The 3 types of sweat on vessels
Atmospheric sweat:
A hold contains a very large volume of air. Any air brought in by ventilators at any instant is very small compared to the volume of the hold. If the temperature of the air in the hold is low, the incoming air will be cooled and, if the incoming air is cooled below its dew point, it will give off moisture. This will condense as sweat on cargo and in the steel parts of the hold and is liable to cause damage to cargo.
Cargo sweat:
Some cargoes give off large quantities of water vapour making the relative humidity of the air inside the hold very high. The slightest cooling would cause sweat to form on the steel parts of the hold. This would drip on other cargoes in the hold, possibly causing damage to them. This can only be prevented by use of large capacity exhaust fans, using dunnage to keep the cargo off the steel and covering the top of the cargo in the hold with bamboo mats or cardboard sheets.
Ship sweat
When the temperature of the sea is much lower than that of the air above it, the underwater parts of the hold would be cooled by contact with the sea. Hence sweat would form on the steel parts of the hold below the waterline. This normally would flow into the bilges. The use of dunnage to keep the cargo off the steel parts of the hold and also to keep it a few centimetres above the bottom of the hold would suffice. Ventilation would also help, in reducing the relative humidity in the hold.
Ref: Marine Meteorology by Capt. H Subramaniam
