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Author: Frank Ross
High performance props aren’t the only factor that can influence your boat’s speed, but it’s one of the first places you should look when you’re trying to pep up your rig.
Once you’ve owned a boat for a while there’s a common experience that overtakes all who captain the helm, no matter how big the craft. It usually occurs shortly after a buddy zips past you, casting an arching spray over your windshield. After pushing the throttle lever so hard that your hand numbs, without gaining an increase in speed, you vow to improve your boat’s speed.
After stripping the boat of all unnecessary weight, stopping just short of leaving the lunch and beverage cooler at the dock, you realize that load is only part of the equation.
A propeller’s relationship to a boat and forward motion in the water is directly related to a car’s tire and the road. Matching the right traction to the available horsepower, load to be moved and top speed desired are just as important in the water as they are on land-based vehicles, or perhaps more so since water provides a liquid footing. Choosing the right propeller affects every phase of a boat’s performance, including handling, comfort of the ride, acceleration out of the hole, engine life, fuel economy, safety and the all-important element - top speed.
Understanding all of the terminology and the way propellers function is the key to the process of matching the right prop for your boat and motor combination.
While the propeller seems like a very simple concept, there are subtle differences in design that can make a big difference at 5,000 RPMs. Maximum all-around performance is achieved when wide-open-throttle (WOT) engine operation occurs near the top of (but within) the WOT- RPM operating range designated by the manufacturer for your engine. An engine that is improperly propped will not reach the rated RPM at WOT, and is over-propped. Over-propped engines are subject to lugging and damage under continued operation. Conversely, an engine that revs past the recommended RPM will have higher than normal wear, damaging and fatiguing parts.
To begin, you need to understand that propellers operate by both pushing and pulling at the same time. As a blade rotates it is actually moving downward as well, which moves water back and downward. As water is pushed in those directions, more water rushes in behind the blade to fill the void left by the moving blade. The result is a pressure differential between the two sides of the blade, with positive pressure causing a pushing effect on the underside and a negative pressure, or pulling effect, on the top side. Since this action is created on all sides of the propeller, the push-pull effect is increased with the speed of the prop.
As these pressures draw water into the propeller from the front and accelerate it out the back, the water is pulled through an imaginary cylinder and exits the prop in a jet stream that is smaller in diameter than the actual diameter of the propeller. This action of pulling water into the propeller and pushing it out in a high velocity jet stream gives the water momentum. The increased momentum caused by acceleration of the water creates momentum and that results in a force, which can be called thrust.
One of the most misunderstood terms related to propellers is slip, and most likely because it sounds like a bad thing. Rather than a measurement of a prop, slip is the difference between actual and theoretical travel distance, resulting from a necessary prop blade angle of attack.
An example is probably the best way to understand this concept. In the real world, theory and math fall victim to the laws of nature and a 13" pitch propeller may only advance 11" in one revolution. That works out to a percentage of 85% of 13", leaving a slip of 15%. A blade with no angle of attack would not slip, however, it wouldn’t push you anywhere because there would be no positive and negative pressure created on the blades - and no thrust.
Thrust is created by some measurable angle of attack or slip. Most props are designed with a "right" amount of slip or angle of attack, which is around 40 plus or minus a few degrees depending on the application.
The "right" amount of slip is accomplished by matching the right amount of blade diameter and blade area to the intended engine horsepower and propeller shaft RPM. Too much of one element in the equation (diameter and/or blade area) will reduce slip that results in lower propeller efficiency and reduced performance.
Diameter and Pitch
Two of the most common terms that are used in prop selection are diameter and pitch. A prop’s diameter is determined by measuring the distance across the circle made by the blade tips as the prop turns. Diameter is a key element for determining the power that will be delivered and is keyed to the RPM rate of the motor. As a general rule, the diameter of props, within a given propeller line, will be larger for use on slower boats and smaller for faster boats. When all other variables remain constant, prop diameter will increase as power increases and diameter should also be increased as the propeller’s RPM decreases. A slower powerhead or engine speed and/or more gear reduction will also call for an increased diameter.
