There are several kinds of cargo ship, each specializing in carrying various goods in tailored ways. But in general they can be identified by three key features.
The first of these are deck cranes, static lifting platforms that specialize in distributing freight on and off the vessel, as well as into the ship’s deep cargo hold. There are two main sorts: single, side-mounted deck arms – which often pivot on a hydraulically powered base, or inverted U-frames, where a grabbing arm moves horizontally between two fixed struts.
Another major feature, not surprisingly, is a large hold. These are typically located in the centre of the deck – though additional holds are sometimes included – and cut through several tiers of the vessel to create a massive, insulated storage bay. Containers are lowered into a hold via deck cranes or others situated on the port.
Lastly, cargo ships can be distinguished by their generally huge size – particularly notable in the height of their hulls when unladen. The latter design works to offset the extreme weight they carry when loaded, with a vast proportion of the hull submerged. This helps to maintain stability while manoeuvring under full load, the physics of which we look at in closer detail in the “How cargo ships stay afloat”.
Mass – The weight of the ship’s cargo acts upon it through the ship’s centre of gravity (G). When level, this is directly through the vessel’s centre point.
Buoyancy – The weight of the cargo is counteracted by the force of the water that’s displaced by the ship’s hull, which acts upward through the centre of buoyancy (B).
Metacentre – When the ship tilts, the centre of buoyancy shifts to one side, creating a metacentre (M) that compensates for the offset, keeping the vessel stable.
The cargo ship family
2. Oil tanker – Oil tankers are notable for their lack of cargo hatches and minimal external handling gear. They consist of a series of internal, discrete tanks, a double hull and they feature very tall hulls.
3. Dry bulk carrier – Long and relatively flat vessels – like big barges – dry bulk carriers specialize in transporting coal, grain, ore and other loose, dry products,
4. Container ship – The easiest carrier to recognize, container ships carry freight containers openly on their large decks. They may or may not have deck cranes, but will sit low in the water when fully laden.
5. Roll-on/off ship – This vessel specializes in carrying moving vehicles, with a hydraulically powered rear ramp leading to multiple tiers of parking. Like ferries they sit high in the water. Read 10 Facts about Shipping Containers