Propeller Design

Prop design has undergone a revolution-and it's only getting better.

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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.

Platzer also maximizes engine horsepower curves. To help engines overloaded at cruise but not at full throttle, for example, propellers with less blade surface but with more camber (airfoil thickness) decrease engine load yet increase efficiency at mid-rpm. He also adjusts a boat's running angle by raking blade tips aft to lift the stern and decrease the need for trim tabs, or reducing rake to raise the bow. Propeller shaft angles make outside, downward-turning blades run at a steeper angle to the water, doing more work than inside, upward-turning blades. This lifts the stern when running, and adjusting that lift fore or aft changes the running angle of the boat.

MIT's Navy software was created not only to design props, but to analyze existing hull and prop combinations, so in just a few moments, Platzer can calculate existing boat deficiencies by knowing a boat's rpm, speed, engine load, and current propellers. "In five minutes I can tell someone what propeller I recommend and what I can do for them in increased efficiency or reduced vibration." Detailed design takes longer, but spreadsheets are e-mailed to

Hung Shen Propellers in China where castings are made and propellers are machined for each yacht. The entire process takes four weeks, including shipping, and costs less than $35,000 for a pair of custom 40- inch five-blade props-a typical choice for an 80- to 100-foot motoryacht. Hung Shen, as well as companies like Michigan Wheel Corporation (www.miwheel.com) and ZF Faster Propeller, a division of ZF Marine Group (www.zf-marine.com), use Navy data to create custom propellers for new production boat models. These designs are adjusted to make "semi-custom" props for five to 10 percent less than Platzer's custom propellers. High-tech "stock" propellers range from around $20,000 to $25,000 for a pair of similar 40-inch five-bladed props.

One drawback to better propellers is the need for more accurate propeller repair. Prop Scan (www.propscanusa.com) and competitor Hale Propeller MRI (www.halepropeller.com) directly measure thousands of points on propellers, mapping each blade shape in a computer. "What would have taken an hour previously takes only five minutes today," says Jim Harrison, president of Frank and Jimmie's Propeller Shop in Fort Lauderdale (www.fjprop.com), and also president of the National Marine Propeller Association (www.nmpa.net). Computers also increase accuracy, particularly in repeating shapes from blade to blade, and replicating exact shapes in subsequent repairs.

Harrison also developed True Torque, which computes actual engine output while the boat is running, using hypersensitive strain gauges to measure the miniscule twist of the metal within propeller shafts. "Engine computer load percentages are calculated based on temperatures and pressures," Harrison says. "The motor is running 100 percent, but 100 percent of what? You were sold an 800-horsepower engine, but what if it is only making 700 horsepower? Strain gauging tells us the actual horsepower of the motor." Since True Torque measures shaft horsepower and engine parameters through all rpm, sea trials also provide accurate horsepower curves for engines not run by computer management systems.

Propeller removal tools have changed, too. "Any five-bladed propellers, and even some with four wide blades, don't have clearance between the blades for a universal propeller puller," says Lou Daniello, my brother and president of Barnacle Busters (www.barnaclebusters.com) in Palm Beach Gardens, Florida, who has worked in underwater maintenance for more than 30 years.

The solution for most boatyards and diving services is PropSmith, which threads onto the propeller shaft and uses three bolts threaded into holes pre-drilled into propeller hubs to remove and also reinstall high-tech propellers (www.propsmith.com). While a PropSmith for a particular size shaft is relatively inexpensive, under $1,000 for a three-inch shaft, a full set covering all boats exceeds $10,000, which is more than many yards or divers will invest for just an occasional need. Boats that travel away from major repair facilities should consider carrying a PropSmith as part of an on board tool kit. They should also carry spares of non-standard parts, like lock washers used on some Asian or European boats in place of traditional cotter pins. "That's all that holds the propeller on," Daniello says. "You might get by reusing them, but it isn't worth the risk." He also recommends acquiring any other specialty tools needed to do the job, such as thin-walled sockets, which are needed to remove prop nuts on some Italian boats.

No matter how you spin it, the props are where your engines gain traction to push you through the water. Custom-built propellers, tuning, and tweaking can give you additional speed, less noise and vibration, and more efficient operation. Maybe it's time your props took a turn for the better.