A boat making 8 knots at 2000 rpm, with a 2:1 reduction gear, completes about 7,500 propeller revolutions every mile and well over 1 million revolutions through 24 hours.
For distance cruisers, it’s interesting to consider how propeller efficiencies can multiply. Beyond better fuel economy, tweaking your boat’s propeller might also increase optimal cruising speed, improve engine health and even facilitate better boathandling.
We turned to two propeller experts for advice about how to wring better performance from that bronze interface between the ocean and your boat’s engine.
Analyzing Propellers
Most prop shops evaluate propeller performance free of charge, says Randy Hale, owner of Hale Propeller in Old Saybrook, Conn. and Beaufort, N.C. That process starts with performance numbers for your boat’s current props, which ideally will include speed and fuel burn at full throttle, again at cruise rpm, and in 200 rpm increments throughout your boat’s normal operating range.
The first thing Hale looks for is engines struggling to turn those props, a problem that is common when boats take on extra pounds over the years. Those overloaded engines burn too much fuel at full throttle and cruise rpm, robbing economy. Accompanying elevated exhaust-gas temperatures may cause serious cylinder, valve, manifold or turbocharger issues.
Hale scrutinizes engine-load percentages on electronic diesels, too. “Even if you reach full rpm, if the engine hits 100 percent load at lower rpm, it’s still overloaded,” he says. Engine manufacturers might also specify maximum fuel burn at cruise rpm.
Propellers are usually matched to boats and engines by adjusting pitch—the blades’ angle in relation to the prop shaft. Because overworked props are likely pushing boats slightly faster at a given rpm, fuel savings are typically minimal.
But that’s where the story takes a turn. Today’s tools for measuring propeller shape detect deficiencies that would have gone unnoticed not all that long ago. The result is props repaired to tighter tolerances that cut more smoothly through the water, increasing efficiency and mitigating noise and vibration.
Benefits of Precision Repair
Hale, along with his father, Randal, a physician who worked in early medical MRI technology, developed the Hale MRI that maps propellers in three dimensions.
“We’re measuring the shape of the blade along any specific radius,” Hale says. A simple repair might check three radii, or 50 percent from hub to tip, again at 70 percent and also 90 percent. The highest-accuracy repairs measure six or more radii.
A computer records pitch and chord length (which defines the three-dimensional shape of each blade). It also indicates blade tracking (how closely blades follow the same rotating path), spacing between blades, rake (how far blades extend forward or abaft the prop hub) and cup height (a slight curve at the trailing edge of the blade).
To avoid propeller vibration, most prop shops aim for ISO Class 1 repairs, where each specific pitch measurement is within 2 percent of spec, and the average for the entire prop is within 0.75 percent. Class S repairs, measuring within 1.5 percent local and 0.5 percent mean pitch, may increase performance, particularly on boats that fully plane. Prop shops retain those computer records to facilitate future repairs, too.
“The more accurate the repair, the smoother the propeller will run,” Hale says. “All of the blades are doing the same amount of work as they go around.”
Static balancing, with the propeller on a stand, or dynamic balancing, where propellers spin 500 rpm on a shaft and accelerometers measure wobble, are the final check to ensure smooth operation.
Science or Art?
Prop repair itself hasn’t changed in decades. Generally, props are heated with a torch and forced into shape with a pair of copper or rawhide hammers, or a backing fulcrum below and hydraulic press above. Technicians alternate between measuring and repair stations several times.
“Fine tuning is best done by hand,” Hale says. “An experienced technician knows just how to hit the blade to move it a few thousands of an inch.”
Technicians repair cracks and replace missing material by welding in new bronze alloy. “With the right welding rod at the right temperature, with good penetration all the way through, welding is just as strong as the original material,” Hale says.
Some damage is beyond repair. Hale’s threshold is around half of replacement cost, although turnaround time, which might be five to 10 weeks for a replacement prop, could be an overriding factor in a short boating season. Particularly for distance cruisers, Hale suggests carrying spares.