The other key factor in prop selection is pitch, which is measured on the face of the blade (see illustration). Pitch is determined with the premise that the prop is moving through a solid material, like a drill bores through wood. Pitch is the measurement of travel that a prop would move in one revolution if it were going through a solid. A propeller identified as 10-1/4 - 13 has a 10-1/4 inch diameter with 13 inches of pitch. In theory, this particular prop would move forward 13 inches in one revolution.
There are two types of pitch, constant and progressive. Constant pitch is also commonly referred to as "true" or "flat", and means that the pitch is the same at all points from the leading edge to the trailing edge. A progressive pitch (also called blade "camber") starts low at the leading edge and as the name implies, progressively increases to the trailing edge.
The actual performance of a prop may vary from the advertised pitch stamped on it from the factory. Possible causes are a minor distortion that may have occurred during the casting and cooling process, or modifications made by propeller repair services; but the most common problem is undetected damage caused by collision with submerged objects.
Progressive pitch can improve performance under high speed and high RPM applications, and when the propeller is operating high enough to break the surface of the water. Progressive props are commonly used on mid- to high-horsepower motors. Think of pitch as another set of gears. For an engine that runs best at a given RPM, the faster the boat can go, the higher the pitch you need. Selecting a pitch that is too low, for the same engine, will cause the RPM to run much higher than the recommended limit, which puts undesirable stress on all of the motor’s moving parts.
A prop with too little pitch may provide greater acceleration but your top speed will more than likely suffer as the propeller’s efficiency drops. Conversely, a prop with too high a pitch will force your engine to lug at low RPMs which is generally at a higher torque level, and this can be just as damaging to your motor. Too much pitch will hurt acceleration and may not help at the top end either.
Propeller lines are normally designed so that the next size pitch will change an engine’s rate by 300 to 500 RPM. Therefore, if the RPM falls too low, try a lower pitched propeller to bring up the RPM, and higher pitched propellers reduce the engine RPM.
There is a simple formula to determine how much pitch change you may require, but you’ll need a tachometer. Just follow these easy steps:
1.Use your owner’s manual to determine the manufacturer’s specifications for your engine’s wide-open-throttle (WOT) range.
2. After adjusting your engine’s trim angle for optimum performance, run your boat at WOT to determine and note its operating RPM with your current propeller.
3. If the WOT RPM is below the recommended RPM range of the engine, take that reading and subtract it from the top end of the operating range listed in your owner’s manual.
4. For every 1" of pitch change, the effect will be approximately 200 RPM. Take the difference between the maximum recommendation and your noted RPM and divide by 200. The resulting number will be the amount of pitch change you need in inches. Or, per the example below 800/200 = 4" less pitch than your current propeller.
Operating range = 5000-5600 RPM
Top end of operating range = 5600 RPM
Tachometer reading = 4800 RPM
Difference = 800 RPM
Theoretical Boat Speed Equation:
When the face of the blade is perpendicular to the propeller hub, the propeller has a 0° rake. Blade rake increases when the slant of the blade is increased toward the aft end of the propeller. A rake of 15° is common for basic propellers on outboard engines and stern drives. Progressive rakes that go as high as 30° are common on higher-raked, high-performance props. In addition to overall performance characteristics, a propeller with a higher rake generally improves the propeller’s ability to operate in a cavitation or ventilating situation, such as when the blades break the surface. In this situation, higher blade rake tends to hold the water as it is being thrown off into the air by centrifugal force, which creates more thrust than a similar, but lower raked prop.
The hub is the center of every propeller. For engines that discharge exhaust through the propeller’s hub, the propeller is called a through-hub exhaust design. Engines that exhaust gases over the hub, the propeller is called an over-the-hub exhaust propeller. This design allows the propeller to wind up quickly, as the propeller bites into water and exhaust at the same time. Top speed may improve slightly, due to a reduction in drag associated with the larger outer hub; however, acceleration will generally suffer slightly.
A word of caution, for some very light, fast boats a propeller with too high of a rake can cause excessive bow lift, making these boats very flighty and unstable. In this instance, a more moderately raked propeller would be a more prudent choice.
