Choosing the Correct Propeller for Your Outboard

Choosing the Correct Propeller for Your Outboard

The right prop will get the best performance out of your boat. Matching the propeller to your boat application, whether cruising, water skiing or performance, maximizes the power from your outboard engine. It also helps with handling and acceleration. 

Use this guide to learn how different propeller designs affect efficiency, power, and speed and then learn how to choose the correct prop for your boat and application.

Here are tips, straight from the experts!

While initial propping for new boats is often spot on, increases in boat weight over time might call for an upgrade.

Choosing the right outboard propeller can be perplexing, but it doesn’t have to be. Selecting the right propeller is part guesswork and part science. 

To home in the science side, we asked four of the top experts in the marine industry for their best tips on choosing the right prop for your boat-and-outboard combination.

Our panel included Gary T. Diehl (GD), Mercury Marine’s senior category manager for props and gauges; David Meeler (DM), Yamaha Marine’s product introduction and information manager; and David Greenwood (DG), Suzuki Marine’s general manager for the Marine Technical Center USA. and Ashlyn McGuire from Marine Parts Warehouse Here is what they had to say.

Dialing in Pitch and Diameter 

Diameter

In propeller designation, diameter is the first number listed, followed by pitch. Diameter is measured across the outer circle of the blade tips, while pitch is the distance a prop theoretically moves forward in a single revolution. So, a 15x17x3 designation ­indicates a 15-inch diameter and 17-inch pitch with three blades. 

In most cases the boat propeller and outboard manufacturers have pre-selected the diameter range needed for your engine. Your selection can be focused almost exclusively on pitch. Your vessel will respond to different pitches the way your car responds to different gear positions in your car's transmission. 

But keeps this in mind when changing propellers, “Diameter can have as much impact as pitch,” McGuire explains. “All things being equal, one-quarter inch in diameter equals the same 150 rpm change ­associated with pitch. That’s because of blade surface area. A larger diameter increases the blade surface, which contacts more water. 

But it’s also more difficult for the engine to move the prop as diameter increases. That’s why in a family of propellers as the pitch increases, diameter generally decreases.” A propeller only performs when water is on the blade. 

Rake

Blades have other important features built into their design. Rake is the angle between the blade and the hub. The rake angle can be flat, staying the same along the entire blade, or progressive, meaning the angle increases as it approaches the tip. 

Props have between 0 and 20 degrees rake. The rake angle determines how much the bow lifts out of the water. High rake propellers are ideal for light-weight, high speed boats. Too much rake, however, will strain the engine and reduce overall performance.

So consider the shape, angle and number of blades. McGuire uses the image of a two-by-four to illustrate the concept of rake. Sliding the board through the water skinny side first is easy. But trying to push it through on the flat side is much more difficult. Having a lot of rake is similar to pushing against the flat side of the board. 

Conversely, the quicker the engine spins up (the less the rake), the faster the boat moves through the water. Moving to a higher pitch will result in a decrease in your full-throttle RPM, just like the effect when your car's transmission shifts to a higher gear. 

The reverse is also true: move to a lower pitch and your full-throttle RPM will increase. Most propellers are sized by two-inch intervals, example 17", 19", 21", etc, and a 2" pitch change will result in an average 300 to 400 full-throttle RPM change.

Pitch 

Pitch is the distance the propeller will move the boat after one revolution (not accounting for slip). A lower pitched propeller will create more power due to more engine RPMs, but the boat will move slower. A higher pitched prop allows the boat to move faster by travelling a farther distance with each rotation. 

When choosing a propeller, choose a pitch that will keep the engine RPM in its recommended operating range. Going under the range will cause the boat to lug, while going over can cause engine strain.“There is no perfect propeller that does it all,” says David Meeler, Yamaha Outboards propeller information manager. “There are too many variables in boat design, and performance. Each propeller is a compromise. You have to give up a little of one aspect, such as top speed, to get more of another, like acceleration.” 

The optimum boat propeller for your particular application is one that will allow your boat's engine - with your normal load aboard - to develop full-throttle RPM within the engine manufacturer's recommended maximum RPM range.

DM: Each inch of the pitch will affect engine speed by 150 to 200 rpm. As the pitch goes up, rpm goes down. When the diameter goes down, rpm ­rises. Each one-quarter inch in diameter has the same effect. A boat should be propped to where it’s properly trimmed out and the wide-open throttle (WOT) rpm is in the top 30 percent of the manufacturer’s acceptable WOT rpm range with the average load in the boat.

