prime element in the risk of collision formula. A collision occurs when the distance of closest approach goes to zero, but a risk of collision may exist when the distance of closest approach is somewhat greater. A passing within one vessel length would certainly involve risk of collision, but how much space is necessary for risk of collision not to exist?

Rule 34 of the Inland (but not International) Rules prescribes whistle signals for power-driven vessels "meeting or crossing at a distance within half a mile of each other." Each vessel indicates whether it intends to leave the other on its port or starboard side. As a general rule then, we can conclude that on inland waters risk of collision exists for vessels whose paths will take them within half a mile and which are within hearing range of each other. What is the hearing range? Annex III prescribes ranges for vessels' sound-signal appliances (horns): one-half mile for vessels less than 20 meters in length, up to two miles for vessels over 200 meters in length.

International Rule 34 prescribes different sorts of maneuvering signals and requires them when vessels are "in sight of one another," although there would be no sense in giving a signal when the nearest vessel in sight is ten miles away, since the sound signal will be very unlikely to travel as far as five miles. On the open ocean, large vessels traveling at full speed should probably consider that risk of collision exists if their projected paths would bring them within a mile of each other.

The type of waterway plays a part in the calculation of risk. On the open ocean the distance of closest approach triggering risk of collision is greater than in confined waters because on the ocean it is easier to keep well clear.

Vessel size and maneuverability have a substantial impact on risk of collision. A small vessel that can stop or turn in its own length has a much smaller zone of risk than a large vessel that may need a mile or more to stop and only begins to turn after the rudder is put over.

Speed expands the zone in which risk of collision exists. Higher speeds give the mariner less time to refine the accuracy of vessel path predictions (remember Rule 6).

Vessels are at risk of collision when they come within a certain distance of their closest point of approach. For medium-size ships moving at average speeds in open water in good visibility, risk of collision would probably become a concern at about five miles out from the closest point of approach. Keep in mind that risk of collision does not arise suddenly like the light from an on-off switch, but rather increases or decreases gradually, like the light from a dimmer switch. For small boats maneuvering in a boat basin the distance out from closest point of approach could be a stone's throw.

An analysis of the Rules themselves will tell us at what distance from closest point of approach a risk of collision arises. As we noted earlier, risk of collision triggers the operation of a number of other Rules. Specifically, the existence of risk of collision obligates certain actions. In crossing situations, for example, the duties are different for each vessel; in a meeting situation, on the other hand, the duties for each vessel are the same. In all cases, however, risk of collision must be discovered early enough for each vessel to be able to carry out its obligations under the Rules.

When one vessel (give-way vessel) is required to keep out of the way of another (stand-on vessel), the give-way vessel must take "early and substantial action to keep well clear" (Rule 16). That doesn't tell us much except that action should be taken soon after risk of collision is established.

Rule 17 is more helpful. It provides that the stand-on vessel may take action to avoid collision as soon as its operator realizes that the give-way vessel is not moving out of the way. Before the stand-on vessel does this, however, it must hold its course and speed long enough for the give-way vessel to predict its path and maneuver clear. The mariner should have a good idea of how long these events will take for the circumstances and types of vessels involved. Remember that the master of the stand-on vessel probably will be more conservative in judging when the give-way vessel should begin taking action, since he or she must hold on and wait, while the master of the give-way vessel knows better what will happen (use you VHF-FM channel 13).

Finally, the relative bearings of two vessels affect the degree of risk. Two vessels meeting on near-reciprocal courses would close relatively rapidly, because their closing speed would be the sum of the two speeds. The risk of collision would arise while while they were still relatively far apart. On the other hand, where one vessel is overtaking another on nearly the same course, the closing speed would be the difference between the the individual speeds. Unless one is traveling a great deal faster than the other, it would take a long time for the overtaking vessel to draw abeam of the other. In the overtaking situation, the vessels would be relatively close together before risk of collision arose. Crossing situations would be somewhere between meeting and overtaking.

All of these factors are interdependent and must be considered as a whole and in context of the circumstances. The above examples and distances are merely ballpark figures for good conditions.

Either vessel is, of course, free to act before risk of collision exists in order to avoid it altogether. Also, in some cases where risk of collision exists, the give-way vessel may not have to alter its course or speed to keep "well clear" (Rule 16) of the stand-on vessel, as long as its course and speed will "result in passing at a safe distance" (Rule 8). The closest point of approach may represent a safe passing distance while at the same time triggering a risk-of-collision situation. Two vessels in a narrow channel is an example. Risk of collision and the need to maneuver can be distinguished; risk of collision and the need for extra care cannot.

Our discussion so far has been based on the premise that each vessel involved knows where the other is and generally knows where it is going and how fast. Rule 7 states that if there is any doubt, if the information at hand is not accurate or complete, then risk of collision shall be deemed to exist.

