Inaccuracies in Draught Surveys Loss Prevention Bulletin Draught surveys by their very nature are not an exact science, and the accuracy of the calculated cargo figure may vary typically by between 0.5 and 1.0%. However, there are occasions when considerable differences in the cargo figures derived from the draught surveys are obtained. The purpose of this Loss Prevention Bulletin is to remind deck officers of the principal sources of errors which may affect the survey results, such that they can be taken into account when shortages in the cargo are found. Draughts It is imperative that the draught marks are read as accurately as possible. Ideally this should be done from a boat so that the marks can be approached closely and read. However, poorly painted, rusty, or draught marks covered with marine growth make reading the draught accurately problematic, therefore it must be ensured, so far as possible, that the draught marks are kept in a readable condition. The presence of wind waves on the water surface will necessitate an estimation of the average draught by observation of the peaks and troughs of the waves against the hull. Darkness and shadows also make accurate reading difficult; therefore good lighting will be required at night. The turn of the hull around the stern leading to elongation of draught marks being viewed from an oblique angle, especially when this has to be done from an adjacent wharf at a higher level, can lead to difficulty in accurately determining the draught. The Club regularly receives claims arising from alleged shortages of cargo as a result of discrepancies between draught survey results. As an example, an error of two centimetres in the mean draught would lead to an error in the displacement of approximately: 68t – Handysize Bulk Carrier (10,000 – 35,000t deadweight) 90t – Handymax Bulk Carrier (35,000 – 59,000t deadweight) 112t – Panamax Bulk Carrier (60,000 – 80,000t deadweight) 300t – Capesize Bulk Carrier (> 80,000t deadweight) It is therefore advised to ensure that the inspection and maintenance of the draught marks be included in the planned maintenance system on a vessel. Density Although there are numerous possible causes of inaccurate dock water density, it must first be confirmed that the hydrometer being used is of the correct type. There are two types of hydrometer in use, the load line survey hydrometer, and the draught survey hydrometer, the latter being used in this case. A draught survey hydrometer, calibrated in air, measures the apparent density of the water and is used for determining the weight of the cargo on board. A load line hydrometer, calibrated in a vacuum, is used to determine the relative density of the water and determine the displacement of the vessel at a given load line. A dock water density reading taken with a draught survey hydrometer showing 1.015 t/m3, would show 1.017 t/m3 when taken with a load line hydrometer, both are correct, the difference between the two of 0.002 is known as the “air buoyancy correction”, however as we want to determine the weight of the cargo on the vessel the density read from the draught survey hydrometer will be used. The draught survey hydrometer is calibrated at a standard temperature. Any deviation from the standard temperature does not require a temperature correction, as the changes in the volume of the vessel and the hydrometer will generally cancel each other out. It must also be ensured that the hydrometer is calibrated regularly Los