A draft survey is a procedure used to determine the mass of dry bulk cargo carried by sea. It is the only established method in the shipping business and is, in fact, the only “accurate” method by which the amount of cargo can be determined at all. It is based on Archimedes’ Principle. The principle states that the buoyancy acting on a floating or submerged object equals the weight of the fluid displaced by that object. In a draft survey, the ship’s displacement, representing the mass of the ship itself along with everything that is stowed on board, is read from the ship’s hydrostatic tables according to her draft. The amount of loaded cargo is then calculated by deducting the lightship mass, the measured or declared mass of ballast water, fresh water, bunker, lube oil, and other masses, and the ship’s constant from the total.
Hydrostatic parameters in the tables are applicable only to a ship floating on an even keel, having no inclination whatsoever. Owing to the longitudinal and vertical asymmetry of the hull, the volume of the bow is not equal to the volume of the stern. To elaborate, the volume acquired on the low side of a trimmed or listed ship is larger than the volume that emerges on the high side. Since the displacement is constant, Archimedes’ Principle only holds if there is no excess of buoyancy, regardless of any trim and list, so the ship rises from the water. Additionally, the water plane experiences a change in both area and shape, while its centroid, the point the axes of inclination pass through, shifts towards the low side of the hull.
As a consequence, the displacement figures given in the tables do not necessarily correspond to the observed drafts. To compensate for that, besides applying the corrections for hull deflection and water density, a couple of trim corrections and one list correction are added to the calculation. The trim and list corrections are derived from the parameters listed in the ship’s tables.
The purpose of the thesis is to evaluate the accuracy of determining the displacement of a trimmed and listed ship by a draft survey. In other words, the focus of the thesis is set specifically on the accuracy of the trim and list corrections. The two subjects of the research are hulls of different types of ships, analyzed at various degrees of inclination on three different drafts. One hull belongs to a crude carrier, which is very similar in design to a bulk carrier’s hull. The other is one of a container ship. The hulls are very distinct from one another, having considerably different characteristics. The former type is mostly used in tramp trade, designed to carry large amounts of cargo at relatively low speeds while easily withstanding heavy loads and stresses. The latter type, a container carrier’s hull, whose fine form allows the vessel to develop the required higher speeds, is predominantly engaged in liner shipping.
The introduction of the thesis is a step-by-step breakdown of the draft survey procedure supplemented by the author’s remarks and hints from experience. The following theoretical part includes mathematical derivations of hydrostatic parameters used in the calculation, an overview of the displacement corrections, and explanations of hydrostatic phenomena because of which the corrections are applied. The key concept of the thesis is the analysis of different hull geometries represented by Rhinoceros 3D CAD models. The underlying method of geometry manipulation is achieved through custom-designed Python algorithms, executing specific commands in Rhinoceros 3D in order to track changes in the hydrostatic parameters in question under particular conditions.
The conclusion is a presentation of results obtained by direct numerical analysis of the hulls compared to results yielded by the standard draft survey calculation. The results are indisputably accurate within the laid set of conditions. However, the use-value of the findings is limited in practice, since the effect of loads on the hull structure is not accounted for. The demanding issue of deformation, i.e., bending of the hull, is not part of the thesis.
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