Part B – Steering and Sailing Rules
Section I – Conduct of Vessels in any Condition of Visibility
Rule-6: Safe Speed
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. In determining a safe speed the following factors shall be among those taken into account:
Safe Speed: The word ‘safe’ is intended to be used in a relative sense. A speed could reasonably be considered safe in the particular circumstances.
Every vessel shall at all times: This rule applies in all conditions of visibility to all vessels. The Officer of the watch should not hesitate to use the engines in case of need. However, timely notice of intended variations of engine speed should be given when possible.
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 manoeuvre 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.
To avoid hydrodynamic effects in shallow waters, vessel operators are expected to be familiar with these effects and to reduce their speed sufficiently to maintain positive rudder control.
Stopping Distance: The second objective of requiring 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 manoeuvring 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.
Other rules: 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.
Factors to consider: Most of Rule 6 presents factors that must be considered in determining safe speed. These factors are not necessarily listed in order of importance, and the list is not exhaustive.
(a) By all vessels:
(i) The state of visibility;
(ii) The traffic density including concentrations of fishing vessels or any other vessels;
Note: Traffic density is important because the probability of a collision increases with the density
(iii) The manoeuvrability of the vessel with special reference to stopping distance and turning ability in the prevailing conditions;
Note: Stopping distances will vary substantially depending on whether the vessel is turning or proceeding in a straight line. The manoeuvring characteristics of most of the larger vessels are required to be posted on the bridge. Operators should learn the characteristics before the information is needed. The distance that a vessel will cover in a crash stop before being brought to rest from full speed is likely to be between 5 and 15 ship lengths, depending upon speed, displacement, type of machinery, manoeuvrability of the vessel, Type of engines (steam, diesel, turbine, etc), Power of the engine, Stopping distance, Turning circle, Fast or slow vessel, Manoeuvrability with regard to Condition of loading, Draught, Trim, Etc.
(iv) At night, the presence of background light such as from shore lights or from back scatter of her own lights;
Note: 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.
(v) The state of wind, sea and current, and the proximity of navigational hazards;
Note: The need to reduce speed in the face of mounting adversity
(vi) The draft in relation to the available depth of water.
Note: Draft restrictions relate to speed in several ways. If there is little under keel 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 within the channel, which is less effective than a combination of slowing or reversing engines and turning away. Hence a lower speed is usually required.
In shallower waters, a vessel’s speed introduces hydrodynamic forces that are not present in deeper waters. The effect on the vessel is called “squat,” and it increases as the underkeel clearance decreases and as the vessel’s speed increases. 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.
(b) Additionally, by vessels with operational radar:
Note: The term ‘operational radar’ means radar in use. In open waters a ship using radar may proceed at a relatively higher speed, provided the speed is adjusted appropriately upon detection of another vessel. Radar equipment varies greatly in power, sophistication, antenna installation, and so forth. The mariners need to understand these qualities and limitations thoroughly.
(i) The characteristics, efficiency and limitations of the radar equipment;
Characteristics: Radar may fail to detect small targets, alterations of course made by other vessels are usually less apparent and the use of radar bearings is more likely to result in a faulty appreciation of risk of collision than visual bearings taken by compass.
Efficiency: The efficiency of the equipment for the purpose of detecting the presence of other vessels and determining whether risk of collision exists must also be related to the competence of those observing it and the way it is being used. Range performance, Minimum range, Diameter of PPI, Scale of display, Plotting facilities, Range accuracy, Heading indicator, Bearing accuracy, Azimuth stabilization.
Limitations: 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.
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.
(ii) Any constraints imposed by the radar range scale in use;
Short range scales give good resolution and enable the detection of small targets; long range scales sacrifice detail to gain early detection. To the extent that different range scales are not available, speed should be reduced.
Constraints may be imposed by every range scale that can be used. When using the longer range scales definition and discrimination are reduced and small targets are less likely to be detected, whereas shorter range scales do not permit early detection of targets and do not enable the observer to obtain an overall assessment when several vessels are in the vicinity. The range scale which is most suitable for the locality should be selected but the scale should be changed at regular intervals. The scale should not be changed when there is a dangerous target at close range. When two radar displays are available and in use it may be advantageous to select a different range scale on each display to avoid the necessity of switching scales.
(iii) The effect on radar detection of the sea state, weather and other sources of interference;
Vessel’s speed should be reduced when interference (caused by large waves, heavy rain or snow, or the like) impairs the performance of the radar.
Interference: The effect of rain clutter is much less when using l0 cm wavelength than when using 3 cm. This is often also the case with clutter caused by sea return. Vessels fitted with two radars, one of each wavelength, would be expected to make use of the 10 cm wavelength for detecting other vessels in conditions likely to cause severe clutter, particularly in heavy tropical rain squalls.
(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.
(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. The greater the number of targets indicated on the radar display the more difficult it may be to determine risk of collision and to assess the effect of possible maneuvers, although some radar systems are capable of providing information of this kind.
(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.
When fog or mist is considered likely to develop, the radar should be in operation. It may be possible to determine the extent of the visibility by observing the radar ranges at which other vessels or navigation marks are first visually sighted, or at which they disappear from view. At night the probable presence of fog may be indicated by failure to see the lights of a vessel which gives a strong echo within the normal visual range.