Blade Cupping

Cup, the curling or bending of the blade tips, will also impact performance because shape influences how water comes off the blade. Cupping can add grip and enhance bow rise, but too much can add drag. With cup, like rake, a happy medium is usually the best combination.

Recommended RPM 

Outboards have a recommended WOT (Wide Open Throttle) and the right prop keeps the engine in that sweet spot. The wrong propeller causes the engine to labor or over-rev, shortening its life and decreasing performance.

Damage to the engine can occur if a propeller that allows the engine to over-rev (attain a full-throttle RPM higher than the specification) is installed. Damage can also occur if the installed boat prop causes the engine to lug - not reach the minimum specification. 

Overall best acceleration and top-end speed will be achieved when your full-throttle RPM is near the midpoint of the manufacturer's recommended maximum RPM range.

Aluminum or Stainless? Why does Propeller Material Matter? 

Factors to Consider: 

What environment will you be in? Deep water is typically safe for aluminum, while you’d want to stick to sandy spots in shallow water unless you’re running a stainless steel propeller on your watercraft. What is your budget looking like?

If you’re concerned about how much you want to spend, take a few minutes of basic price research, then let that information guide your decisions on what kind of budget you should have. What kind of performance level are you looking for? 

If you’re just tooling around doing a bit of fishing on the weekend, not having an ultra-efficient propeller may not be a problem. For a racing boat, on the other hand, you’d want a premium stainless steel propeller to ensure superior performance. 

Exactly how will you be using your boat? As mentioned above, an aluminum propeller is sufficient for most boats, with only boats that demand the best quality, performance, and top speed often rating a stainless steel propeller.

Advantages of Aluminum Props 

To start, aluminum props are very economical, often costing a third of what a comparable, stainless steel propeller will run. This is one of the top reasons why they’re selected so commonly for boats and are used by manufacturers, due to the lower cost of outfitting a boat. 

Given their low cost of acquisition, it’s pretty common to see them used as a spare propeller as well because, for a low price, you gain a little insurance in the off chance you run into issues with your boat, such as a prop blade shearing off due to an impact while you’re out and about. 

With a few simple tools onboard, you can make repairs to get back to shore, where you can get a repair or replacement for your existing prop. An aluminum propeller can also be easy to repair, especially when compared to a stainless steel propeller.

Disadvantages of Aluminum Props 

Though there are some notable advantages, there are also some disadvantages to aluminum props. To start, aluminum is a strong but flexible material, which is among the reasons why it’s used in many types of boats and aircraft. However, this flexibility can cause the pitch on your propeller blade to change, which can reduce performance, especially on higher horsepower boats at high speeds. 

If performance is vital to your boat’s purpose, it may be worth looking at the more rigid stainless steel prop options. If you’re expecting to pull all the performance you can from your outboard engine, another potential disadvantage is the thickness of an aluminum prop blade. Because aluminum blades are thicker than stainless propeller blades, they create more drag in the water, which lowers your top speed. 

If higher speeds are important to your operation, a stainless steel prop will make it easier to reach a higher top speed due to reduced drag. The third disadvantage of aluminum propellers is that they are more likely to be damaged. If you’re only running in sandy conditions, this may not be an issue, but if there’s a lot of driftwood, rocks, or underwater obstacles, you’ll find yourself digging out a spare prop much faster with an aluminum prop on your outboard motor that you’ll see on a stainless steel prop.

Stainless Steel Propellers 

Though you may initially balk at the higher cost of an average stainless steel propeller at your local prop shop, that cost also provides you with a range of options that will boost your boat’s performance and provide superior durability.

Advantages of Stainless Steel 

To start, a stainless steel propeller will deliver better performance in an average 3-blade prop. At the same time, you won’t lose the better handling and superior hole shot that you’d expect from a lower-pitch propeller. This makes this type of propeller more efficient in the long run, making up some of the expense of purchase. 

A stainless steel prop will also see superior durability and longevity. The same blow that may snap an aluminum propeller blade will often simply glance off of a stainless steel propeller, due to the higher level of durability as a simple aspect of using stainless steel. 

The high performance that these propellers produce is caused by thinner blades that reduce drag and a stiffer, stronger profile that is able to cut through the water more efficiently. 

This loss of drag makes it easier for your motor to reach its true top speed, with superior efficiency. Because stainless steel props operate more efficiently, you’ll experience better performance that leads to exceptional fuel efficiency in the motor, allowing you to get more boating in for less expense. These aspects can often defray much of the cost of upgrading to a quality stainless steel prop.

