Maritime pilotage consultant Captain Don Cockrill dissects the dimensions and forces at play in Ever Given’s troublesome transit of the Suez Canal
In an analysis of the Ever Given grounding, it is worth putting the scale of the navigational challenges of ultra-large container ships (ULCSs) transiting the Suez Canal into some sort of understandable context.
The canal was deepened about 12 years ago to accommodate revised Suezmax vessel limits which are based on a ratio of beam to draft. According to the limits, the maximum allowable beam would be 77.5 m at a draft of 12.2 m and 20-m draft at 50-m beam.
Ever Given thus falls well within these criteria with a 59-m beam and 15.7-m draft.
Similarly, the Suez Canal Authority (SCA) consider Ever Given’s length of 400 m the general maximum for a vessel transiting the canal although longer vessels may be granted special consideration.
These dimensions are set with regard to the canal’s measurements, and the width of the canal is of particular relevance for this incident.
At its surface, the canal is about 313 m wide, however its construction profile means that the sloping sides give a width of about 225 m at an 11-m draft, decreasing to about 121 m across the bottom of the canal. Thus, at 400 m, ships are significantly longer than the navigable width of the canal waterway.
Taking into account speed of travel, the usual transit speed for a ship in the canal and indeed any canal is about 8 knots. A combination of factors dictates this, predominantly the hydrodynamic effects of water displacement as the ship moves along the canal and the resulting huge pressure forces that are generated.
In simple terms, relatively slow speeds are required combined with maintaining a central track within the canal cross-section in order to minimise and maintain a balance of the hydrodynamic forces acting on the sides and bottom of the hull.
However, in addition to these forces, there are the effects of wind and tidal currents to consider.
The estimated lateral surface area of a loaded ULCS such as Ever Given is about 16,000 m2. At 30 knots, wind such as that which has been reported at the time of the grounding – if on the beam – will generate a force on the ship side of about 300 mt.
In terms of tidal forces acting on the vessel, although minimal compared to other parts of the world, there is nonetheless a tidal cycle and associated water flow within the Suez Canal. Even a low current of 0.5 knot will impart a force on a ULCS the size of Ever Given, at a similar draft of 75 mt.
Acting in unison, a combined external lateral force on the vessel of about 375 tonnes (and possibly more) is conceivable under the worst conditions.
These measurements represent more than enough force to cause a deviation from the very specific track that the transit demands.
Once any deviation from the intended track occurs, then an imbalance of hydrodynamic forces simply adds to the problem and thus creates a huge challenge for correcting the vessel’s course and resuming its intended track. It is debatable what effect applied rudder and engine power alone would have under these circumstances.
That said, navigating over-sized vessels in restricted waterways is nothing new, and even the ULCS has become ubiquitous in many ports where they are navigated and manoeuvred in waterways designed for much smaller vessels.
In many waterways, dynamic escort towage – that is the use of a tug made fast aft and which is utilised to assist very effectively in both speed and directional control of the vessel – is commonplace. Media reports to date suggest that dynamic escort towage was not in use on Ever Given’s transit of the Suez Canal.