These vessels are designed to take containers only, and range in size from the small coasting container vessels up to the large and fast ocean going container vessels. Their holds are “cellular”, that is to say they have vertical frames or guides into which the containers are slotted. They will be able to handle containers of International Standards Organisation (ISO) dimensions, generally 20 or 40 feet in length. The size of the vessel will be expressed by the number of “twenty foot equivalent units” (TEU) she will be able to carry. Many small and medium sized container vessels are “Self sustained” – that is, they have cargo gear on board with which they can load and discharge their containers. All large container vessels cater for refrigerator-containers by supplying electric power outlets to which may be connected the container’s integral refrigeration compressor. Highly specialised container ships for trades where there is a high demand for chilled cargo are equipped with central refrigeration machinery which delivers cooled air to insulated containers through ‘portholes’ in the units although this system is now being phased out . A feature of container ships is the arrangement of their holds. They are completely open hold ships which means that the hatches give access to the whole of the hold area, so that there is no need for any stowage work and as the cell guides are erected in the holds no lashings are required below decks. The hatch covers of the holds are either slab or pontoon type and are fitted out so as to carry containers up to six or seven tiers high. Below deck, the boxes are secured by the cellular arrangement but on the hatch covers they must be lashed in order to prevent movement. The modern deep sea gearless container ship carries between 3000 TEU and up to about 8000 TEU. It will have a service speed of between 22 and 26 knots. Panamax vessels are the largest that can navigate the Panama Canal and are about 4500 TEU, less than 295 metres LOA, 32 metres beam and 12 metres draft on tropical loadline. Post-Panamax vessels are too large for such transit. The largest vessels of about 8000 TEU have dimensions of 347m LOA, 42.8m beam, 14.5m draft.