By Charles Plueddeman
Outboard Expert: The Right Propeller, Part 2
Today's props are highly specialized, so it's important to find the right one.
The Right Propeller series opened with the steps an outboard owner can take to determine if a boat is running the correct-size prop. In this second installment, we’ll explore some of the propeller alternatives that are available, and the effect they may have on boat performance.
Years ago, every boater started with an aluminum prop, and maybe stepped up to a stainless steel model, which was much more expensive but improved performance and was more durable. Today, like everything else in the world, propellers have become highly specialized. Outboard manufacturers and the aftermarket are developing props for specific boat types, props designed to work well with certain motors, and others designed to enhance one aspect of performance, such as acceleration. So there may be a prop out there that will optimize the performance of your boat, while others won’t work well at all. But, unless your current prop is way off the mark, simply changing propellers is not going to have a dramatic effect on boat performance – for example, an instant 5-mph boost in top speed. No propeller is a silver bullet.
Aluminum vs. Stainless Steel
The chief virtue on an aluminum prop is its low price. Name-brand aluminum props cost about $180. A very basic stainless prop starts at about $500. A stainless prop is stronger and more durable. It also will deliver better performance because its blades can be thinner and thus produce less drag, and because manufacturers put more design effort into a stainless prop. An aluminum prop is also subject to much more blade flex than a stainless prop. I’ve seen high-speed underwater video of props in action, and it is amazing to see how the blades flex as they unload near the water surface and then come under load again as they submerge. When subjected to too much flex, aluminum blades can break right off the prop.
“We generally don’t recommend an aluminum prop for our motors with the 4.75-inch gear case, which starts at 135 hp,” said Dirk Bjornstad, Brand Manager for Mercury Propellers. “And I would say that if your boat is running a more than a 21 pitch prop, it should be stainless.”
That said, aluminum props have really improved in recent years. New alloys and casting techniques have allowed the production of aluminum props with thinner, stronger blades that can work as well as a stainless prop on boats with more modest power.
“I think the owner of a boat with a 70- to 90-hp motor is the one who has to make a choice between aluminum and stainless,” said Suzuki applications manager David Greenwood. “If it’s a planing hull, and not a pontoon, the stainless prop is going to give more top speed just because the blades don’t flex. That said, the aluminum prop still works pretty well and if hitting bottom is not an issue, it’s a great value.”
Three Blades vs. More Blades
Things get more complicated when we start comparing stainless props with three and more blades. One reason to add a blade to the prop is to get more total blade area, which is sort of like putting wider tires on a sports car to gain traction. If a motor has enough power, the added blade area will help it hook up with the water and improve acceleration. However, you can also get more blade area by simply designing larger blades into a three-blade prop. Propellers designed to push large, heavy boats (so-called barge props) usually have big blades shaped like elephant ears. These props work best at lower speed and when the prop is always submerged in the water. A trolling kicker on a walleye boat, or a pontoon boat, for example, benefit from a three-blade prop with lots of blade area.
But a four-blade prop offers other advantages, especially in a situation where the motor is trimmed out at speed (like a bass boat) and the prop blades clear the water surface at the top of each rotation. In this application, a three-blade prop is going to have two blades “unloaded” at some point, while the four blade prop is going to always have two blades biting the water. The four-blade prop is going to be more effective at higher trim angles, and you may be able to raise the motor higher on the transom to reduce drag and gain speed. The four-blade prop will also produce more lift at the stern, improving hole-shot acceleration and speed on boats that are heavy aft, like a bass boat or a flats skiff. The four-blade prop may also feel smoother and hold the boat better in turns.
The four-blade disadvantage should be increased drag. A decade ago, the conventional wisdom held that a four-blade prop would improve acceleration but sacrifice some top speed. That’s not always the case with the latest four-blade designs.
“Our tests have shown that a four-blade prop can be 20 percent more efficient in some situations,” said Greenwood. “You may be able to raise the motor on the transom, and the added stern lift allows the boat and motor to achieve a better angle of attack on the water. More power is available to push the boat forward, rather than hold the boat up.”
That advantage diminishes on boats with less performance potential, or which keep the prop in the water, like an offshore fishing boat or a family runabout. In those applications, the old rule usually still applies – four blades deliver better acceleration and smoother steering, at some cost to top speed. So if you are trying to pull a big tube or skier with a 150-hp outboard, in a boat also loaded with people, a four-blade prop might be worth a try just to get the boat on plane more quickly.
Demo to Decide
The number of props on the market today can make the selection process an overwhelming task. Many prop manufacturers offer a web program that uses information on your boat and motor to get you in the ballpark. Mercury Marine has just launched a new website with lots of great prop information and a very slick prop-selection feature. But, it’s really hard to predict how any prop will perform until you test it on your boat.
“Just because it works on your buddy’s boat does not mean it will perform the same on yours,” said Greenwood. “There are so many variables, including boat weight due to gear and passengers, the mounting height of your motor, and even the way water flows off the hull, that you just need to bolt on the prop and try it.”
Dedicated prop shops seem to be more willing than some dealers to let customers test-drive a prop, but if I’m going to spend $700 on a new wonder-prop, the dealer has to let me try it. Many keep demo props on hand for that purpose. Mercury currently has about 110 dealers participating in its prop demo program, which supplies dealers with a range of props (they were painted blue when the program started) they can use for customer testing. You can use the Mercury dealer locator to find one. Prop Gods, a Florida service, will meet you at the water and for $95 an hour let you test drive all the props you care to try, and then sell you the one you like the best. That sounds like the ultimate way to fine-tune your boat’s performance.
Need a new propeller for your boat? Visit the Propeller Finder in the Boats.com Gear & Parts Store.
Editor’s Note: This is the second of a two part series. Read The Right Propeller, Part 1
- Charles Plueddeman is Boats.com's outboard, trailer, and PWC expert. He is a former editor at Boating Magazine and contributor to many national publications since 1986.