Propellers and All That Goes With Them

    Propellers comprise various areas that create or affect certain performance attributes and the boat’s overall performance.  For instance, the propeller hub can dictate what type of boat application the propeller is best suited for, while a blade’s trailing edge can dictate how well the propeller holds the water in turns. Each part of the area of a propeller affects performance. Marine propellers accomplish a few things: hole shot and acceleration, top speed, turning and handling, load-carrying ability, bow lift, and stern lift.

    Because so many factors affect boat performance, no single propeller can simultaneously maximize all of these attributes. In order to gain more of one attribute (for example, hole shot), you may need to compromise in another area (in this example, top speed). Each boat and customer is different. This is one of the reasons why so many different designs of propellers exist.

    Blade geometry refers to the actual shape of the blade (sometimes called an “ear”). By manipulating the shape of the blade, different characteristics are gained or lost to create a unique performance signature for each propeller. Propeller blades are designed for a specific objective and boat application.

    Blade surface area refers to the total surface of the blades, and it’s a large part of the performance equation. A marine propeller can only affect the boat’s performance when water contacts the propeller’s blades. The more blade surface area a propeller has, the more contact it has with the water and the more water it can push. This means there’s greater potential for performance (i.e., the more water you can move, the more performance you can realize if you have the horsepower to push it). A propeller with more blade surface area can push more water than one with less. This means it has better potential for the strong hole shot performance and increased planning efficiency.  This may also allow for increased bow or stern lift as it provides more grip on the water and decreased potential for proposing and ventilation. Water contacting any surface creates drag. Having too much blade surface area for an application can significantly restrict the engine’s RPMs, making performance suffer and potentially causing negative boat-handling issues.

    To properly choose a propeller and predict its performance, you must understand how different propellers affect performance.  These performance modifiers are the areas and parts of a propeller most commonly adjusted to affect overall boat performance. Knowing what they are and how they affect performance will help you make sound decisions about propeller requirements.

    Pitch is the distance in inches a particular propeller would theoretically travel in one full revolution, as if traveling through a solid. Each inch of pitch is equal to approximately 150 +/- RPMs at wide-open throttle. The correct pitch of a propeller will allow an engine to reach the upper portion of the operating range specified by the engine manufacturer with a normal-to-heavy load. A lower pitch will create greater acceleration and pushing power but a lower top speed. This will also increase engine RPM at WOT (wide open throttle). Generally, lower the pitch as the total boat weight increases.  Higher pitches will provide less acceleration but a greater potential for higher top speeds (if the outboard can properly maintain WOT RPMs). The more pitch a propeller has, the greater the drag on the engine and the more horsepower the engine needs to use it effectively. All things being equal, engine RPMs will decrease as the pitch increases. 

    Please note that the same propeller, or different propellers with the same pitch, will perform differently on different boats. Each propeller style is different, even though the pitch may be the same. The blade shape and blade surface area can be vastly different between propeller styles, giving completely different performance numbers with the same pitch. The overall boat weight and hull style are large contributors to these differences.

    Because water is not a solid and moves as the propeller pushes against it, the actual distance a propeller travels is one revolution is actually less than its stated pitch. This is known as slip. Slip is the amount of wasted energy a particular propeller experiences and is normally expressed as a percentage of inefficiency. A certain amount of slip is engineered into each line of propellers to create different performance characteristics. Slip can be measured using a slip calculator by knowing any four of five parameters (speed, gear ratio, WOT RPMs, propeller pitch, and/or percent of slip). Slip calculators can be very helpful in verifying a customer’s performance claim, but they can also quickly lead to an incorrect finding if the input data is incorrect or used incorrectly. Although these calculators are based on mathematical formulas and are easily obtainable, use caution when using them. 

    In most applications, a slip value of 7%=14% is considered good, although slightly higher numbers and those approaching zero aren’t necessarily bad. It depends upon the application. A negative slip is not possible and likely means the input data is bad. Very high slip means all available engine power is not used to create thrust, as WOT will be attained before potential top speed is realized. 

    Different propeller designs have different slip values in different applications. An example is exhaust venting. Holes, or vents, in the forward propeller tube, allow exhaust to escape into and around the propeller blades at hole shot or hard acceleration from slow speeds. Since marine propeller blades encounter very little to no resistance in air, aerating the blades creates a controlled ventilation condition which dramatically lessens the initial drag on the engine and propeller at hole shot. This allows the engine to develop RPMs faster. As the propeller moves the boat forward and speeds increase, these vents seal off via water pressure or are expelled directly into the air when the vent is not below the water’s surface.

    Exhaust vents are most common on high-performance two-stroke outboards, as they develop most of their power in the middle RPM power band. Four-stroke engines develop their power differently, and because of this, they don’t always require vented propellers to achieve their performance. It won’t hurt to use a vented propeller on a four stroke, but non-vented propellers should not be used on high-performance two-strokes, or hole shot and acceleration will suffer.

    With so many components and characteristics coming into play, it can take time to choose the right propeller for your application. Mercury and Yamaha both have online propeller slip calculators and propeller selection guides. And, as always, your best resource is to contact your local dealer for guidance and recommendations. At Power Implements, we not only service and repair boats, but we are boating enthusiasts ourselves.