I was walking the Miami Boat Show with a former client from my yacht-captain days when we happened across a display showing some of the latest high-tech seven-bladed propellers.
"Would new props help my boat?" he asked. My short answer to his complex question was, "Probably." His 76-foot Lazzara was built in 1994, when engines offered a third less horsepower for their size and weight. His engines struggle just below cruise rpm and the propellers vibrate as the boat comes on plane. It runs much better, though, at higher rpm-a good indicator that new props would make better use of his engines, and also his fuel budget.
New propellers probably won't increase his cruising speed beyond its current 18 knots, but today's propeller designs better match individual boats and engines and improve fuel economy, run more smoothly and quietly, increase performance in the mid-rpm range, and even reduce the need for trim tabs.
High-tech props for recreational boats are a surprisingly new development. The Wright brothers recognized that propellers were really airfoils-wings flying circularly through air or water and creating lift on a propeller's forward-facing side. Unfortunately, applying this knowledge to yachts took a full century. Even with the Cold War resources of the U.S. Navy, prop design required tedious trial and error, at least at first.
"The Navy funded MIT to develop software to design propeller geometry and also to predict performance of a given design," says Greg Platzer, president of Platzer Marine Propulsion in Fort Lauderdale (www.platzermarine.com). "Initially these codes were proprietary, used to design Navy ships," he says. Then, like a scene from a Cold War spy thriller, "The Navy moved to a different propeller technology, and the codes became available [to the public] in the early 1990s." He would know; Platzer headed the U.S. Navy's surface ship propeller design group through 1990. This software, tweaked at taxpayers' expense, enabled precise propeller design and instant computer evaluation of propeller and hull combinations, advancing the science rapidly over just a few years. The first of these new props were for commercial ships, but high-tech yacht propellers arrived with the new millennium.
Platzer still designs propellers for military and commercial craft, but the thrust of his business is creating custom airfoil-shaped props for individual yachts, focusing on issues boat owners most want to improve. I first encountered his company when interviewing George Sawley, a top light-tackle marlin captain who wanted more speed in reverse. With new custom props, Sawley reports better backing and a cleaner wake for trolling. He also gained about a knot at the top end. At 2000 rpm the boat maintained its 30-knot cruise, but fuel burn dropped from 75 gallons per hour to 65 with Caterpillar C18 engines, a gain partly due to correcting original overloaded props.
"Typically we can gain four to six percent, but that can go as high as 10 percent," Platzer says. "We can reduce vibration by 75 percent, down to about a quarter of what it was."
How can he be so sure? "Propellers are almost universally smaller than optimum to keep draft down," Platzer says. "The optimum diameter of a five-bladed prop is always going to be smaller than a 4-blade, so by increasing the number of blades we gain efficiency."
Platzer also manages cavitation, which is inevitable on high-speed boats. Most older propellers concentrate load, and therefore cavitation, at the leading edge of blades, but by varying a propeller's pitch to change a blade's angle of attack at different radii, and also by progressively changing both pitch and camber to tweak the shape of the airfoil along each radius, Platzer spreads the load over more of the surface of the blades, reducing cavitation and mitigating what remains. This increases propeller efficiency and reduces vibration.