Doubt commonly arises under conditions of restricted visibility. Rule 19(e) implies that in restricted visibility when another vessel is detected ahead, risk of collision shall be deemed to exist until the mariner can positively determine that it doesn't--a guilty-until-proven-innocent standard.

Doubt can also occur because of instrument and measurement errors in tracking another vessel. Know the limitations of your instrument and measurement techniques and include them in the assessment of the situation (much as celestial navigation positions are plotted as circles rather than points).

Many other factors can cause doubt--wind and currents, the movement of another vessel in a busy harbor, and the like. For answers, use your radiotelephone.

 

INTERNATIONAL

INLAND

(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.

(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.

 

The second paragraph of Rule 7 covers use of radar in assessing risk of collision. The requirements extend Rule 5 and 6 to define further the use of properly operating radar in avoiding collision. Rule 5 introduced the idea of long-range scanning for maintaining lookout, and Rule 6 made specific mention of using different radar range scales.

The value of radar in assessing risk of collision in poor visibility is obvious. Rule 19 (Conduct of Vessels in Restricted Visibility) requires that a vessel in restricted visibility determine whether risk of collision exists when it detects by radar alone the presence of another vessel. But radar is also valuable in clear weather after a target has been sighted visually, being better able than the human eye to measure range and other distances.

If the vessel's radar is capable of both true-motion/north-up display and relative-motion/head-up display, the operator must select the display better suited to the operating conditions. Because of the adoption of both national and international standards, the accuracy of both displays is the same. In general, true-motion (sea- or ground-stabilized) radars are preferred for navigation and piloting in confined waters as the position of the oberver's own ship moves in accordance with its own path. On the other hand, relative-motion displays allow the observer to assess more quickly the movement of other vessels in relation to his or her own movement. All but the smallest vessels are required to have radars stabilized in azimuth (that is, in the horizontal plane). Radars without compass stabilization are almost useless for determining the actions of other vessels without highly accurate maneuvering board plotting or unless the vessel's heading does not vary by a degree or two at the very most.

Radar plotting (or equivalent systematic observation) is required by Rule 7. It is not enough to just look at the radar, unless plotting would not be helpful, as for example, on meandering inland rivers where observations of the vessel's position relative to the channel or banks may be more informative. Plotting is usually appropriate in relatively open water.

Plotting is not required if "equivalent systematic observation" is used. These other observation techniques include manual and automatic (computerized) radar plotting aids or the listing of bearing, range, and time at regular intervals. Plotting by the vessel's operator in congested waters may take so much time that it becomes counterproductive. In such cases automated radar plotting aids (sometimes improperly called collision-avoidance systems) are especially appropriate.

 

INTERNATIONAL

INLAND

(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.

(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.

 

While Rule 7 requires the full and proper use of radar, it also warns against relying on radar for more information than it can realistically give. The mariner who assumes an approaching vessel will pass well clear after making a couple of long-range radar observation is inviting danger and violating Rule 7.

Distances magnify small errors, and errors are almost inevitable because of the imprecision of observations made from a moving vessel. Nor can you assume that the other vessel is maintaining a constant course and speed. Regular and consistent checking of observations is imperative.

Many collisions continue to happen because vessel operators base their actions on faulty assumptions. Rule 7 calls attention to the danger of basing actions on scanty information, requiring that the operator be patient and monitor other vessels in the vicinity until the risk of collision can be determined with a satisfying degree of certainty. (And remember, as long as there is doubt, you must assume that risk of collision does exist!)

 

INTERNATIONAL

INLAND

(d) In determining if risk of collision exists the following considerations shall be among those taken into account:

(i) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change;

(ii) such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range.

(d) In determining if risk of collision exists the following considerations shall be among those taken into account:

(i) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change;

(ii) such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range.

 

The classic test of risk of collision is given special attention in the final paragraph of Rule 7: if your vessel is holding course and speed, and you take several compass bearings on another vessel and those bearings are all about the same, then you will collide with the other vessel if it is also holding course and speed and if one does not take evasive action. Even if the compass bearings do change, there may still be the potential for collision.

If the other vessel is maneuvering, the compass-bearing test doesn't work. Also, the test works only when the size of the vessels is small when compared to the distance between them. If you take compass bearings to the bow of an approaching ship from the bridge wing at the stern of your ship and note that the compass bearings are changing, then all you know is that a collision is probably not set up between your stern and the other vessel's bow--your bow and its stern, however, may have other ideas.

If the vessel does not have a compass suitable or convenient for taking bearings, other reference points on the vessel can be used to sight on approaching vessels. If, as the other vessel approaches, it remains lined up with the chosen reference points, then risk of collision exists. This technique, of course, is simply a tool to aid the mariner. Like all tools, it has its limitations and should not be relied upon as conclusive.

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