In April 2001, an experienced yachtsman, having previously owned both catamaran and mono-hull yachts, approached NWBS to find out more about Triumphant. During June 2001 this prospective client and his family participated in the extensive trials program. NWBS was subsequently tasked to investigate the feasibility of a trimaran yacht – particularly with respect to sea keeping and hull efficiency.
Equipped with TWIN FOILS
TRIMARAN VERSUS MONOHULL
Comparison studies of the motions of a trimaran compared to an equivalent mono-hull indicated that the triamaran has lower heave and vertical acceleration characteristics in seas forward of the beam, and lower roll motions at all headings. In seas aft of the beam, accelerations of the trimaran hullform over a monohull.
TRIMARAN VERSUS CATAMARAN
Both the catamaran and trimaran hullforms are inherently stable, however the trimaran offers several advantages.
Catamarans are susceptible to bow slamming. This can be offset to some extent by he addition of a wave piercing bow underneath the bridging structure. However this adds weight while offering little additional benefit other than reserve of buoyancy. The trimaran by comparison shows remarkable resistance to slamming, and reduced vertical accelerations.
With its highs metacentric height, a catamaran requires more controlling forces to actively stabilize it. The trimaran hull with its lower metacentric height abd softer roll period, requires less stabilizing force.
Another benefit of the trimaran shape is that it allows the inclusion of large motion control foils between the centre and side hulls. It is the combination of the inherent stability of the trimaran hull and the controlling forces of the foils that combine to create an exceptional ride.
The fast ferry Triumphant was designed to operate at a Froude number of 0.92. The yacht was required to operate at a more sedate pace compared to the fast ferry. The yacht’s hull design had to provide fast ferry. The yacht’s hull design had to provide optimum efficiency at a Froude number of 0.44.
The formulation of a displacement trimaran hull involves more than simply optimizing the shape for minimum resistance. The geometry of the hull also affects the sea keeping performance.
Greater hull immersion increase the hull resistance and decreases the efficiency, however greater hull immersion also improves the sea keeping. Also the immersion of the side hulls in this displacement configuration is critical to optimum performance.
Combining hull theory with the knowledge gained from 5 years of research and experience into the tri-hull form, the hull design for White Rabbit was finalized. This new hull was optimized to provide the required combination of efficiency with sea kindliness. A model was built and subsequently tank-tested, confirming the high efficiency of the new hull design.
Theory, coupled with experience indicated that a significant saving in resistance over an equivalent sized monohull was achievable, thus giving the trimaran a 1-2 knot speed advantage for an installed power of around 3000kw.