Dunnage is the name for the materials used in holds and containers to protect goods and their packaging from moisture, contamination and mechanical damage.
Dunnage may include plastic films, jute coverings, tarpaulins, wood (wooden dunnage), rice matting, nonwovens, liner bags or also inlets etc.. Depending on the use to which it is put, dunnage may be divided into floor, lateral, interlayer and top dunnage.
On general cargo ships, the most important of these is floor dunnage. It protects moisture-sensitive cargoes, such as bagged cargo (e.g. coffee, cocoa) or bales (e.g. tobacco, tea) from sweat, which forms on a ship’s sides and runs off over the decks. The floor dunnage „lifts“ the cargo off the deck, so that water is able to flow off without coming into contact with the cargo. In the case of very sensitive cargoes, the space between cargo and deck has to be enlarged or at least great care must be taken to ensure that the cargo does not under any circumstances come into contact with the deck over which sweat may flow. This is achieved by a double layer of dunnage or criss-cross dunnage. The first layer of dunnage must be laid in the correct direction, depending on the design of the ship. In ships with lateral water drainage (bilges), the first layer must be laid crosswise, while in ships with fore and aft water drainage (wells), the first layer must be laid lengthwise. Criss-cross dunnage must be laid close enough together to prevent bags, bales or other cargo from sinking into the holes under the pressure of the cargo and making contact with the deck.
To protect sensitive cargoes from contamination, additional dunnage consisting of jute coverings, paper, matting etc. is also laid. The same applies to lateral and top dunnage. Most general cargo ships have spar ceilings. These are wooden laths connected securely to the ship, which prevent direct contact between the cargo and the ship’s side and allow the sweat to flow downwards over the steel ship’s side. Sweat may form in particularly large amounts in holds under the water line during voyages from hot to cold climates. In the absence of spar ceilings, dunnage or criss-cross dunnage must be used. This criss-cross dunnage then consists of nailed grids, cross-wise wooden dunnage being nailed to vertical uprights (vertically positioned squared beams).
Wooden dunnage must be dry (water content 12 – 15%, air-dried).
Wooden dunnage which is too fresh or has been stored in the open air and thus exposed to precipitation must not be used as dunnage. Moisture may lead to considerable damage.
Where spar ceilings are present in a general cargo ship, the purpose of side dunnage is to protect the cargo from contamination. Top dunnage generally fulfills a two-fold function:
|protection against moisture (sweat dripping from the deck, in particular the weather deck)|
|protection from contamination (dust, cargo residues, hydraulic oil etc.).|
The hygroscopicity of the generally sensitive goods means that top dunnage must allow a minimum of air circulation and evaporation. For this reason, air-permeable materials are used, such as paper, rice or bamboo matting and jute coverings.
Standard containers are constructed so as to be spray-tight. For this reason, they do not have water drainage holes, since they would impair tightness. As the sweat has no way of draining way, as it does in a general cargo ship via wells and bilges, wooden dunnage is not very appropriate for use in standard containers. Floor and side dunnage is used to protect the cargo from contamination. Top dunnage has become increasingly significant in containers. Since a standard container cannot be ventilated, humidity levels become very high inside a container. Hot air rises upwards and moisture contained in the air condenses on the underside of the container ceiling during the night (cooling of the external air) or in transit from hot to cold climes.
During transport of coffee or cocoa for example in standard containers 80 to 100 liters of sweat may be produced in a few days. If this sweat drips in large quantities straight onto the cargo, the bags in the top layer, against the container side walls and in the bottom layer may quickly become wet through. The cargo then becomes unusable as a result of mold and the entire cargo may be lost. Whatever top dunnage is used, condensation on the container ceiling cannot be prevented, but a proportion of the sweat may be retained and moisture may be prevented from penetrating through the cargo for a certain period. In addition to paper and matting, special nonwovens are used, which exhibit very high water absorbency. Another option is plastic tarpaulins, which are suspended under the container ceiling (not in contact with the cargo if at all possible) and have „reservoirs“ in which the sweat may collect as it drips.
Interlayer dunnage may fulfill two functions:
|protection of an underlying cargo from contamination by a top cargo|
|segregation of individual batches of cargo.|
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