Real Life Incident: Hydrodynamic Interactions While Passing Leads To Collision
A container ship (A) had closed to approximately 8 cables astern of a loaded tanker (B) in a restricted waterway. The pilots of the two vessels had made overtaking arrangements; the tanker would move to the north side of the channel and reduce speed, and the container ship would also reduce speed, move to the south side of the channel, and overtake the tanker on its port side.
Ten minutes later, the helmsman on vessel A found it necessary to use a considerable amount of port helm (up to 23°) to maintain the desired heading of 235° Gyro (G). However, this information was not relayed to the pilot, nor did the pilot detect it from monitoring the rudder angle indicator. About one minute later, after passing a green channel buoy, the vessels were beginning to draw parallel to each other. They were now about 75 metres apart. Vessel B had reduced speed and was making 7.3kt, and vessel A was proceeding at 10.7kt.
A few minutes later, vessel B sheered suddenly to starboard. To regain control, the pilot ordered hard-a-port helm and half ahead followed by full ahead. Once the vessel steadied on a course of 236° G, the engine telegraph was reduced to dead slow ahead. Shortly afterward, there was no longer any apparent speed difference between the two vessels; both were proceeding at approximately 8kt.
The pilot on vessel A then requested that vessel B further reduce speed so the vessel A could complete the overtaking manoeuvre. The pilot on vessel B agreed to the request, adding that he had just used ‘full ahead’ power to correct a sheer to starboard. For the next five minutes, vessel A’s propeller pitch was modified incrementally on several occasions, resulting in an overall increase in speed from 8.2kt to 9kt.
The changes were carried out by the OOW, who used his discretion to interpret the pilot’s orders, which were delivered in unquantified terms such as ‘faster’. As its speed increased, vessel A began to experience bank suction aft. The helmsman maintained the desired heading and prevented the bow from moving to starboard by applying more port helm. Again, this information was not communicated to any other members of the bridge team.
A few minutes later the pilot of vessel A asked the pilot on vessel B to further reduce speed. Pilot B replied that he was unable to comply without losing manoeuvrability. Moreover, vessel B’s speed had now increased from 7.3kt to 8.2kt despite the fact that there had been no change from the previous command of dead slow ahead. Despite full starboard helm at this point, vessel B continued to move towards vessel A. For the next two minutes, the distance between the vessels continued to decrease. Even with vessel B’s engine telegraph set to stop, the tanker continued to accelerate to more than 8.5kt. Aboard vessel A, the pilot requested greater speed and eventually full ahead.
With the vessels closing, pilot B asked for full ahead, in an attempt to pull away. Notwithstanding this action, the two vessels collided, making parallel body contact about 9 minutes 40 seconds after the overtaking manoeuvre had begun.
Some of the findings of the official report were:
Neither pilot appreciated early enough the strength of the hydrodynamic forces at work, nor the need for early and decisive action to prevent the vessels from drawing together.
Ineffective bridge resource management and poor communication between the vessels prevented both bridge teams from recognising the developing situation and taking timely action.
When in the confines of a narrow channel, hydrodynamic forces between vessels are greater than when in open water due to the reduced flow capacity around the vessels and through the channel.
When two ships pass or meet in the confines of a narrow channel, the squat experienced by each vessel increases by a considerable percentage.
Hydrodynamic forces experienced by the vessels are proportional to the speed of the vessels through the water and inversely proportional to the distance between the vessels and the under-keel clearance of each vessel.
The overtaking ship’s resistance increases once past the overtaken ship, and the latter’s resistance decreases. This can lead to a ‘trapping situation’ for the overtaking vessel.
It is difficult to predict the onset and magnitude of hydrodynamic forces in the confines of a channel when manoeuvring large vessels.
The hydrodynamic pressure zones around vessels can extend farther than the 100 metres commonly assumed.