Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015
Figure 1: Profile view (top) with corresponding water lines for light, medium and heavy displacement and plan view (bottom) with symmetry line of hulls under consideration.
scaled with length squared, the component weight was scaled linearly with respect to length and weight of the machinery was assumed to be constant for all vessels under consideration. Assuming that the displacement force equals the combination of lightship weight and deadweight, the
vessels will have a comparable
deadweight at the light displacement. The hull form properties are summarised in Table 1 and 2.
2.1 (a) Implications of Design Rules For the medium and heavy
load case the higher
deadweight can be achieved for the longer models, because an increase in length at constant demihull beam increases the waterplane area. For the heavy loading conditions, the shortest hull will be able to carry an additional 180% of its deadweight at light
loading
conditions, while the longest hull will be able to carry an extra 280% of its deadweight at light loading conditions.
In an earlier study [19], a further requirement was to keep the deck area
(LBoa) constant to compare
catamaran designs of different length. It was found to be more practical to keep the overall beam constant. Note though that if the required deck area was insufficient, extra decks could be added for a vessel of this size.
(L/1/3) ranging from 9 to 15 and transom immersion ratios from 0.21 to 0.28. These three ratios depend on the length of the hulls, whilst the latter two also depend on the loading condition (light, medium, or heavy).
The resulting hull form family featured the 6 models shown in Figure 1 with lengths ranging from 110 to 190 m; varying displacements; different demihull separation ratios (s/L) varying from 0.13 to 0.23; slenderness ratios
0.19. This enabled the effect of transom immersion and separation ratio on the drag to be investigated as well, since configurations with similar slenderness ratios, but different transom immersions and s/L exist.
The concept of this hull form family allows analysing resistance properties from a hydrodynamic point of view, by considering the drag with respect to the corresponding displacement. Furthermore, it allows looking at it from a design point of view by considering the drag with respect to the deadweight the vessel is able to carry. Latter on is proportional to the inverse of transport efficiency stated in earlier work [19].
2.2 SIMULATION TECHNIQUE
A RANSE-based (Reynolds-Averaged Navier-Stokes Equation) solver featuring transient, viscous, multiphase flow and dynamic mesh motion (interDyMFoam) of OpenFOAM 2.3 was used for simulating the flow around the catamarans. It allows the build-up of free-surface waves and turbulent boundary layers, partial transom immersion and rigid body motions of the vessel to include effects of heave and trim. The SST (shear stress transport) turbulence model was used in accordance with wall functions and an eddy viscosity ratio of 10 was chosen. A symmetry plane was utilised at half demihull separation distance, i.e. the centreline of the complete vessel. All hull forms under consideration were scaled to 2.5 m length during the analysis.
2.2 (a) Mesh Generation
The demihull separation ratio influences the resistance of the catamaran and is one of the most important design parameters for catamarans ([7, 22]. In this study the demihull
separation was kept constant but the
characteristic demihull separation with respect to vessel length (s/L) decreased as length increased. The demihull separation was altered for the 130 m and 170 m hulls by half a demihull beam in both directions for two Froude numbers (Fr = 0.37,
0.45) at light
displacement, to study its influence on the resistance. For the 130m hull the separation ratio resulted in s/L = 0.15, 0.20, 0.25 and for the 170m hull it was s/L = 0.11, 0.15,
©2015: The Royal Institution of Naval Architects and heavy
A hybrid mesh consisting of a block-structured background mesh with a hexagonal unstructured mesh featuring hanging nodes in proximity to the vessel was generated using blockMesh, snappyHexMesh and refineMesh from the OpenFOAM toolbox. The block- structured background mesh allowed a higher mesh concentration at the free surface and around the vessel with smooth transitions into coarse cells close to the domain boundaries. The mesh was refined around the vessel with special attention paid to refinements between the demihulls, around the transom stern area and within the Kelvin wave angle. The inlet was situated two ship lengths in front of the vessel and the outlet 5 ship lengths aft. Figure 2 shows the principal setup of the mesh in terms of cell level, which is a measure for mesh density. It
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