By Charles Plueddeman
Outboard Expert: The Right Propeller
In part one of a two part series, our Outboard Expert explains how to test your current prop to make sure it matches your boat and motor.
The propeller is the single, critical link between an outboard motor and the water. The only way all that horsepower gets turned into forward motion is through the prop, which is why it’s important for both performance and fuel economy to have the correct propeller on your boat.
Getting your boat propped right is not a difficult task. In this article we’ll walk you through the steps of testing your current prop to see if it’s a good choice for your boat-and-motor combination. The second article in this series will go a step further and explore the potential of different propeller options.
This basic propping exercise applies to any boat, even one that’s brand new. One veteran technician who works for a major outboard manufacturer and specializes in application issues recently told me he frequently sees brand-new boats that are delivered with the wrong-size prop.
“It’s more of a problem with dealers that are not on the water, because it’s more trouble for them to test the boat before delivery to the customer,” he said. “But I also see boat builders sending out new boats with the wrong prop, because they tend to prop for an impressive top speed, and they also frequently test the boat lightly loaded. So by the time you fill the fuel and water tanks, the live well, load in all your gear, and add a few friends, the boat is over-propped.”
He adds that the performance of perhaps 50 percent of the boats powered by smaller motors that are shipped from the outboard manufacturer with a “standard” prop – generally 40 horsepower and smaller – could be improved with a prop change.
So here’s some advice for those buying a new boat – it is often the dealer’s responsibility per his agreement with the outboard manufacturer to sell you the boat with the correct-size prop. Before you take delivery of the boat, ask the dealer if he has tested the boat, or one like it, to make sure it’s propped correctly. An then ask to be able to come back and exchange the prop that’s on the boat with a larger or smaller prop of the same type if you find that’s its outside the operating specs for the motor. Of course, you are going to make this exchange promptly, and you are going to bring back the original prop in pristine condition.
The first step in the propping process is to establish a performance baseline with the prop you’ve presently got on the motor. Of course, you are wasting your time if that prop is a mess. Bent or otherwise damaged blades should be repaired by a good prop shop first.
A prop is described by its diameter and pitch, always in inches, and always in that order. So a 14 x 24 prop is 14 inches in diameter, and has a pitch of 24 inches. The diameter is simply the distance across a circle scribed by the blade tips. Pitch refers to the distance forward the prop would move as it makes one revolution. Imagine the prop is a screw turning in wood. If you rotate that 24-pitch prop one revolution, it would move 24 inches into the wood. The dimensions of the prop are usually etched in the tube or stamped in the hub. So find those numbers and make a note. This is where you are starting.
Next, you’ll need to know the WOT (wide open throttle) operating range of your motor. Check the owner’s manual, the manufacturer’s website, or a dealer can look it up for you. The WOT rpm range is determined by the outboard manufacturer, and is where you want to motor to be under full throttle and optimal trim. A 2.6-liter Evinrude E-TEC, for example, has a WOT range of 4850 to 5850 rpm. For older two-stroke motors the WOT range is typically 4500 to 5500 rpm. On newer four-strokes and direct-injected two-strokes, the range is often at higher rpm and is narrower, 800 to 600 rpm rather than 1000 rpm. Ideally, you’ll prop for an rpm right in the middle to upper part of that range.
To do a baseline test, you need to be equipped with a tachometer, which displays engine rpm. If your boat does not have a tach, you could use a portable “shop tach” that’s designed for testing motors, and usually clamps one a spark plug lead. It’s also good to do this test with a GPS to get an accurate boat speed.
To make the test as “real world” as possible, the boat should be loaded as it will be for a typical outing. Fill the fuel tank, have your usual gear on board, and bring along a few passengers if that’s how you’ll usually be boating.
You want to find a nice stretch of open, smooth water where you can run for a long distance without turning, and where boat traffic is light. Get the boat on plane, trim the motor out a little, and then take it up to full throttle. Now trim the motor out in very gradual increments, just bumping the trim button. Note boat speed on your GPS. As you trim out, the bow will rise and the boat should gain speed, but eventually you may “over trim” the motor and the prop will begin to loose bite, and you’ll note the speed start to drop on the GPS. Trim back down and you may be able to here the pitch of the motor change as the prop regains traction with the water and speed increases. Now note your rpm, and remember the boat speed for later reference. Make a pass in the opposite direction if there’s significant current or wind that could affect boat speed, and then average the data for both runs.
You can also use a stop watch and the GPS to measure acceleration. We often talk about time to plane, but “on plane” is a hard state to pin point. Better to simply clock some zero-to-30 mph times (or pick a speed that’s appropriate for your boat). Start at idle, trimmed down, with the motor in gear. Have a helper manage the stop watch and GPS while you drive. Just do a ready-set-go and nail it. Do four or five runs and if the times are consistent, you’ve got good data.
That’s all there really is to do. If your boat is running in the middle to upper portion of the WOT rpm range, and its on-plane performance is satisfactory, all is well with your current prop. By landing in the middle of the range, you’ve got some leeway for the rpm to drop when the load is heavier, or go up when the load is lighter.
If you are at the upper end of the WOT rpm range, or exceeding the range, you are “under propped.” Imagine riding a multi-gear bicycle in a lower gear – your legs are spinning furiously but you are not going very fast. That’s your boat. By moving up one pitch size (say from a 15 to a 17), you will lower your WOT rpm to the middle of the range, and may pick up some speed. However, acceleration from a stop may not be as quick. Now, there are plenty of boaters who never push the throttle to wide open. And if you like extra-snappy acceleration, or your boat is under-powered or heavy and thus slow to plane, there’s nothing wrong with running the smaller-pitch prop, as long as you don’t over-rev the motor. Except…your fuel mileage will probably be better with a bigger prop, because the engine will turn at a lower rpm for a given boat speed.
If your test shows that as propped you are running below the WOT rpm range, you are “over propped.” You are on the bicycle in too big a gear, grunting and straining to get down the street. If you spend much time at wide-open throttle, this is bad for your motor. It raises combustion temperatures, and stressed the connecting rods and main bearings. You want to try moving down one pitch size (from 19 to 17 perhaps). You should also see a nice improvement in acceleration.
Most propellers are sized in two-inch increments. A 14-inch-diameter prop, for example, might be offered in 15, 17, 19, and 21 pitches sizes. Each two-inch increment results in a WOT change of about 400 rpm up or down. Suzuki was the first manufacturer to tighten that pitch spread, to 1.5 inches, and the new Mercury Enertia props are offered in one-inch pitch increments (from 14 to 22) because its Verado outboards have a WOT rpm range of 5800 to 6400, or just 600 rpm. Each increment on the Enertia props is good for 150 to 200 rpm, so WOT rpm can really be fine-tuned.
In our next column, we’ll look at how some of the available prop options – aluminum vs. stainless steel, three vs. four or more blades, high rake vs. low rake – can affect boat performance.
Need a new propeller for your boat? Visit the Propeller Finder in the Boats.com Gear & Parts Store.
- 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.