You want another mph or two on the speedo so you can sneak ahead of your buddy's boat. Can you get it by switching props?
Or, say you're tired of sluggish hole shot. Is a new prop the answer?
Yes, say the experts, but ... there's a trick to it.
"Everybody always wants an ironclad answer on what prop to use," says Yamaha propeller expert Ted Zahorski, unabashedly compounding his absolutes. "But finding the answer is always a trial-and-error process."
Most people think of speed first, but usually acceleration is more important. "You want to feel something when you hit the throttle," says Zahorski.
So with a million props and a million variations in pitch, diameter, rake and cup — whatever those mean — how do you find a good starting point for improving your ride? Vito Palazzolo rigs boats with Yamaha engines and suggests you go online to find test reports.
"I'll look for a similar hull of nearly the same weight and horsepower and use their performance reports for a baseline propeller," says Palazzolo. "Then I'll experiment from there."
If Palazzolo's test boat comes close to other boats with, say a 19-inch pitch prop, he'll try both a 17- and a 21-inch prop. Zahorski agrees.
Next, both experts say they'd experiment with cup and positive rake to get better "grip" on the water.
What is "rake?" A prop has positive rake if its leading edge leans toward the stern of the prop hub's centerline. Negative rake means the blade edge leans forward.
"A positive rake tends to grab the water and pull it toward the center of the prop," says Zahorski.
Then what does "cup" mean? A prop is cupped when the trailing edge of the blade is curved backward. Both Zahorski and Palazzolo agree that cupping also tends to pull water to the center of the prop, improving bite on the water and helping with acceleration.
"Lift" is another important term. Lift comes from the downward pull of the prop turning in the water. Both gurus agree that getting enough "bow lift" from the prop to get as much of the boat out of the water as possible will give better speed and performance.
TERMS YOU NEED TO KNOW
Specified rpm: Every engine has a specified rpm range. Choose the prop that gives the performance you want without exceeding either the high or the low end of that range.
Pitch: Theoretical distance a prop will move through the water in one revolution if there is no slipping.
Diameter: Distance from the center of the hub to the tip of the blade times two. Large diameters sometimes give stronger hookup for slower-moving boats.
Cavitation: Water passing the leading edge of the prop vaporizes from the pressure change, then devaporizes, striking the trailing blade edge or the gear case. Cavitation erodes metal much like a sand blaster.
Ventilation: In turbulent water, the prop can grab air pockets, causing it to lose thrust. Larger-diameter props with less rake and cup are more forgiving on turbulent hulls like pontoons.
Ventilation holes: On performance craft like bass boats, ventilation holes can increase the acceleration by causing air to form around the hub, letting the prop spin up faster.
Aluminum props: Less expensive metal is more forgiving on impact, but it flexes, slowing acceleration.
Stainless-steel props: Thinner, stiffer blades give better acceleration but are less forgiving in impact, increasing risk of gear-case damage.
DECISIONS, DECISIONS
Even armed with information, there's a huge swamp of variables to slog through — and getting the experts to agree on a few rules isn't easy. But here are some good starting points for maximizing your boat's performance.
Pontoons
Pontoons create a lot of turbulence ahead of the prop, and because of the hull shape, prop design doesn't impact lift much. To keep the prop hooked up try these:
• Wide diameter prop
• Positive rake
• Little or no cupping
Bowriders and Deck Boats
These boats are about fun and thrills, so acceleration to pop skiers and yank tubes takes priority over top speed. Test props of varying:
• Cupping to improve hole shot
• Positive rake for added lift
• Moderate pitch for acceleration
Bass and Performance Boats
Speed, speed, speed. Now is the time to go long on pitch and forget a snappy hole shot. The goal is to get the most mph from rpm. Try:
• Cupping to improve acceleration
• Rake for strong hookup
• Long pitch for maximum speed
• Ventilation holes in hub
Offshore Boats
Fuel economy and mid-range control — especially in following seas — are most important. Sometimes saltwater boats are prop-tested in fresh water, but saltwater is more dense and tends to slow the boat down with the same equipment. Try:
• Lower pitch to handle load and fuel
• Four blades to assist in lifting the heavier rig
• Cupping for acceleration
• Positive rake
Sources: solas.com, michiganwheel.com, turningpointpropellers.com, yamaha-motor.com