“If you can swap a prop and keep going, versus sitting two weeks in a port you might not want to be in, the few thousand dollars you spend for a spare will pay off,” he says.
Advanced Propeller Design
Prop shops can often improve existing props: Adding a bit of cup to the trailing edge of propeller blades might increase performance at cruise rpm without overloading engines, for example. But the science behind propellers has advanced leagues in just a couple of decades, and improved propeller designs have followed suit.
At the end of the Cold War, U.S. Navy computer propeller performance models became available to the public. Before then, civilian propeller working faces were flat, like a fan blade. This often overloads tips and edges, causing cavitation, yet leaves most of the blade underperforming. With those once-secret computer models, propeller manufacturers began varying prop pitch across blades from leading to trailing edges, like an airplane wing, and radially from propeller hub to tip. This spreads the load across the entire blade to better match engine horsepower curves, mitigate cavitation, and decrease noise and vibration.
“In general, a sophisticated prop design is more efficient than a prop with constant-pitch ogival sections,” says Kevin Mitchell, a naval architect working at Frank & Jimmie’s Propeller in Fort Lauderdale, Fla.
Bigger gains can come, though, as horsepower and speed increase. Foil-section props might be adjusted to add stern lift to improve performance for planing and semi-displacement boats, for example, where a flat-profile prop would tend to cavitate along blade tips and edges on the same boat.
“Displacement boats hit hull speed with relatively low power,” Mitchell says. “A good prop might be 4 or 5 percent more efficient, but it isn’t like a sportfish where the boat is suddenly going a knot faster.”
Performance Gains
Boats burning perhaps a gallon a mile may never recoup $10,000 to $20,000 for a complex-geometry prop, but when new props might resolve other deficiencies, that equation can change.
Less-sophisticated designs, at two-thirds or even half of those prices, often score big improvements just by selecting props better suited to a boat. Increasing from four blades to five, for instance, almost universally increases performance or economy at cruise rpm, although often with a slight performance loss at full throttle. Additional blades also mitigate vibration where they track through water disturbed by a boat’s keel or running gear.
Skewed blades, which are curved like bananas instead of being symmetrical like clover leaves, further reduce the effects of disturbed water. Adjusting blade rake affects a boat’s trim underway. Propeller noise might be decreased dramatically by adding more blades while decreasing diameter. New props can even improve thrust in reverse.
It’s also not uncommon for boats to have suboptimal props right from the start, Mitchell says. To reduce draft, boatbuilders might choose props with a smaller diameter than what’s ideal, so a new prop with more blades will improve efficiency. Boatbuilders don’t necessarily invest in the best propellers, either, and prior owners may have replaced damaged props without considering efficiency.
Tougher Props
Traditional manganese bronze propellers have steadily given way to NiBrAl, an alloy of nickel, bronze and aluminum. NiBrAl holds its shape better under load, offering theoretical gains in performance.
Perhaps more important, it stands up better to an occasional whack from floating debris or nudge against sand or mud, potentially avoiding a costly haulout and prop repair. Abrasion, cavitation and corrosion resistance are also better for NiBrAl, and price has come down compared to manganese bronze.
Comfort Underway
With all the spin on propeller efficiency, it’s easy to overlook crew comfort, which might be the real bottom line, particularly when decreasing noise and vibration benefits distance cruisers for days on end, versus just a few hours between ports for weekenders.
Propeller noise comes from cavitation caused by overworked propeller edges. This is exactly what Navy computer models were intended to prevent. The sound (like gravel hitting the boat bottom) is actually propeller cavitation bubbles imploding.
Accurate repairs ensure that props perform as they should. In fact, on long passages, a propeller that shakes with each revolution offers a million reasons every day to seek improvement.
Sea Trial Guide
Starting with a clean bottom and trim tabs retracted, with a normal load and half-full fuel tanks, make two runs on reciprocal compass headings. Note the speed and running angle, as well as electronic engine load and fuel consumption for each engine. Do this at full throttle, normal cruising speed and 200 rpm increments through the boat’s operating range. Also record wind and sea state.
