Selecting the best prop for your boat can be a mind-boggling task, given the number of prop options available for most outboard-powered rigs, and the number of variables to be considered.
Last summer, in part one of “The Right Propeller” I offered some guidance to Boats.com readers with step-by-step instructions on how to evaluate prop performance. In part 2, we explored propeller alternatives and the effect they may have on boat performance.
This time, we’ll put that process into live action with an on-the-water test session, comparing the performance of four propellers provided by Mercury Marine. The point here is not to judge these props, but rather to go through the routine of prop comparison just to see how it’s done, and to demonstrate the kind of results you might get doing this with your boat.
Mercury helped me set up the test by providing not just the props, but also from its Oshkosh, Wis., test facility, a 2005-model Lund 1700 Fisherman boat powered by a Merc 115 four-stroke outboard. Along with the boat, I got Chuck Blatz (is that the perfect Wisconsin name?), who has worked for 39 years as a Mercury test technician. Blatz had the boat rigged with a laptop connected to data loggers that measured boat speed, engine speed, and the rate of acceleration.
The first step was choosing four props. To do that, we used the Prop Selector mini-site that’s part of the Mercury Marine website. Launched a few months ago, this site is a big improvement over the one it replaces, and leads the boater through five pretty easy steps, prompting you to select a boat type, power specs, and whether you’d like to prop for speed, economy, or just all-around performance. At the end the site suggests a number of prop options. In our case, those props were the Black Max, Lazer II, Trophy Plus, and the Vengeance – options with three and four blades, in aluminum and stainless steel, ranging in price from $165 to $598. The recommended prop size was an 18- or 19-inch pitch.
With that, we went out on the water. Mr. Blatz had topped off the 27-gallon fuel tank, and our load also included three people with a combined weight of 420 pounds. We motored to smooth water, where there was no wind and very little current. We made two top-speed runs in each direction, adjusting trim to get the best speed as we watched the speedometer. And we made three acceleration runs in each direction, averaging the combined times. If you try this at home, you won’t have Mercury’s lab-rated test gear, but you can measure speed with a hand-held GPS, clock acceleration with a stopwatch, and gather equally useful data.
The chart above shows our results. There was a spread in top speed of just 1.14 mph, or about three percent. In acceleration, the spread was about 0.6 seconds, or 10 percent. I’m afraid that most of us would be hard-pressed to notice this range of performance through the seat of our pants.
But there’s more to picking a prop than just the raw data. Here are notes I made on each prop after I drove the boat. Remember – test results with your boat-and-motor combination might be different.
This is the base-line aluminum prop that might be shipped with many boats powered by this motor. The prop has less blade area and less blade cup than the stainless props, and for those reasons would not take as much trim, either. The Black Max would abruptly lose its bite on the water when it was over-trimmed, and it would slip and let the boat bog in moderate turns unless the motor was trimmed down before the turn. This due in part to its blade shape, but also because the aluminum blades flex a lot more than stainless blades as they load and unload through a rotation.
This is Merc’s “affordable” stainless steel prop, designed more for durability than for performance. It has the same 13-inch diameter as the Black Max, but comes in even pitch increments, and we tested an 18. Less pitch is like a lower gear, and it helped the Vengeance produce the best acceleration. But it was also the slowest in top speed, and will run at a higher rpm, and use more fuel, at any boat speed compared to the 19-pitch props. It might have been interesting to try a 20-pitch Vengeance, but with that prop we’d be running on the very low side of the motor’s 5800-6400 rpm range. Underway, the Vengeance seemed to produce a little less bow lift than the other stainless props, and you could feel it slipping in turns, though not as badly as the Black Max.
This prop has been in the Merc line for years. Its thin blades have quite a bit of rake to produce the sort of bow lift that really works on a bass boat but doesn’t matter as much on a Lund. It comes with 12mm PVS (Power Vent System) holes at the root of each blade that vent exhaust over the blades on acceleration, causing some slip that helps a two-stroke outboard rev up into its power. Those holes don’t have much function with a four-stroke, according to Merc. The Lazer II didn’t accelerate any better than the Black Max. It does hold well in a turn and takes trim.
The Trophy Plus not only has four blades, it also has the greatest diameter of the props tested here (a combination that gives it 54 percent more blade area than the Black Max, for example). It also has PVS holes, but they come plugged in this prop size. The prop was clearly the best in terms of handling. Steering felt smoother, and the Trophy didn’t produce a hint of slip in even high-speed turns, with the motor trimmed out to a good cruising angle. Four blades is like all-wheel drive for your boat.
What can we conclude from this test? For one thing, modern aluminum props work pretty well on a boat with mid-range power. The best all-around choice in this bunch would be the four-blade Trophy Plus, just because makes the boat handle better. But, alas, the Trophy is also the most expensive option.
My opinion? Unless you are frequently running among stumps or touching the bottom and chewing up aluminum props, you might was well live with the Black Max and pocket the $350 price difference. That’s like a season’s worth of gas. However, if you need or want the durability of a stainless prop, I’d spend $100 more for the Trophy Plus, because you might as well go for the gusto. But you wouldn’t make that choice without actually testing, because you need to feel the Trophy to appreciate its benefits. So before you spend this kind of money on a new prop, find a dealer that will let you do a little testing with your boat and motor.
Editor’s Note: This is part 3 of a series on Choosing the Right Propeller.
Read Choosing the Right Propeller, part 1
Read Choosing the Right Propeller, part 2