Occasionally, surveyors report a ‘negative’ stores variable which is theoretically impossible unless, in extremely rare instances, the vessel had been subject to modification, and large quantities of structural steel removed, without being subject to a further inclining experiment and commensurate correction of the relevant data contained in the vessel’s stability book.


Charterparties often contain reference to an approximate quantity for the vessel’s ‘constant’, which may well create a discussion between master and surveyor should the constant found by survey to be substantially larger than that quoted by the owners. The surveyor, after relevant checks, should remain confident in the figure obtained, but always record on documents issued to the master and clients, any unusual factors or difficulties experienced during survey. These include any differences between surveyors, should owners, charterers or shippers each appoint separate survey companies to act on their behalf.


Documentation ●


At completion of survey, a ‘survey work sheet’ or computer printout should be placed on board the vessel recording the data and calculations used to obtain the cargo loaded / unloaded quantity. This document is usually produced by individual survey companies, or by shipping companies for use by their officers.


●


A formal ‘survey report’ should be submitted to clients at a later date. Specific formal documentation has been drawn up, amongst others by IMO, United Nations Economic Commission for Europe and various P&I Clubs.


●


The formal report document should not only include details of the survey, but also:


● Dates and times of surveys ● Vessel particulars ● Ship’s location ●


●


survey to be outside acceptable limits of normal draught survey measurement error.


Expert opinion


Surveys must be carried out to the very best of the surveyors’ ability, with each part of the survey conducted as accurately as possible in order to minimise procedural and/or measurement errors which could effect the quantity of cargo recorded by survey as being loaded or discharged.


The final report should include details of any defect or circumstance regarding weather, surface water, tides / currents or on board conditions which the surveyor considers might well influence the result adversely.


Cumulative errors ●


Errors can occur when reading and correcting the draughts. The final fully corrected 3/4 mean draught should be within +/- 10 mm of the true mean draught.


●


Errors of calculation. The main error to be avoided in this section is that of incorrectly positioning the LCF relative to LBP/2 the amidship point.


●


Error of the water density in which the vessel is floating. Always ensure an average sample, or alternatively the average of a number of water samples are obtained and the correct type of certificated hydrometer is used to obtain the density.


●


Sounding of tanks. Leaving aside documented tables which may not be accurate, the way of avoiding the main errors in this section of the survey is by ensuring, as best possible, that all volumes of liquids, especially ballast water, on board are both correctly quantified and attributed with correct densities. These factors, particularly when applied to ballast water, undoubtedly contribute to the largest number and degree of errors likely to be encountered in draught surveying.


Weather conditions (and whether these were within acceptable limits)


Sea conditions (and whether these were within acceptable limits)


●


Tidal/current conditions (and whether these were within acceptable limits)


●


A record of any difficulties or defects in a ship’s documentation or equipment which might cause the calculated weight by draught displacement


24


A full version of this article is available in the Loss Prevention – Carefully to Carry section of the Club website, www.ukpandi.com, under Miscellaneous – Measurement of bulk cargoes


Bearing these reservations in mind, a well conducted draught survey under reasonable prevailing conditions is capable of achieving an absolute accuracy of +/- 0.5%.


See following pages for a worked example


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