The hull of Steve and Linda Dashew’s new vessel, an 83-foot powerboat, what they call their “unsailboat,” resembles that of their sailboats, featuring a long, narrow light-displacement shape that is intended to cruise at 12 knots and still comfortably ride through rough weather — at least according to their computerized tests.
The Dashew approach to sailing, which embraces use of technology in both boat design and seamanship skills, has gained a certain ubiquity in no small part because of the couple’s prolific publishing efforts as well, including the Offshore Cruising Encyclopedia, Surviving the Storm and Practical Seamanship. And so it is that the Dashews have somewhat sheepishly announced that they will be building a new kind of boat, one that is fast and technically advanced, to be sure, but that is lacking a certain something that their previous boats have all had, namely a sailing rig. Yet the Dashews have not admitted, either to themselves or the public, that the boat is a powerboat. “It’s an ‘unsailboat,’” Steve Dashew said in an email interview. Indeed, the 83-foot aluminum hull of the vessel, now in the construction phase at Kelly Archer in New Zealand, resembles each of their previous designs, a long, very narrow hull with limited displacement and a nearly plumb bow. The vessel features twin rudders and a pair of stabilizers mounted just aft of a short keel, all of which the Dashews hope will allow safe operation in extreme conditions, whether tropical storms or high-latitude gales. How will such a unique vessel perform?
“The trade-offs involved in optimizing a power design for crossing an ocean are considerably different than what you find with sail,” Dashew said. “Under sail, stability is the critically important factor for speed and comfort. Under power, the need for initial stability to offset the heel induced by the rig is removed. This allows the designer to aim at a different package of hydrostatic characteristics. In our case, the focus has been on survivability in heavy weather and comfort on long passages (4,000 to 6,000 miles).”
The computerized version of the design, its “computational fluid dynamics” (CFDs), was tested by Lee Hedd at Ocean Consulting in St. John’s, Newfoundland, whose past clients include several America’s Cup syndicates.
“This allowed us to vary hull shape, fins and the distribution of weights around the hull to optimize motion and controllability in a variety of sea states. We tested up to sea state 7 — 20-foot significant wave heights,” Dashew said. “While we were going through this process, we constantly looked at the drag output of our VPP (velocity prediction program) software. At the point where we thought we had an optimum package of comfort, heavy-weather capability and powering efficiency, we decided to test the resulting hull in the tank. This provides us with an added safety factor in the design process, while generating drag data to compare with our own VPP-generated information.
“During the tank-test series, we investigated a series of refinements in the design. These resulted in a drag reduction of 13 percent at our cruising speed of 12 knots. It is interesting to note that the tank-test data had close correlation to what we were seeing with our computer-generated drag figures.”
The new Dashew vessel, call it what you will, will likely head for the tropical South Pacific and eventually, perhaps, Tierra del Fuego.
“But as you know,” Dashew explained, “with boats and cruising, the only thing that is certain is that the plans will change.
