A numerical study of resistance components of high-speed catamarans and the scale effects on form factor

Abstract

Resistance and powering of ships is one of the most important aspects for Naval Architects and it still heavily relies on scale model testing. The catamaran is a particular case that is treated slightly different from the monohull due to the interference effects between demihulls. There are various works from previous researches dealing with this challenge. Since catamarans have been introduced, resistance components and form factor have been evaluated by focusing on the influence of many aspects such as hull form, speed, and hull separation. The methods by which the resistance is evaluated, either experimentally or numerically are mostly driven by the size of physical test facilities or computational power available. This thesis investigates the resistance components of high-speed catamarans and the effect of scale on form factor using a commercial CFD code.

The numerical investigation into resistance components and form factor of the high-speed catamarans focuses on the hull geometry, separation to length ratio (S/L) and scale. The hull geometries include the Wigley III and NPL round bilge 5b catamarans. The CFD results are compared against Insel’s experimental series. The investigation into the effect of scale on form factor (1+k) are made for the Wigley III and NPL 5b catamarans S/L = 0.3. Three different models of those hull configurations are created, which are 1L, 2L for both hulls, 4L for Wigley III catamaran and 10L for NPL 5b catamaran respectively. The numerical domain to ship dimension ratios are constant for the Wigley III catamarans. The influence of domain size, recreating two towing tanks and the unbounded condition, are investigated. Wave elevations, resistance components and form factors are evaluated and compared.

The CFD results show good agreement with the original experimental results, and allow deeper understanding into the resistance components, form factor and free surface wave elevations. The CFD simulation domain size has an important influence on the resistance. To some extent the ITTC recommended blockage corrections can correct for this, but the wave system is significantly different and cannot be corrected. The CFD investigation into the influence of scale on form factor shows that form factor is both speed and model length dependent. So, it is noted that a larger model produces more accurate predictions of form factor and that the form factor should be evaluated at the design speed rather than determined by either low speed or high-speed runs.

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ORCID for Dominic Taunton: orcid.org/0000-0002-6865-089X

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