Disadvantages of Stainless Steel Props 

For all these advantages, there are disadvantages as well. The main disadvantage of a stainless steel propeller is its cost. As a more expensive option, these propellers will cost more at the onset, but this cost is often defrayed through lower overall operational expenses, such as superior fuel economy, better performance, longer lifespan, and better durability.

Aluminum makes a great lightweight spare to have on board, which is ­highly ­recommended. Also, when navigating unfamiliar, shallow or debris-strewn waters, an aluminum propeller will absorb an impact by bending or breaking. 

GD: Boats with smaller outboards tend to run in shallower waters and are more prone to hitting bottom or structure, so it can make sense to use an aluminum prop to keep from breaking other gearcase components. 

The Biggest Mistakes in Selecting Props 

DM: Many boaters say, “I ­never run wide-open, so what does it matter?” It matters because, otherwise, you don’t know if the boat is wasting rpm and not giving total available speed (not enough prop) or if it is pushing too hard and the engine is lugging (too much prop). 

DG: The most common mistake occurs when the boat gets propped correctly initially, then owners add extras onto the boat over time, and now the rpm and hole shot are greatly affected when the boat has a lot of passengers. 

What to look for in a propeller to maximize your boat's performance.

Modern casting techniques allow precise detailing in propeller manufacture. Courtesy Mercury Marine

Three Blades: Three-bladed props, the most common configuration, are the most efficient. They spin up faster and produce less drag.

3-blade props are perfect for recreational boats. They’re especially good for boats with 3, 4, and 6 cylinder outboard and I/O engines. Fewer props allow for a higher top speed but less horsepower. Since recreational boats are often lighter, you aren’t losing a lot. You’ll be able to achieve a higher speed and have more fun on the open water.

Four Blades: Four blades aid initial acceleration and stern lift, advantageous for flats boats in skinny water or the center engine in a triple setup.

When to Go With Four-Blade Props 

DM: Four blades are ­specialty props and should be used when your boat needs extra load-carrying capacity, maximum hole shot, and better control in rough water. 

DG: One caution about four-blade props is they tend to create stern lift that can drive the bow down and make the boat run too level. 

The Right Prop for Varying Loads 

DM: This is the ideal situation for owning two propellers, one to address each scenario. However, if only one prop is used for two widely disparate applications like this, I would prop for the heavy application. Lugging a marine engine is more damaging over time than running it light. 

DG: I would prop the boat to have good or acceptable performance with a lot of people. When the hole shot is poor, everyone remembers it. Compromise by not using full-throttle movement and staying out of the rev limiter. 

GD: Prop the pitch selection at the very top end of the recommended engine rpm when you are in a light-load condition and hope that the engine is still within the recommended rpm range when operating heavy. If the WOT engine rpm ends up below the recommended rpm in a heavy payload condition, lower the pitch to prevent engine lugging.

Five Blades: Five-bladed props are top performers in rough conditions or when torque and acceleration are priorities.

Adding a fifth blade further increases blade area, reduces vibration and improves the prop’s ability to hold its grip on the water. The five-blade design allows it to maintain a solid bite on the water, even in the highly aerated water flow produced by performance boats with stepped hulls.

The total number of propeller blades will also impact overall performance and top speed. Fewer blades are more efficient. Again, the choice must be balanced against the performance objectives, boat design and normal use. 

Consult engine performance bulletins for your boat model and dealer recommendations before making a final propeller decision. If necessary, have a prop shop fine-tune them some more after extensive testing under normal loads and conditions. 

And remember: Dialing in a propeller is an art, not an exact science. In need of new propellers? Marine Parts Warehouse offers a full line of Quicksilver, BRP, Michigan Wheel, Solas and Turning Point propellers. 

Propeller Terminology

Back - the low pressure side of the blade closest to the boat 

Cavitation - vaporized air caused by water flow disturbances 

Cup (blade) - the lip on the edge of the blade Diameter- width of the circle made by the tips of the blades Face (blade)- the high pressure side of the blade farthest from the boat 

Leading Edge (blade) - the blade edge closest to the boat 

Pitch - distance travelled by the propeller in one revolution 

Rake - angle between the blade and the hub 

Root (blade) - the edge of the blade attached to the hub 

RPM - revolutions per minute 

Slip - distance lost due to inefficiency 

Surface Area - the total surface of the blades 

Tip - the point separating the leading edge and trailing edge farthest from the hub 

Trailing Edge (blade) - the blade edge farthest from the boat 

Ventilation - air bubbles surrounding the blades 

WOT - wide open throttle