What is Overtaking?
Overtaking means coming up from a direction more than 22.5° abaft of another vessel.
Thus, the two essentials are:
1. Distance must reduce.
2. The faster vessel must be in the stern light sector of the vessel being overtaken.
If any one of the above two conditions are not satisfied, the situation cannot come under purview of rule 13. Thus, if a ship is in the stern light sector of another vessel but is on a course that increases the distance between the two vessels, it is not governed by this rule. On the other hand, there might be a very large difference of course. But the ships may be governed by this rule if the above two conditions are satisfied.
How do you know if other vessel is overtaking you?
A vessel is overtaking you if her relative bearing is more than 112.5° and the distance is reducing.
How do you know if you are overtaking another vessel?
In the night time a vessel can consider her to be overtaking another vessel if she can see only stern light of another vessel and the distance is reducing. In daytime she will know this only by radar plotting (calculating the aspect of vessel ahead).
The rules are founded on common sense and good seamanship. Frequently they state that if you are unsure about a particular situation, then you should err on the side of caution. (Examples are: if you are in doubt whether you are technically the overtaking vessel, assume that you are; and if you are not sure whether a collision risk exists, assume that it does.)
Rule 13: Overtaking
a. Notwithstanding anything contained in the Rules of Part B, Sections I and II, any vessel overtaking any other shall keep out of the way of the vessel being overtaken.
b. A vessel shall be deemed to be overtaking when coming up with another vessel from a direction more than 22.5 degrees abaft her beam, that is, in such a position with reference to the vessel she is overtaking, that at night she would be able to see only the stern light of that vessel but neither of her sidelights.
c. When a vessel is in any doubt as to whether she is overtaking another, she shall assume that this is the case and act accordingly.
d. Any subsequent alteration of the bearing between the two vessels shall not make the overtaking vessel a crossing vessel within the meaning of these Rules or relieve her of the duty of keeping clear of the overtaken vessel until she is finally past and clear.
The overtaking rule dominates over all the other ‘steering and sailing’ rules other than Rule 19. This means that a fast-moving sailing vessel may well be required to give way to a slower power driven vessel. So much so that a vessel not under command and making way through water, if overtaking another vessel must keep clear of the vessel being overtaken. The idea that the vessel which is overtaking has to keep clear is simple, straight and logical. It is quite the same on road as well. At sea, you are regarded as ‘overtaking’ if you are approaching another vessel from anywhere within an arc of 67.5° either side of dead astern (stern light sector i.e. relative bearing more than 112.5°).
Responsibility of vessel ‘A’:
- ‘A’ should observe caution while ‘B’ overtakes, till ‘B’ is finally past and clear.
- With the prevailing data available, ‘A’ should assume that most probably ‘B’ would alter to port at some stage (towards common waypoint. It is always good to keep situational awareness on, completely, particularly in coastings and approaches). A, in observance of good seamanship, anyway assume that ‘B’ became reasonably clear at 1830, (there could be a new vessel in place of B) ‘A’ should reduce own speed to allow ‘B’ to reach first, treating her as a crossing vessel under a new situation. Use of attraction seeking signals must be made in case of doubt. General principle of ROR, that whenever a vessel is in doubt, she shall assume responsibility. Extreme caution and reduction of speed to allow more time are the effective tools. Where a clear understanding of rules is so important, existence of gray areas can’t be ruled out.
‘B’ must closely monitor the speed and distance of ‘A’, approach with caution. She may use signals to attract attention if needed. This is because there has been a change of status and responsibility. Now ‘A’ has the responsibility of keeping clear. ‘B’ being a ‘stand on vessel’ now must not alter further to port for a crossing vessel ‘A’ on her port side. Even if A had incorrectly analyzed the situation, B would eventually be required to take action under the rules even as the stand-on vessel in a crossing situation. Rule 17 would allow B to take action to avoid collision by its maneuver alone when it became apparent that A was not taking appropriate action and would require B to take action to avoid collision when it became apparent that collision could not be avoided by the action of A alone.
Ship to ship interaction – Overtaking:
Ships floating free in water have their area of influence, which must be respected by the ships in vicinity. Once close enough in this zone, even if the ships try to steer away, the separating part of other ship acts as an attracting magnetic pole. Under such influence the ships may find themselves losing the maneuvering capabilities.
It is never enough to ‘just miss’ another vessel. Rule 8 specifies that any action taken to avoid collision should result in the vessels passing ‘at a safe distance’.
This is particularly true when a large vessel is overtaking or being overtaken by a much smaller one, because it is under these circumstances that interaction caused by pressure waves around their hulls is likely to occur.
When a relatively large ship is overtaking a smaller one, the latter tends to sheer across the bow of the former. Where the two vessels are very close, the effect can be so great that the smaller vessel loses all control. A Master, in order to impress his ship mates, try and go close to a ship from same company, or a ship with familiar crew. These experiments can prove very costly.
In the three figures showing the Hydrodynamic interaction the ship ‘A’ is a faster vessel, overtaking ship ‘B’ from her starboard side. The minus sign (-) indicates fall in water level and plus sign (+) indicates rise in water level, decreasing and increasing the pressure respectively. A corrective helm may be needed. A vigilant officer must take the appropriate action.
In these figures bow of the overtaking ship is seen experiencing sheer towards ship being overtaken. While passing abeam, both of the ships will experience strong turning force.
Counter helm is necessary. In the last figure, bow of ship being overtaken will experience sheer towards the stern of the overtaking ship. Corrective helm is necessary.
Variation of pressures occurs due to the relative positions, sizes and speeds of the two vessels during overtaking. The ships’ officers must be well versed with these effects. This will enable them in identifying dangerous hydrodynamic interactions while executing a maneuver in an overtaking situation. One should also remember that the shallow water effects and bank effect may also be present in addition to the inter-ship interactive forces. Ships, in general, must remain in the centre of the channel (depth wise). Failure to do so, could expose either ship to bank effect, leading to a sheer across the path of the other ship or grounding subsequently. Speed should be planned to be low to reduce the interactive forces. The reason being there is, plenty of reserve power for corrective ‘kicks ahead’.
When overtaking a ship, the following points also should be considered:
- the duration for which the other ship will be in influence field;
- available room for each ship during above period;
- relative speed;
- sensitive phases of interaction;
- if any alternative action would be better; and
- if any common way point or termination of a TSS is being approached.
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