A propeller is said to have a cup when the trailing edge of the blade is formed or cast with an edge that curls away from the boat. Cupping was originally done to gain the same benefits created by a progressive pitch and curved or higher rake. However, the positive benefits of cupping are so desirable that nearly all-modern recreational, high-performance or racing propellers are made with some degree of cup. Conversely, cupping is of little value on propellers used in heavy-duty applications where the propeller remains fully submerged.
To achieve maximum effectiveness, a cup should be completely concave on the pressure side of the blade (face) finishing with a sharp trailing edge. Any convex rounding of the trailing edge of the cup, on the pressure side, detracts from its effectiveness.
Cupping will usually reduce an engine’s full-throttle revolutions by 150 to 300 RPMs below the same pitch propeller without a cup.
Rotation or ("Hand") Reference
Although the most common outboard and stern drive propellers are of the right-hand rotation design, there are some motors that rotate in the opposite direction. To differentiate between the two, look at your propeller and note the slant of the blade. A blade rotates in the direction of the slant toward the forward end.
Number of Blades
The number of blades a prop has is a balance between efficiency and vibration. Practically speaking, a two-bladed propeller is the most efficient, but tends to vibrate more than a blade with three or more blades and a five-bladed propeller is the most vibration free. A majority of propellers manufactured today are of the three-blade variety, striking a compromise between the two evils of vibration and efficiency.
With the growing number of propellers being operated closer to the surface, four- and five-bladed propellers have become more common. In addition to a higher level of vibration suppression, they improve acceleration by putting more blade area in the water. The additional blade area also helps to make the rake more effective in getting the bow out of the water for less drag and additional speed.
You may have noticed that some blades sweep more radically than others. A blade that sweeps back is said to have skew. A more dramatic skew is helpful in allowing a propeller to shed weeds. Also, when the propeller surfaces, a higher skew will reduce the pounding vibration of the blades re-entering the water.
Ventilation and Cavitation
When your engine winds up high RPMs it can be caused by one of two problems - ventilation or cavitation. Ventilation happens when air from the water’s surface or from the exhaust outlet is drawn into the propeller blades. The additional air/exhaust reduces the water load and the propeller over-revs, losing much of its thrust. In addition to the potential damage that over-revving can cause, this also causes massive cavitation. Most often, ventilation occurs in turns, especially when trying to plane in a sharp turn or with an engine that is trimmed out excessively.
Marine engines are designed with a large "anti-ventilation" plate, directly above the propeller. This plate is often mistakenly referred to as an anti-cavitation plate. Its purpose is to prevent air from being sucked into the propeller’s blades from the surface. For through-hub exhausts, the problem is mitigated with a flared trailing edge, which funnels the escaping gases away from the blades.
Cavitation is basically boiling water. While water boils at 212°F at sea level barometric pressure, but it will boil at room temperature if the pressure is low enough. When a propeller’s blade passes through the water at an increasing speed the pressure on the sides and back of the blade drops. When the water temperature and pressure drop are just right the water begins to vaporize and boil. This occurs most often near the leading edge of the blades, and can cause damage to the propeller called cavitation burn. When the speed is reduced the pressure rises again and the boiling will subside.
Cavitation can be aggravated by nicks in the leading edge of the blade, too much cup, sharp leading edge corners, improper polishing and sometimes, poor design. Excessive cavitation is rare and is usually caused by a severely bent or damaged blade, or one that is too small in diameter for the engine.
Propellers are classified by the way the construction method and material used, such as aluminum or stainless steel. Pleasure boat propellers can be generally divided into six basic types: basic aluminum, basic stainless steel, high-reverse thrust, cleaver-style, chopper-style and other high-performance stainless steel propellers.
General Blade Types
Propeller design varies in appearance mainly due to the shape of their blades; however, all propellers can be classified in one of three general blade types, conventional, weedless and Cleaver™.
Conventional blades are distinctive due to their round-eared blades. Their rounded contour as a very slight sweep back or skew with various shapes based on the type and application. Conventional blades are designed to run fully submerged but can be used in a slightly surfaced application in some cases, with a light load.
Weedless propellers are designed with varying degrees of weedlessness, and most propellers have some degree of weed-shedding ability.
