Part B – Steering and Sailing Rules
Section I – Conduct of Vessels in any Condition of Visibility
Rule-7: Risk of Collision
(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.
The existence of risk of collision is implicit to the operation of other rules such as Rule 13 (Overtaking), Rule 16 (Action by Give-way Vessel), Rule 17 (Action by Stand-in Vessel), and Rule 18 (Responsibilities between Vessels).
Risk of collision in Head-on situation: 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 they were still relatively far apart.
Risk of collision in Overtaking situation: Where one vessel is overtaking another on nearly the same course, the closing speed would be the difference between the individual speeds. Unless one is travelling 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.
Risk of collision in Crossing situation: Crossing situations would be somewhere between meeting and overtaking.
Avoid Development of Risk of Collision: Either vessel is, of course, free to act before risk of collision exists in order to avoid it altogether. If RoC with another vessel doesn’t exist, then there is no necessity to take action as a give way vessel.
Doubt: If there is any doubt or the information at hand is not accurate or complete, then risk of collision shall be deemed to exist. In case of doubt, the OOW must call Master immediately as per STCW Code, Ch-VIII. It’s understood that doubt begins when an OOW starts to feel confusion about a situation and feels whether he should call Master or not. Beyond this point, the OOW’s delay in calling Master could be considered as negligence and at times, calling Master late might result in a critical situation when Master may not have many options on hand to avoid a close quarter’s situation or collision.
All available means: Examples are –
• Look out
• Compass repeaters
• Sound / light signalling equipment
• Radars / ARPAs
• The radiotelephone may be used to advantage in certain circumstances for the purpose of clarifying a situation involving two vessels and indicating intentions via well regulated VTIS. However, collision avoiding actions shall not be taken basis VHF agreement between two vessels since it’s never guaranteed that the second vessel is the vessel that you’re trying to communicate with as many vessels make funs out of it. Valuable time may be wasted in attempting to make radio contact instead of concentrating on the assessment of collision risk and the need for action. Reference is also made to the further danger of proposing, by VHF radio, to take action which is not in compliance with the Collision Regulations. In RoR, there is no mention about the use of RT to determine RoC, rather appropriate sound signals are prescribed for the same.
• As AIS can be used to advantage for collision avoidance, such as in determining the identity of another vessel and in more rapid detection of changes of heading, vessels may be expected to make use of the equipment in appropriate circumstances. The AIS target data (CPA, TCPA etc.) MUST not be taken as correct as it is found accurately only in ARPA/Radars. An OOW should acquire the targets on Radar/ARPA using Radar functions instead of just simply clicking the AIS Symbol of a target echo on radar screen to get CPA, TCPA, etc.
Appropriate to the prevailing circumstances: The use of the available means to determine risk of collision depends on the prevailing circumstances and conditions, such as daytime or night time, low traffic or heavy traffic, coastal waters or ocean waters, clear visibility or restricted visibility, etc.
(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.
Proper use of Radar Equipment: Rules 6, 7, 8 and 19 contain specific references to the use of radar and there is an important implied reference in Rule 5. Proper use of radar may include optimum setting of all controls, appropriate range scale & display, etc.
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.
Long range scanning: In any kind of weather, long range scanning is a very useful tool to an OOW. If a target is detected at long range, an OOW can do little alteration of course / speed to avoid a RoC. However, RoC in a head-on situation remains the same even at long range. If two high-speed vessels meet head-on, their relative speed becomes very high and TCPA reduces significantly, therefore an OOW gets less time to takes actions. But if the situation is detected at long range, the OOW gets sufficient time to take early action. That’s why the OOW should cultivate a habit of doing long range scanning of targets frequently in his/her watch.
True Motion Display: The OOW must select the radar display better suited to the operating conditions. In general, true-motion (sea- or ground-stabilized) radars are preferred for navigation and piloting in confined waters as the position of the observer’s own ship moves in accordance with its own path. An alteration of course made by another vessel moving at fairly high speed is likely to be more readily apparent on the true motion display from the change in direction of the echo trail. True motion is generally more suitable for use with the lower range scales in congested waters rather than in the open sea.
Relative Motion Display: 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.
Radar Plotting: It is not enough to just look at the radar. Plotting is usually appropriate in relatively open waters.
True & Relative Radar Plotting: The principles of relative plotting should be understood by all observers, as this is the method which enables the closest position of approach to be determined.
The true plot is simpler to understand, and is considered by many to be superior when there are several targets on the screen. Alterations of course, or speed, by the observed vessel, carried out simultaneously with, or shortly after, an alteration by own vessel, are likely to be more readily detected by a true plot than by a relative plot.
Use of Vectors: The use of vector is very common. You can use this function for multiple targets on the radar screen. Usually, the user selects relative vector to see how a target will pass her own ship actually. Alteration of course and speed, however, don’t get updated in vector immediately, it takes sometimes. A prudent user sometimes changes the vector setting from relative to true to see a target’s actual course and thus it helps to find target’s aspect. The use of appropriate vector length is also very important. Longer vector-length of numerous targets on the radar screen lead to an unnecessary clutter and confusion to the OOW.
Equivalent Systematic Observations: 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 are especially appropriate.
(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.
Assumption & Scanty Information: The word ‘Scanty’ stands for ‘Insufficient’. Assumptions made on the basis of scanty information have been a contributory cause of many collisions in both clear and restricted visibility.
Rule 7 warns mariners 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.
Several observations should be taken at short and regular intervals to reduce the effects of these random errors when there is a possibility of a close quarter’s situation developing. Bearings taken relative to the ship’s structure can be very misleading in determining whether risk of collision exists.
(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;
The closest distance of approach is perhaps the prime element in the risk of collision situation. A collision occurs when the distance of closest approach goes to zero. Sighting an approaching vessel against components of the ship’s structure may give a rough indication of whether there is risk of collision and may provide sufficient basis for deciding whether to make a bold alteration to pass astern of a vessel being overtaken or crossing from the starboard side.
A number of factors are involved in such an assessment:
Closest distance of approach
• Type of waterway – 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 manoeuvrability – Vessel size and manoeuvrability 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 – 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).
• Distance out from closest point of approach
• Relative bearings
Appreciably change of bearing: Here bearing means true bearing but not relative bearing. Due to appreciable change of bearing, the change in target data and aspects will become evident, such as a noticeable change in CPA, TCPA, BCR, etc. However, an appreciable change of bearing at greater ranges does not necessarily mean that there is no risk of collision. The other vessel may be making a series of small alterations which have not been observed.
(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.
Large vessel or tow: Even if the compass bearings between two vessels do change, there may still be the potential for collision. A tow has two components – the towing vessel and the vessel being towed. If a RoC doesn’t exist with the towing vessel, it may exist with her tow. The same explanation goes for a large vessel – bearings between bow and stern differ significantly. Mariners must consider the length of an approaching vessel before determining that a RoC doesn’t exist with the other vessel’s bow and stern or with the towed vessel and her tow.
Risk associated with changing bearing at close range: An appreciable change of bearing at close ranges does not necessarily mean that there is no risk of collision. An appreciable change of bearing at short range may be associated with a dangerously close passing distance. The rate of change of bearing increases as two vessels pass too close to each other. Such a passing distance will bring danger of collision.
If an overtaking vessel with greater speed, approaching from starboard quarter of a vessel being overtaken with lower speed, suddenly starts crossing the bow of the vessel being overtaken assuming that the overtaking vessel can execute this action safely, a collision may happen if overtaking vessel’s steering or M/E fails at that point due to the fact that both vessels are at close range. Also, at close range in confined waters, the effect of interaction between two vessels is significant and a collision is always possible.