A suggestion based on the fact that it seems odd that hatch covers should be charged for

Hatchless container ships first appeared around the beginning of the 1990s. They were intended to make cargo handling more economic. In 1993 and 1994, Howaldstwerke-Deutsche Werft AG (HDW) delivered four hatchless container ships to the Swiss shipping company Norasia Line. These vessels were equipped with an innovative cargo protection system. These 2,780 TEU vessels are a further development of the "ship of the future". Except for holds 1 and 2, which are equipped with pontoon hatch covers to allow the carriage of hazardous materials, these vessels have no hatch covers. Since, on their usual route between Europe and Far East, these vessels are exposed to heavy tropical rain, the shipping company decided to equip holds 3 to 7 with twelve rain protection roofs of lightweight steel construction. Each rain roof rests on the transverse coaming of the container cell guides and is secured by rapid fastenings. The coamings extend in each case up to the highest container which, in some holds, amounts to twelve tiers. Before and after cargo handling, the roofs have to be removed and replaced by on-shore lifting gear. Moreover, the roofs constitute part of the aerodynamic hull shape. Windtunnel testing revealed that the shape of the forecastle deck in conjunction with the rain roofs and the deckhouse will save fuel. The rain roofs protect not only the stowed containers, but the ship as well, as large quantities of rain water in the holds cause stability problems. Other owners of "open top" container ships use high power bilge pumps to combat this problem.