Dewpoint, Ventilation & Paint Calculator

The dewpoint rule. Compare the dewpoint of the outside air with the dewpoint of the air in the hold. If the outside air is drier — a lower dewpoint — ventilating brings in air that will not condense on the cargo, so ventilate. If the outside air has the higher dewpoint, ventilating brings moisture in and it condenses on colder cargo. That is cargo sweat, and it is the one that damages the goods.

The three-degree rule. For when you cannot measure the hold air. If the outside dry bulb is at least three degrees below the average cargo temperature at loading, ventilate; otherwise do not. It is a rule of thumb standing in for the hold reading, and it assumes the cargo temperature has not moved much since loading.

Ship sweat and cargo sweat. Ship sweat is condensation on the steel — frames, beams, the underside of the hatch — which then drips back down onto the cargo. It forms when the hold air is warmer and wetter than the structure, which is what happens sailing from a warm port into colder water. Cargo sweat is condensation on the cargo itself, from letting warm wet air into a hold holding a cold stow. Ventilation cures the first and causes the second.

Coal: gas first, moisture second. Coal is ventilated to manage gas. It gives off methane, which has to be cleared, and it can self-heat — and a self-heating cargo must not be ventilated, because airflow through the stow feeds it. Carbon monoxide is the early warning: a level at or above the figure set, or a rising trend, points to self-heating. The trend is read over the last four readings for that hold, taken in date order — two steps of the set size anywhere in that window, or an overall climb of twice it, counts as rising. The steps do not have to be next to each other: for a self-heating detector the looser test is the safer one.

• Self-heating indicated — close up and inform the company. This overrules everything, including a wet hold.
• Methane above the level — ventilate, surface only. This overrules the moisture reading too: gas comes before condensation, so you ventilate even if the outside air is the damper of the two.
• Gas readings clear — only now does moisture matter. A hold that is much wetter than the outside air will sweat onto the structure and drip back onto the cargo, which feeds oxidation, corrodes the tank top, and adds moisture to a cargo that may be liable to liquefaction. A short, watched period of surface ventilation to dry the air out is worth considering — with the gas readings taken again afterwards.

Ventilation methods.

• Surface ventilation — air passes over the top of the stow without going through it. The method for coal and for most bagged and general cargo.
• Through ventilation — air passes through the body of the cargo. Never for coal or other self-heating cargoes.
• Exhaust only — drawing air out so replacement comes in through openings.
• Closed up — hatches and ventilators shut and secured; the old term is “battened down”, from the wooden battens once used to hold canvas hatch covers in place. For a self-heating cargo this is what starves it of air.

Record whichever was done, and put the reason in the remarks. A log showing why ventilation was stopped is worth as much as one showing it was run.

Paint coverage. Theoretical coverage is volume solids × 10 ÷ dry film thickness in microns, giving square metres per litre. Wet film thickness is the dry thickness divided by the volume solids — that is the figure the comb should read while the paint is still wet. Practical coverage takes off the loss factor for overspray, wind, staging and the shape of the structure.