High-cube (HC) boxes contain the secret that reveals that today's mega ships are even bigger than they look, according to French shipping analysts at AXS-Alphaliner.

"Many large containerships are today seen with deck cargoes dominated by 40-ft high cube boxes, that is, with a 9-ft 6-in height instead of the standard 8 ft 6 in on which the TEU capacity counts are based," said an Alphaliner research note.

"With this in mind, a deck cargo of HC boxes on the Emma Maersk class ships totals 3,672 units, which takes into account eight tiers of HC boxes combined with prismatic stowage," said the analysts.

Prismatic stowage is seen almost systematically on these extra wide ships of 22 rows. "Of note, prismatic stowage is usually not taken into account on sketches provided by shipyards offering wide ships (17 to 20 rows), although a common sight on today 17-18 rows ships," they said.

Converted into standard TEU, the HC deck capacity of the Emma Maersk class ships stands at 8,208 TEU, adding to the AXS-Alphaliner estimate of 7,004 TEU below decks and raising total capacity of the ship to just over 15,200 TEU.

But containerships that maximise deck capacity usually reserve the areas for empties for stability sake and to limit the strain on the lashing gear.