International Journal of Small Craft Technology


EXPERIMENTAL INVESTIGATION ON TWO DISPLACEMENT CATAMARANS: SYSTEMATIC VARIATION OF DISPLACEMENT, CLEARANCE AND STAGGER. F. Caprio and C. Pensa, Università degli Studi di Napoli Federico II, Italy SUMMARY


The aim of this study has been the evaluation of the effects of varying longitudinal stagger, transverse separation and displacement on the resistance of two symmetric hull forms suitable for catamaran configuration over a speed range corresponding to Fn < 0.5.


To achieve this aim at the Dipartimento di Ingegneria Navale di Napoli (DIN) an intensive experimental investigation has been carried out on two models in three configurations: monohull, symmetric and staggered catamaran.


In particular, for one model, 6 clearances in the symmetrical configuration and 4 longitudinal staggers have been tested; for the other one 3 clearances and 3 staggers have been tested.


The influence of displacement on resistance has been evaluated by testing the models for three different displacements. NOMENCLATURE


Demihull: One of the hulls which make up the catamaran; Stagger: Longitudinal shift between demihulls L; LWL: Length on waterline b: Breadth of demihull B: T: S:


Breadth of catamaran Draught


Separation demihulls


distance between centrelines


SW: Wetted surface area (static condition) AT: Transom stern area AM: Area of midship section : Displacement volume


L/1/3: Length-displacement ratio V:


Speed


Fn: Froude Number Rn: Reynolds Number LCB: Distance of Centre of Buoyancy from transom CB: Block Coefficient CP: Prismatic Coefficient CT:


CF: Frictional Resistance Coefficient (ITTC ’57)


CR: Residuary Resistance IT: Total Interference Factor :


UCT: Total Uncertainty BCT: Bias Limit PCT: Precision Limit


1. INTRODUCTION


The interest in catamarans has grown in the last decades. In fact, this kind of ship is often chosen for building ferries and workboats. Not many systematic experimental


Staggered Catamaran Efficiency Factor of


investigations have been carried out to evaluate the influence


of displacement and separation on the


resistance of catamarans. On this subject the two most important research studies are by Molland et al. in [1] [2] on round bilge hulls and the comprehensive study on the systematic series “Series 89” of hard chine catamaran hull, reported by Muller-Graf et al. in [3].


Drastic reduction of resistance is possible when using a staggered


catamaran configuration (also called


asymmetric in this paper). In this configuration the demihulls have a longitudinally shift between them. The working principle is based on the advantages gained from the positive interferences of the transversal waves systems generated by the shifted demihulls. It is easy to observe that


the wave systems generated by the Total Resistance Coefficient (Resist/0.5SWV2)


CT : Total Volumetric Resistance Coefficient (Resist/0.52/3V2)


demihulls have equal size and phase and, in the tunnel of symmetric catamarans, the amplitudes of the transversal waves add up constructively and almost doubles. But applying a sizeable shift between the transverse wave systems of demihulls, the resulting wave pattern is largely reduced, also reducing the wave resistance.


In [4] Söding reported the results of the numerical investigation on the catamaran Supercat-Haroula and highlighted a reduction of up to 50 % of total resistance with a 50 % LWL stagger for Fn = 0.4. Theoretical results were also confirmed by an experimental investigation carried out on a SWATH model.


2. DESCRIPTION OF MODELS


The two tested models C932 and C925 [5], illustrated in Figures 1 and 2, have symmetric hull form and transom stern. Their hull surfaces are also developable. The model C932 presents three hard chines, while the model C925 only one.


©2007: Royal Institution of Naval Architects


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