Innovation & Design: Slippery When Wet

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In the inaugural offering of its Innovation & Design annual issue, Yachts International explored some of the latest advancements in marine propulsion and power-generation technology, cornerstones of a global effort to harness ecologically sound energy sources while reducing our collective dependence on fossil fuels. Equally critical to achieving that dual objective is a parallel effort to design and develop new hull forms—or refinements to existing ones—that make the most efficient use of those energy sources to maximize speed and range. Indeed, some of these improvements even yield an increase in interior volume.

While many of the latest innovations originally were developed for commercial maritime applications, notable examples of these have found their way into the large-yacht category as well. This is a logical extension since all vessels are subject to the same physical constraints that govern speed and propulsion efficiency. Only a part of the energy that a propulsion system produces is applied directly to moving the hull; the rest is expended in overcoming resistance from friction (drag) and in moving the pattern of waves that form a vessel’s wake. To the extent, therefore, that a hull’s design and condition can reduce drag and wake size, less energy will be required to move it through the water, an improvement that translates to lower fuel consumption, higher speed from a given horsepower output and/or greater range.

Probably the most widely recognized among refinements to the design of power vessels is the bow bulb. Introduced early in the 20th century and in various forms fitted to such early adapters as the Navy’s USS Delaware and, in 1932, the storied French liner Normandie, the bulbous bow has become a standard fixture on nearly every oceangoing ship built in the past few decades, and more recently on a growing number of smaller commercial vessels, as well as on motoryachts and cruisers in the 50' range and larger. The effect of the bow bulb, a rounded appendage extending forward from the hull’s forefoot just below the waterline, is to create a wave that interrupts and partially cancels the vessel’s normal bow wave, in the aggregate reducing the volume of water that the hull must move while underway. A classic example of the resulting efficiency gain compares the conventional hull of the British liner Queen Mary to the Normandie, which developed speeds comparable to the former while using about 30 percent less horsepower and achieving a commensurate reduction in fuel consumption.

Multihull designs also have proved effective in increasing overall fuel efficiency. By virtue of their typically narrower hull cross sections compared to monohulls, catamaran designs reduce drag and wave height, a characteristic that led New Zealand naval architect Craig Loomes and his LOMOcean naval architecture and design company to specify this type for his design of Turanor PlanetSolar, an experimental vessel featured in the previous issue of Yachts International’s Innovation and Design. At press time, the vessel was about halfway along on its landmark circumnavigation voyage.

Notwithstanding its dramatic appearance, Turanor PlanetSolar already has offered a practical demonstration of solar power as a viable energy source for offshore cruising; an achievement also due in no small measure to the efficiency of its wave-piercing catamaran design. Of the project, Loomes notes that “immense focus and effort were put into refining the hull form to minimize resistance/drag as much as possible. Our hard work was rewarded by this (design) being the most efficient form ever tested in the history of the Australian Maritime College tow test facility, a facility that tests a large proportion of the world’s multihull craft.” Other multihull types include the wave-piercing trimaran perhaps best exemplified by the biodiesel-fueled 78' Earthrace, another Loomes creation that in 2008 completed a circumnavigation of its own in fewer than 61 days, a record-shattering attempt. A distant cousin to wave-piercer geometry is the axe-bow design (Yachts International, July 2008), a monohull variant that combines narrow forward sections, a deep forefoot and vertical prow and hull sides to produce about a 15-percent efficiency advantage over conventional flared-bow geometry. Beyond reducing fuel consumption, these types also offer a smoother ride compared to conventional V-hull forms, typically allowing more comfortable operation in a heavy sea state.

More recently, researchers have introduced other monohull modifications, like the bow bulb, aimed at improving efficiency through reductions in wave height. Among these initiatives is the development of parabolization, a process that extends midship hull sections outward to produce wider, more rounded contours than those of a hull with parallel sides. Following extensive theoretical studies and tank testing at the University of British Columbia, Professor Sander Calisal and his team of naval architects and hydrodynamics experts report substantial efficiency gains directly related to this modification. “A 10-percent reduction in wave height,” Calisal says, “can provide about a 20-percent reduction in wave resistance and roughly a 10- to 12-percent reduction in total resistance.” Calisal’s study has demonstrated that increasing the beam by 10- to 15-percent and eliminating the parallel middle body can produce a significant reduction in wave height—ergo resistance—and a corresponding improvement in propulsion efficiency. The augmented beam, he adds, also aids lateral stability—and what designer could fail to appreciate the resulting increase in interior volume? Calisal notes that parabolizaion may effectively be applied to existing vessels—including those with bow bulbs—as well as to new builds. Canadian yacht designer Patrick Bray currently is at work on a 45-meter tri-deck featuring a variant of the parabolic form, a pair of teardrop-shaped extensions, one on either side of the hull just below the waterline and, like the convex hullside geometry, positioned to diminish wave height by negating the formation of the midship “hollow” typical of displacement hulls underway. Bray, an early adapter and a developer of fuel-efficient hull forms, indicates that computer modeling of the new design has yielded promising results and plans tank testing to confirm the data obtained so far.

The simplest, most tried-and-true means of reducing drag are still available to all boatbuilders and owners virtually without regard to hull type or size. Designer Ron Holland, perhaps best known for his lithe sailing yachts capable of wringing every last fractional knot out of any wind condition, extols the virtues of single-engine installations for motoryacht designs, which was implemented in his Marco Polo design. “A hull powered by a single engine turning a large-diameter propeller can be up to 30 percent more efficient than a twin-screw hull of equivalent horsepower,” he says, citing the added weight and resistance produced by multiple engines, and the transmissions, propeller shafts, struts and rudders.

And lest we forget, the next increment of boat speed is nearly always the most costly: The simple expedient of throttling back by a knot or two can yield agreeable results in terms of fuel burn. All told, the means exist, both old and new, for motoryacht devotees to trim a sizable proportion of both fuel costs and emissions from their cruise agendas, to the benefit of their vessel operation budgets, the environment and the trade balance. ■