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Outboard propeller Guide

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Outboard propeller Guide

Issue Time:2019/09/10

Apropeller’s relationship to a boat and forward motion in the water is directlyrelated to a car’s tire and the road. Matching the right traction to theavailable horsepower, load to be moved and top speed desired are just asimportant in the water as they are on land-based vehicles, or perhaps more sosince water provides a liquid footing. Choosing the right propeller affectsevery phase of a boat’s performance, including handling, comfort of the ride,acceleration out of the hole, engine life, fuel economy, safety and theall-important element - top speed. 

Understanding all of the terminology and the way propellers function is the keyto the process of matching the right prop for your boat and motorcombination. 

While the propeller seems like a very simple concept, there are subtledifferences in design that can make a big difference at 5,000 RPMs. Maximumall-around performance is achieved when wide-open-throttle (WOT) engine operationoccurs near the top of (but within) the WOT- RPM operating range designated bythe manufacturer for your engine. An engine that is improperly propped will notreach the rated RPM at WOT, and is over-propped. Over-propped engines aresubject to lugging and damage under continued operation. Conversely, an enginethat revs past the recommended RPM will have higher than normal wear, damagingand fatiguing parts. 

To begin, you need to understand that propellers operate by both pushing andpulling at the same time. As a blade rotates it is actually moving downward aswell, which moves water back and downward. As water is pushed in thosedirections, more water rushes in behind the blade to fill the void left by themoving blade. The result is a pressure differential between the two sides ofthe blade, with positive pressure causing a pushing effect on the underside anda negative pressure, or pulling effect, on the top side. Since this action iscreated on all sides of the propeller, the push-pull effect is increased withthe speed of the prop.

As these pressures draw water into the propeller from the front and accelerateit out the back, the water is pulled through an imaginary cylinder and exitsthe prop in a jet stream that is smaller in diameter than the actual diameterof the propeller. This action of pulling water into the propeller and pushingit out in a high velocity jet stream gives the water momentum. The increasedmomentum caused by acceleration of the water creates momentum and that resultsin a force, which can be called thrust. 

One of the most misunderstood terms related to propellers is slip, and mostlikely 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 realworld, theory and math fall victim to the laws of nature and a 13" pitchpropeller may only advance 11" in one revolution. That works out to apercentage of 85% of 13", leaving a slip of 15%. A blade with no angle ofattack would not slip, however, it wouldn’t push you anywhere because therewould be no positive and negative pressure created on the blades - and nothrust. 

Thrust is created by some measurable angle of attack or slip. Most props aredesigned with a "right" amount of slip or angle of attack, which isaround 40 plus or minus a few degrees depending on the application.

The "right" amount of slip is accomplished by matching the rightamount of blade diameter and blade area to the intended engine horsepower andpropeller shaft RPM. Too much of one element in the equation (diameter and/orblade area) will reduce slip that results in lower propeller efficiency andreduced performance.

Diameterand Pitch
Two of the most common terms that are used in prop selection are diameter andpitch. A prop’s diameter is determined by measuring the distance across thecircle made by the blade tips as the prop turns. Diameter is a key element fordetermining the power that will be delivered and is keyed to the RPM rate ofthe motor. As a general rule, the diameter of props, within a given propellerline, will be larger for use on slower boats and smaller for faster boats. Whenall other variables remain constant, prop diameter will increase as powerincreases and diameter should also be increased as the propeller’s RPMdecreases. A slower powerhead or engine speed and/or more gear reduction willalso call for an increased diameter. 

The other key factor in prop selection is pitch, which is measured on the faceof the blade (see illustration). Pitch is determined with the premise that theprop is moving through a solid material, like a drill bores through wood. Pitchis the measurement of travel that a prop would move in one revolution if itwere going through a solid. A propeller identified as 10-1/4 - 13 has a 10-1/4inch diameter with 13 inches of pitch. In theory, this particular prop wouldmove forward 13 inches in one revolution. 

There are two types of pitch, constant and progressive. Constant pitch is alsocommonly referred to as "true" or "flat", and means thatthe 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 theleading edge and as the name implies, progressively increases to the trailingedge. 

The actual performance of a prop may vary from the advertised pitch stamped onit from the factory. Possible causes are a minor distortion that may haveoccurred during the casting and cooling process, or modifications made bypropeller repair services; but the most common problem is undetected damagecaused by collision with submerged objects. 

Progressive pitch can improve performance under high speed and high RPMapplications, and when the propeller is operating high enough to break thesurface of the water. Progressive props are commonly used on mid- tohigh-horsepower motors. Think of pitch as another set of gears. For an enginethat runs best at a given RPM, the faster the boat can go, the higher the pitchyou need. Selecting a pitch that is too low, for the same engine, will causethe RPM to run much higher than the recommended limit, which puts undesirablestress on all of the motor’s moving parts. 

A prop with too little pitch may provide greater acceleration but your topspeed 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 RPMswhich is generally at a higher torque level, and this can be just as damagingto your motor. Too much pitch will hurt acceleration and may not help at thetop end either. 

Propeller lines are normally designed so that the next size pitch will change anengine’s rate by 300 to 500 RPM. Therefore, if the RPM falls too low, try alower pitched propeller to bring up the RPM, and higher pitched propellersreduce the engine RPM.

Pitch Formula

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 specificationsfor your engine’s wide-open-throttle (WOT) range. 