Cleaver blades have a trailing edge that is cut in a straight line, usually along the rake. A cleaver blade is usually very thin at the leading edge while the trailing edge is the thickest point. Cleaver propellers are best suited for elevated engine installation that allows the propeller’s blades to break the surface of the water.
Standard aluminum props are available in a large variety of diameters, pitches and rakes for a wide range of both outboard and stern drive applications, and are excellent for general purpose use.
Large diameter aluminum propellers offer enhanced mid-range performance, fuel economy, top-end performance, exceptional holding in turns and positive reverse thrust for improved stopping power and maneuverability on large, slower boats.
Large blade aluminum propellers (having an even-numbered pitch, 12", 14" and 16") are designed for stern drives and V-6 outboards. These propellers offer high thrust for large workboats, pontoon boats, houseboats or cruisers.
Stainless Steel Construction
Stainless steel is often mistakenly associated with speed; however, stainless steel alone doesn’t make the propeller any faster. It’s the design of the propeller and features that determines its efficiency and not its composition. Stainless steel is stronger than aluminum and makes it possible for manufacturers to design thinner blades, but its superior cup design, progressive pitch and sharper leading edges that are an advantage.
Stainless steel propellers are an excellent choice for saltwater because of its resistance to corrosion.
Remember to inspect your engine’s propeller on a regular basis, checking for obvious signs of damage or burning from excessive cavitation. Even small dings in the blades can lead to blade failure if not dressed or repaired. Worse yet, a damaged propeller significantly reduces performance as well as fuel economy and can severely damage your engine. While a trained technician can repair some damage, extreme damage can be more costly to repair than the cost of a new replacement.
Also, before installing your new propeller, it is a good idea to coat the propeller shaft spline with a quality anti-corrosion grease to aid in removal, should that become necessary in the future.
Finally, check your propeller’s self-locking prop nut periodically to assure that it is secure. Paddling back to the dock is not pleasant on a hot summer day, especially when every stroke of the paddle reminds you that you should have done a routine inspection.
Basic Propeller Terminology
A. Blade Tip
The area defined as the blade tip is at the maximum length of the blade, measured from the center of the propeller hub. The tip is the point of the blade that separates the leading edge from the trailing edge.
B. Leading Edge
The leading edge is the part of the blade nearest the boat that cuts through the water first.
C. Trailing Edge
The trailing edge is the portion of the blade that is farthest from the boat, or the edge of the blade that is the last to touch the water as it is propelled aft. The trailing edge extends from the tip of the blade to the hub.
The cup is a small curve or lip on the trailing edge of the blade that enables the blade to hold water better. The cup normally adds from 1/2" to 1" of pitch to the propeller.
E. Blade Face
The positive pressure side of the blade, or face, is the side of the blade that faces away from the boat.
F. Blade Back
The blade back, or negative pressure (suction) side, is the side that is facing the boat.
G. Blade Root
The blade root is the point where the blade attaches to the hub.
H. Inner Hub
The inner hub contains the rubber hub (where applicable).
I. Outer Hub
The blades are attached to the exterior surface of propellers that have a through-hub exhaust, and the exterior surface is in direct contact with the water. The inner surface consists of the exhaust passage and ribs that attach the outer hub to the inner hub.
Through-hub exhaust propellers use various numbers of ribs to strengthen the connection between the inner and outer hub. While three is the most common number used, there can be as many as five, depending on the size and requirements of the motor it is designed for.
K. Shock-Absorbing Rubber Hub
To minimize damage on impact with submerged objects, some propellers use a rubber hub between the hub and propeller’s splined shaft. The rubber also serves to minimize the impact between the gear and clutch mechanism during normal gear shifting.
L. Diffuser Ring
The diffuser ring is simply a slightly outward curve of the outer hub that reduces exhaust backpressure and helps in preventing exhaust gas from back feeding into the propeller blades.
M. Exhaust Passage
On through-hub exhaust propellers, the hallow area between the inner hub and the outer hub through which engine exhaust gases are discharged into the water. This passage only carries air on some stern drive installations using a through-transom exhaust system.
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