This concerns the Roll on / Roll off (Ro/Ro) principle in which mobile material is driven onboard a ship, directly or on mafis, without the use of cranes. Self propelled materials can either be driven or towed directly on board. To this end, special Ro-Ro vessels have been developed, not to be confused with car carriers. The difference between the two is that a ro/ro vessel does not exclusively accept these kind of cargoes.