The center of buoyancy of an object is located at the center of the volume of the liquid that the object displaces. It is the point through which the upward buoyant force acts.
The following diagram shows a rectangular form floating in water. The center of gravity (point 1) is at the geometric center of the shape. The center of buoyancy (point 2) is at the center of the volume of the displaced water.
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1 - Center of Gravity 2 - Center of Buoyancy |
For a ship, there is no single center of buoyancy value. Rather, there is a center of buoyancy for every floating position of the ship. This includes both non-heeled and heeled conditions up to and including the design draft (the maximum submerged floating position). Normally, calculations are done for a range of drafts at the 0° heeled state. These are reported in a tabular form or depicted graphically on a drawing along with other computed values. The collection of these values is commonly referred to as Hydrostatics.
Smart 3D does not directly calculate the center of buoyancy. The data used to compute the center of buoyancy of a ship is provided by the hull plate system. Because a ship is designed to meet conditions of performance (speed, carrying capacity, and so on) the hull form shape is usually developed before the three-dimensional model is started. Usually, the three-dimensional model is built based on preliminary hull designs.
Smart 3D allows you to use the third-party hull design tool that best meets your needs to create this preliminary design. You can then import that hull form as either an Initial Graphics Exchange Standard (IGES) file or a Standard ACIS Text (SAT) file. See Import ACIS Command and Import IGES Command.
For floating offshore structures, there may be multiple hull plate systems, such as the pontoons and columns on semi-submersibles.
The tubular legs and braces of offshore jacket structures can be treated as hulls for floating calculations after the jacket is launched and positioned and before it is flooded. In these cases, you must know which structural objects are designed to be watertight and which are not. Smart 3D allows you to set properties on plate systems to indicate their tightness.
Smart 3D allows you to query structural system objects in the model database for the type (such as hull, deck, transverse bulkhead, longitudinal bulkhead, tube, and so on) as well as for their tightness (such as watertight, non-tight, weather tight, airtight, and so on). Such a query helps to identify which objects contribute to determining a center of buoyancy. These objects must be defined by the designer creating the model.
You must know what objects contribute to the floating position, the density of the fluid (salt or fresh water), the weight and center of gravity, and which objects are going to produce buoyant forces before you can calculate the center of buoyancy.
Calculations of this nature, and use of the computed data, are very specialized. They are done by professionals and experts in the field of naval architecture. Thicknesses of the structural members, such as pipes and tubes, have one volume when they are not flooded (volume of outer diameter) and another when they are flooded (volume of inner diameter). Submerged objects must take into account the volume of the structure itself because the net volume is what determines the actual buoyancy. Hexagon assumes no liability for use of data from Smart 3D for this purpose.
The three-dimensional model is built from early designs and it is rare to take a 3D model and try to compute these values. We recommend that you contact the group or company that was responsible for the design to get this type of data.