Some owners have introduced hatchless vessels. In one type of design the cell guides are extended above main deck level so that on deck lashing is also unnecessary. In another type there is also a heightened and ‘streamlined’ foc’sle with a ‘wave breaking’ device intended to keep water away from the main deck of the vessel. In both cases the intention is to avoid the additional labour and delay involved in removing and replacing hatch covers as well as the tedious securing of the containers carried on deck. Ro-Ro vessels (Roll on/Roll off). These vessels are suitable only for cargo which can be driven on/off the ship, such as cars, lorries and cargo on trailers which can be either road trailers or trailers designed solely for shipborne use, being loaded/unloaded in dockside terminals. (“Mafi” trailers). Pure Ro/Ro’s are usually ferries, although on certain trades vessels which are part container vessel/part Ro/Ro can be found. The size of a Ro/Ro is measured in lane metres, which indicate the total length of the marked parking lanes (3.5m wide) available on board. Of importance is also the size of the entrance ramp, its length, width, height of the openings leading into the hull and the total weight it can bear at any one time. The so-called “pure car carriers” also fall into this category. Refrigerated Vessels. These vessels, customarily known as ‘Reefers’, are very specialised ships indeed. They have two or more decks and, as the name implies, are basically ocean going deep freezes. Their holds are insulated, they have an extensive compressor system with which they keep the temperature in the holds at the required (cold) level; different cargoes requiring different temperatures. They are usually very fast (up to 22 or 23 knots), thus reducing the transit time for the frozen or chilled cargo. They also have small hatches and cargo gear designed to operate speedily, thus limiting the time the cargo is exposed to the elements.