transverse bulkheads, between cargo holds – must be checked when safe to do so, and proved in same condition at initial and final surveys.


These voids often contain the manhole access covers to the adjacent double-bottom tanks. If these covers are not totally watertight, then the voids will flood, or partially flood, during ballasting or pressing up of the tanks, potentially resulting in huge errors in the lightship or ballast survey.


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As noted above, the calculation of the weight of ballast water is undoubtedly the most usual source of errors which may result in very large, and unacceptable, inaccuracies of the cargo quantity as calculated by draught survey.


Density of the ballast water ●


It should be established, with the chief officer, where the various ballast tanks were filled. If from a single source, the sea, a few random samples of the water will confirm its density. If from different sources, docks or rivers, etc samples must be taken from the tanks containing water from these various sources and relevant densities of the water in individual tanks established.


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Do not overflow the tanks substantially to obtain samples unless local regulations permit; instead use sampling equipment suitable for tanks that are only partially filled.


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When small samples are obtained, use a salinity refractometer to establish density (see below). When larger samples have been obtained, a draught survey hydrometer may be used. See details above.


Establishing the correct weights of oils on board ●


This can be established either by sounding or ullaging of the tanks or, in the case of the engine room daily service and settling tanks, by reading the gauges.


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The volumes of oils in each and every tank should be measured and recorded.


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The relative densities of the most recently delivered oils on board can be obtained from the bunker delivery certificates. However bunkers are almost inevitably mixed with oils already on board, the densities of which are likely to differ. The relative density of the contents may be calculated using the following formula:


23 ●


RD of tank contents at survey =


(Old oil volume x Old RD) + (New bunker volume x New RD) Total volume of oil in tank


After completion of the bunker survey the totals of each oil found must be agreed with the chief engineer and the master.


Calculations and associated corrections of vessel’s displacement from draught readings ●


Before extracting hydrostatic data from the vessel’s stability book, care should be taken by surveyors to familiarise themselves with the format and methods used to display the various particulars, especially the means of depicting positions of LCF (longtitudinal centre of flotation) etc, relative to amidships or alternatively the after perpendicular.


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When using a recommended draught survey computer programme or alternatively calculating directly from data extracted from the hydrostatic particulars contained within the vessel’s stability book it is essential that the data is carefully and properly interpolated or, in what should prove to be a rare event, extrapolated.


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As mentioned below, one of the areas where significant errors often result is from the incorrect application of the sign in respect of the position of the LCF (in the first trim correction).


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When undertaking initial and final ‘displacement draught surveys’ to establish weight(s) of cargo loaded, or alternatively unloaded, the difference between the net displacement weights provides the ‘total cargo’ quantity. Nonetheless it is recommended for a cross check that, at the light ship / ballast survey, the vessel’s light ship weight is deducted from net displacement found. The resultant then provides the vessel’s ‘constant’ at that time. These unknown weights might also be termed the vessel’s ‘stores variable’. Although variable, for a number of reasons as later discussed, it should serve as a guide to the accuracy of the light ship / ballast survey.


Comparison between ‘stores variable’ quantities, or mean thereof, established at previous surveys should be treated with caution unless the variable is a direct comparison that can be made. For example, all surveys include a check and a record of the engine lubricating oil held in storage tank(s), etc.


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