Source: Adapted from Clarkson Research.
Larger ship capacities require deeper drafts since capacity is a function of weight, and that weight influences buoyancy. Depth requirements are deeper than design drafts because a ship in movement in a harbor usually requires an additional two feet of draft because of hydrodynamic factors (known as the “squat effect”). While for a 1,000 TEU containership, the average draft is 8.3 meters, these figures reach 16 meters for ships above 14,000 TEU. Smaller ship classes are associated with a larger variation in draft since they have several design specifications, such as coastal ships, reefer ships, or geared ships. Larger ships above 8,000 TEU have a uniform design with little draft variations being observed. The 4,000 TEU range is the class having the largest variation, with an average draft of 12.5 meters, which corresponds to the service depth of the old locks of the Panama Canal (Panamax). The New Panamax standard that was set in 2016 with the expansion of the Panama Canal has a draft of around 15 meters, allowing ships with a capacity of about 12,000 TEU.
The draft appears to be peaking around 16,000 TEU as larger capacities are obtained by lengthening ships, but not much by widening them. For instance, a 16,000 ship of the Marco Polo class is about 25% longer than a ship of about 9,000 TEU, but only 18% wider. Ships of the “Triple E” class, which was introduced in 2013, require less draft (14.5 meters) than ships between 10,000 and 16,000 TEU. The reason is that they are not much longer (only a few meters) but are close to 60 meters in width. This substantially improves their buoyancy but requires portainers (container cranes) with a greater reach to handle their 23 container width. The limitations of the Suez Canal (Suezmax) at around 17 meters are an important draft constraint for containerships.