Measurement of bulk cargoes


Draught surveys – practice ●


The master of a vessel should be advised in adequate time that a draught survey will be taking place. If it is an initial light ship survey, he should be requested, subject to the safety of the vessel, to ensure that individual ballast tanks are either fully pressed up or empty – that the vessel is upright, and with a trim which is within the limits of the tank calibration tables.


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When draught surveys are undertaken by independent surveyors, co-operation of the ship’s officers is essential.


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Independent surveys should be undertaken together, during the relative survey sections, with the vessel’s chief officer and chief engineer or their appointed respective deputies.


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Before undertaking the survey, it is recommended that the surveyor makes time to inspect a general arrangement plan in order to confirm the number and position of the various ballast, fresh water and oil bunker tanks on the vessel.


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Equipment which may be used in the survey: ●


Strong torch ●


Patent draught mark indicator or measuring devices (draught tubes, indicators etc)


● Calibrated inclinometer or manometer ●


Steel tape measure with plumb bob / stainless steel sounding tape with brass plumb bob (preferably calibrated)


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Sea water sampling bucket or can of sufficient volume


● Calibrated patent draught survey hydrometer ● Calibrated salinity refractometer ● Ballast water-sampling device ● Computer / calculator.


Reading the draught marks ●


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The vessel should ideally be lying in still, calm water. Otherwise errors, without ease of correction, from reading the draught marks can result. For example:


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Vessels lying at exposed berths or anchorages where wave and swell surface disturbance is almost inevitable; even to the extent that the vessel may be rolling and pitching. In these circumstances it is usual to assess the actual mean water level over a number of readings to be at two-thirds of the distance between the lowest and highest levels of water as seen against the draught marks. Some experts advocate that, after studying wave patterns, a mean of the average highest and lowest draught readings should be used


Vessels which are lying at a river berth or in tidal conditions when strong currents are running. Under these conditions the draught marks should ideally be read over periods of slack water (provided that at a low water slack there is sufficient under-keel clearance)


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Currents of appreciable strengths are likely to cause the vessel to change trim or pitch slightly and/or sink bodily into the water from her static draught (‘squat’). This phenomenon becomes more pronounced in shallow waters (shallow water effect)


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Strong currents will result in raised water levels against the leading edge of a stationary vessel lying in flowing water. This is especially true when the flow is in the direction of a vessel’s bulbous bow.


Draught marks must be read on both sides of the vessel: forward port and starboard; amidships port and starboard, and; aft port and starboard or, alternatively, if additional marks are displayed on large vessels at all the designated positions.


Should draught marks not be in place amidships, distances from the deck line to the water line on both sides of the vessel must be measured. The amidships draughts can then be calculated from load line and freeboard data extracted from the vessel’s stability booklet.


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Draught marks should be read with the observer as close to the water line as is safe and reasonably possible, in order to reduce parallax error.


At the time of reading the draught marks, the vessel should be upright with a minimum of trim. The trim at survey should never exceed the maximum trim for which corrections may be included in the vessel’s stability book.


21 ●


Although it is common practice to read the offside draught marks from a rope ladder, a launch or small boat provides a more stable environment and brings the observer to a safer position closer to the water line.


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