Tweendecker

Tweendeckers are general cargo ships with two or sometimes three decks. The upper deck is called the main deck or weather deck, and the lower deck is the tweendeck. Cargo such as bales, bags, or drums can be                                  stacked in the tweendeck space

Today the tweendecker is more likely to be referred to as a multi-purpose ship, a term that is quite vague and has different meanings to different people. While some do apply it to traditional tweendeckers, others believe that for the term to be valid, some additional feature must also be present. There are, for instance, vessels that have folding or moveable tween decks and some also which have a RORO ramp included into the design.

The term general purpose ship is usually applied to 'tweendeckers, which are sometimes referred to as multi-purpose ships. The word 'tweendecker' comes from the fact that these ships have decks fitted between the floor of the ship and the main weather deck. They usually comes in sizes between 5,000 and 25,000 tonnes dwcc.

Multipurpose cargo types Virtually all multi-purpose ships are able to carry containers both in the holds and on deck. Almost certainly a multi-purpose ship will be equipped with cargo handling gear. Today most vessels are equipped with cranes, although derricks are by no means obsolete. Crane capacity will vary, but around 30 tonnes is probably the most common choice unless the vessel is intended for trades where heavy lift cargo is frequently carried.

Tramp voyages As well as being employed in general cargo liner services, multi-purpose ships are often chartered for tramp voyages to carry cargo that is not easily containerised. They are used in the bulk trades for cargoes such as steel and forest products and, for vessels equipped with folding tween decks, grain, fertilisers and coal are also popular cargoes.

Stowage & Holds To allow for the variation in stowage factors, many multi-purpose ships also have moveable bulkheads that can be adjusted to prevent cargo shift or to allow separation. The number of holds will vary between two and five and folding MacGregor hatches are the norm. Ships with a low number of holds often include a very long hold that can accommodate cargoes of exceptional length that needs under deck stowage.

Tween decks add to a ship's versatility because apart from the obvious need to have a simple way of separating consignments, there is a limit to how many bags, drums, crates etc one can place one on top of another before the bottom tiers collapse under the weight of those above. Heavy lift ships Heavy lift ships are very similar in design to multi-purpose ships but are specially constructed to carry heavy loads such as very large pieces of machinery. This requires strengthened decks and cranes that can lift much more than the multi-purpose ship. These ships usually have two cranes, each capable of lifting around 200 tonnes alone or 400 tonnes when working in tandem. One such ship has cranes that can lift 600 tonnes apiece or 1200 tonnes when worked together.