6 SEPTEMBER 2017 • FOGHORN FOGHORNFOCUS: TECHNOLOGY A fundamental design dating back thousands of years to early Polynesian craft, the characteristics of modern catamaran hulls are well known to PVA members. Catamarans are commonly employed to meet re- quirements in which traditional monohulls fall short. For many operators, minimizing fuel efficiency is everything, while for others, keeping wash levels to a minimum might be the driving factor. For longer or rougher routes, high speed or high levels of passenger comfort in waves might be the most important aspects of vessel design. In the four decades since co- founding BMT Nigel Gee, we have seen catamaran designs steadily evolving, not only regarding hull form design, structure, and propulsion system technologies, but also in the tools and techniques used in design development of these vessels. In that time, BMT has designed over a hundred cata- marans spanning a vast range of vessel types. Some of our most notable catamaran designs can be seen in service or under construc- tion in the U.S., including the 128ft/38kt Finest and Bravest designed for New York Fast Ferry and the 236-ft/38 knot Fairweather and Chenega operated byAlaska Marine Highway System, the first full HSC vessels to be built in the U.S. More recently, BMT has been busy completing design work on four ultra-low-wash passenger ferries for Entertainment Cruises in DC, and two more ultra-low-wash passenger ferries for the New Orleans RTA, which are being constructed at Metal Shark’s facility in Jeanerette, LA. In discussing some case studies, we use various scenarios to describe how catamaran technology has evolved to meet challenging design requirements, and how it can be used to develop optimum vessel designs. Case Study #1 – Designing for ultra-low wash Vessels operating on inland waterways or conservation areas are often required to meet stringent wake wash height and/or energy requirements. From independent tests, the BMT-designed Finest and Bravest have been proven to offer the lowest wash levels in the U.S. for ferries of their size and speed. Low wash has also been a concern for some European operators, with BMT designs in the Netherlands providing parent hull data for the Potomac River and NOLA ferries currently under construction. Typical methods to minimize wake wash include maxi- mizing waterline length and hull separation, and minimiz- ing vessel weight and transom immersion. For low-wash vessels, hull length is often prescribed by service speed and wash height requirements, rather than passenger numbers or deck area requirements. It is a common misconcep- tion that fast boats generate high wash levels. By way of example, a 100-ft low-wash catamaran operating at 20 knots will cause considerably more wash than if running at 24 knots. For minimal wash with such catamarans, a small increase in service speed is therefore often preferable. In recent years, BMT has adopted computational fluid dynamics (CFD) techniques to investigate design solutions beyond what has already been tested using a physical model. With CFD, the effects of varying hull separation, length-beam ratio, beam-draft ratio, and transom immersion, among other parameters, can quickly be assessed to derive a fully- optimized form. These predictions can provide accurate pre- dictions of wash height (verified by full-scale trials), with a confidence level that allows us to agree to stringent con- tractual wake wash limitations for future vessel designs. As computer processor speeds increase, it is possible that CFD simulations could become a day-to-day naval architecture activity in the development of new vessel designs. Case Study #2 – Designing for high fuel efficiency When the U.K. ferry operator Wightlink planned to replace their existing fleet of catamaran ferries, they ap- proached BMT with a challenge to develop a new design that would offer significantly reduced fuel consumption with no impact on service schedule. In the early stages of designing and planning, a decision The Evolution of Catamaran Design By John Bonafoux, BMT Nigel Gee, U.K. (TOP) Ultra-low wash Potomac River ferry. (ABOVE) Ultra-low wash NOLA ferry.