Uses of dunnage -
To provide protection to the cargo from water damage caused by contact with water from the bilges, other cargo or double bottom tanks
It protects moisture sensitive cargoes such as bagged cargo ( eg. coffee, cocoa) or bales (eg tobacco, tea) from sweat, which forms on a ship’s sides and runs off over the decks.
to provide air channels between stows to aid ventilation, mainly for refrigerated cargoes
to prevent damage to goods through contact with either the ship’s structure or with other cargo
to aid in the effective distribution of weight on tank tops of hatch covers
to aid in filling void spaces between cargoes, especially unevenly shaped cargoes

Types of Dunnage -

Floor Dunnage
This is mainly used to lift the cargo off the tank top
This prevents cargo damage from sweat or moisture running off from other cargo
Floor dunnage is laid crosswise and the direction depends on the design of the vessel
In ships with large lateral water drainage (bilges), the first layer must be laid cross wise, while ships with fore and aft drainage(wells), the first layer must be laid lengthwise. The dunnage should be laid close together to prevent the cargo from making contact with the tank tops.
On modern vessel’s especially reefers permanent deck gratings, made of grooved and perforated aluminum or plywood gratings, are fitted

Lateral Dunnage
This usually takes the shape of a spar ceiling on a general cargo vessel.
These are wooden beams securely connected to the side of the cargo hold.
Lateral dunnage prevents contact of the cargo with the ship’s side and so prevents cargo damage due to sweat
In the absence of spar ceilings, wooden or criss cross dunnage must be used.
Criss Cross dunnage consists of nailed grids, cross wide wooden dunnage being nailed to the vertical uprights (vertical positioned upright beams)

Permanent Collapsible Dunnage (PCDs)
This comprises of painted or varnished 8’ x 4’ plywood sheets that may be fitted to ship’s sides
These sheets are hinged at their base and open out to be secured in a variety of ways
Their main purpose is to square off curved ship’s sides and they are particularly useful for the palletized cargo

Interlayer Dunnage
This separates cargoes from each other, either for the protection of of an underlying cargo from contamination by a top cargo or for the segregation of individual batches of cargo

Top Dunnage
This generally protects the cargo from contamination damage from the water, hydraulic oil, etc dripping from the weather decks.
Top dunnage must also allow for some air circulation and evaporation, especially for 3. hygroscopic cargoes
For this reason, top dunnage is usually made from an air permeable material such as paper or jute

Side dunnage
This is similar to Spar ceiling and provides protection against contamination from sides and prevents mechanical damage

Inflatable dunnage
This is being used widely on refrigerated ships. This dunnage can be re-used an inflated using compressed air which is readily available on board.

Any wooden dunnage that is too fresh or has been exposed to the atmosphere must not be used. While using Wooden dunnage it is prudent to obtain a fumigation certificate for the Dunnage being used stating that the Dunnage has been properly fumigated.