sweat damage under some circumstances is inevitable. It will be seen from the foregoing sections that moisture migration within, and from, a water-holding cargo must occur as the vessel moves through different climatological regions. The purpose of ventilation is to minimise damage to the cargo resulting from this moisture migration. However, it will also be seen that such ventilation cannot always be completely effective and under some circumstances can be at least partially self-defeating. It follows that with certain cargoes, especially those where moisture movement is rapid, such as bagged rice and cocoa beans – which have been taken as examples in this article – no normal ventilating system can prevent cargo damage occurring as result of the conditions encountered during certain types of voyage.


The rate of moisture migration and the amount of moisture moving in water-holding cargoes will vary between the two extremes of bulk grain on the one hand and rice or cocoa beans on the other; but because of the wide variety of voyages undertaken and types of product carried, it is impossible to give precise recommendations (except under special circumstance) as to when ventilation should be practised. Many surveyors at present, work on the ad hoc basis that ventilation should be practised whenever weather conditions permit and, if under these circumstances sweat is formed, they consider the ship’s personnel have taken all reasonable steps to ensure a sound outturn. This is subject to the qualification that what is crudely termed ‘moisture migration in reverse’ does not occur; in other words, that the ventilation air introduced into the hold does not give up its moisture to the cargo. Moisture will be absorbed into cargo whenever the dewpoint of the interstitial air is lower than the dewpoint of the ventilating air. Unfortunately, however, it is virtually impossible onboard ship to measure the dewpoint of the interstitial air, and thus the decision of when or when not to ventilate cargoes of this type must still be based on a compromise between the scientific theory of the text book and the practical experience of those engaged in the trade.


Ventilation experiments Introduction


When maize arrives damaged by heating, the cargo interests frequently allege that the damage is caused by unsatisfactory ventilation. Thus it may be suggested that inadequate ventilation has permitted sweat to form on the ship’s structure, with the result that the cargo has been wetted on the surface. Alternatively it may be claimed that, because the ventilation is inadequate, the heat produced in a cargo was not removed, with the result that the damage becomes progressive.


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Damage as a result of ship’s sweat is readily recognisable, and takes the form of a layer of mouldy grain on the surface of the cargo.


In many instances, particularly where the amount of damage is appreciable, a defence against a claim for such damage is to demonstrate that if in fact ventilation had been practised, the air would have been sufficiently cold to have cooled the top layer of grain with the result that moisture in vapour form would have migrated from within the bulk to the cooler surface. But, on encountering the cooler cargo in the surface layer, the vapour would have given up its moisture in the form of condensation on the cargo.


Thus instead of ‘ship’s sweat’, there would have been ‘cargo sweat’ – the total damage however, about the same .


This is, simply stated, the theory of ‘moisture migration’, which was judicially considered in John v The Turnbull Scott Shipping Co. Ltd. (The Flowergate) [1967] 1 Lloyd’s Rep.1 – see in particular, the evidence of Dr Milton, quoted on pages 32 and 33 of the judgement.


Serious claims for damage however, normally arise from heating up within the bulk of the cargo and the question of efficacy of ventilation in removing heat and minimising progressive heating up in such instances had never been thoroughly examined on a scientific basis. It was therefore decided to examine the changes in temperature within the bulk of maize cargoes during shipment from South America to Europe.


Conclusion


These experimental shipments indicated that maize of low moisture content, loaded cool, will carry well over a relatively long voyage whether or not the surface of the bulk is ventilated. They showed further that it is only the surface of the cargo that responds to ventilation, the bulk of the stow remaining unaffected. This perhaps, is not surprising when one remembers how resistant maize is to the transfer of heat; its coefficient of thermal conductivity is less than that of asbestos, and as an insulator, it is about one-third as good as cork.


In general it can be said that the experiments confirmed the view previously expressed by Dr Milton, namely that ventilation is irrelevant to the carriage of maize in bulk.


This article and full details of these experiments are available under Moisture migration and surface ventilation in the Loss Prevention – Carefully to Carry section of the Club web website www.ukpandi.com


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