Catamaran demi-hull interference

One downside of RANS codes in general is their tendency to be diffusive. In the case of a vessel with multiple hulls like a catamaran, this can be a huge source of uncertainty as the interference between the wave patterns of the two hulls has a pronounced effect on the resistance of the vessel as a whole. To determine the suitability of RANS in resolving these effects a set of simulations was conducted, calculating the resistance of a catamaran for a series of demi-hull spacings.

A symmetric “Wigley Hull” was used as the base model. It was simulated as a monohull and in a catamaran configuration with spacing to length ratios (s/l) of 0.2 to 0.5 and at Froude numbers from 0.2 to 0.7.

Qualitatively the results were consistent with literature on the subject. The monohull had the lowest coefficient of drag and the drag coefficient of the catamaran reduced as the demi-hull spacing increased at s/l = 0.5 the drag coefficient is very close to that of a monohull. Figure 14 shows plots of the free surface deformation around the models at constant froude number.

Quantitatively, the results show some traits of the diffusive behaviour inherent in RANS codes. While the trends are clearly in the right direction the magnitude of the interference effect is much greater in the experimental data than in the CFX results. More work needs to be undertaken, particularly in mesh density studies, to ascertain how this effect can be minimised. In this particular case invicid panel methods will give a much better result in terms of interference but the comprehensive nature of the RANS solution, taking into account all the viscous effects and details offer a tantalising alternative.