2. After adjusting your engine’s trim angle for optimumperformance, run your boat at WOT to determine and note its operating RPM withyour 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 rangelisted in your owner’s manual. 

4. For every 1" of pitch change, the effect will beapproximately 200 RPM. Take the difference between the maximum recommendationand your noted RPM and divide by 200. The resulting number will be the amountof pitch change you need in inches. Or, per the example below 800/200 = 4"less pitch than your current propeller. 

For example:

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 propellerhas a 0° rake. Blade rake increases when the slant of the blade is increasedtoward the aft end of the propeller. A rake of 15° is common for basicpropellers on outboard engines and stern drives. Progressive rakes that go ashigh as 30° are common on higher-raked, high-performance props. In addition tooverall performance characteristics, a propeller with a higher rake generallyimproves the propeller’s ability to operate in a cavitation or ventilatingsituation, such as when the blades break the surface. In this situation, higherblade rake tends to hold the water as it is being thrown off into the air bycentrifugal force, which creates more thrust than a similar, but lower rakedprop. 

The hub is the center of every propeller. For engines that discharge exhaustthrough the propeller’s hub, the propeller is called a through-hub exhaustdesign. Engines that exhaust gases over the hub, the propeller is called anover-the-hub exhaust propeller. This design allows the propeller to wind upquickly, as the propeller bites into water and exhaust at the same time. Topspeed may improve slightly, due to a reduction in drag associated with thelarger outer hub; however, acceleration will generally suffer slightly. 

A word of caution, for some very light, fast boats a propeller with too high ofa rake can cause excessive bow lift, making these boats very flighty andunstable. In this instance, a more moderately raked propeller would be a moreprudent choice.

A propeller is said to have a cup when the trailing edge of the blade is formedor cast with an edge that curls away from the boat. Cupping was originally doneto gain the same benefits created by a progressive pitch and curved or higherrake. However, the positive benefits of cupping are so desirable that nearlyall-modern recreational, high-performance or racing propellers are made withsome degree of cup. Conversely, cupping is of little value on propellers usedin heavy-duty applications where the propeller remains fully submerged.

To achieve maximum effectiveness, a cup should be completely concave on thepressure side of the blade (face) finishing with a sharp trailing edge. Anyconvex rounding of the trailing edge of the cup, on the pressure side, detractsfrom its effectiveness. 

Cupping will usually reduce an engine’s full-throttle revolutions by 150 to 300RPMs below the same pitch propeller without a cup. 

Rotation or ("Hand") Reference
Although the most common outboard and stern drive propellers are of theright-hand rotation design, there are some motors that rotate in the oppositedirection. To differentiate between the two, look at your propeller and notethe slant of the blade. A blade rotates in the direction of the slant towardthe 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 tendsto vibrate more than a blade with three or more blades and a five-bladedpropeller is the most vibration free. A majority of propellers manufacturedtoday are of the three-blade variety, striking a compromise between the twoevils 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 ahigher level of vibration suppression, they improve acceleration by puttingmore blade area in the water. The additional blade area also helps to make therake more effective in getting the bow out of the water for less drag andadditional speed. 

You may have noticed that some blades sweep more radically than others. A bladethat sweeps back is said to have skew. A more dramatic skew is helpful inallowing a propeller to shed weeds. Also, when the propeller surfaces, a higherskew will reduce the pounding vibration of the blades re-entering thewater. 

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’ssurface or from the exhaust outlet is drawn into the propeller blades. Theadditional air/exhaust reduces the water load and the propeller over-revs,losing much of its thrust. In addition to the potential damage thatover-revving can cause, this also causes massive cavitation. Most often,ventilation occurs in turns, especially when trying to plane in a sharp turn orwith 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 ananti-cavitation plate. Its purpose is to prevent air from being sucked into thepropeller’s blades from the surface. For through-hub exhausts, the problem ismitigated with a flared trailing edge, which funnels the escaping gases awayfrom the blades. 

Cavitation is basically boiling water. While water boils at 212°F at sea levelbarometric pressure, but it will boil at room temperature if the pressure islow enough. When a propeller’s blade passes through the water at an increasingspeed the pressure on the sides and back of the blade drops. When the watertemperature and pressure drop are just right the water begins to vaporize andboil. This occurs most often near the leading edge of the blades, and can causedamage to the propeller called cavitation burn. When the speed is reduced thepressure rises again and the boiling will subside. 

Cavitation can be aggravated by nicks in the leading edge of the blade, toomuch cup, sharp leading edge corners, improper polishing and sometimes, poordesign. Excessive cavitation is rare and is usually caused by a severely bentor damaged blade, or one that is too small in diameter for the engine.

Blade Types
Propellers are classified by the way the construction method and material used,such as aluminum or stainless steel. Pleasure boat propellers can be generallydivided into six basic types: basic aluminum, basic stainless steel,high-reverse thrust, cleaver-style, chopper-style and other high-performancestainless 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. Theirrounded contour as a very slight sweep back or skew with various shapes basedon the type and application. Conventional blades are designed to run fullysubmerged 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 mostpropellers have some degree of weed-shedding ability.

Cleaver blades have a trailing edge that is cut in a straight line, usuallyalong the rake. A cleaver blade is usually very thin at the leading edge whilethe trailing edge is the thickest point. Cleaver propellers are best suited forelevated engine installation that allows the propeller’s blades to break thesurface of the water.

Aluminum Construction
Standard aluminum props are available in a large variety of diameters, pitchesand rakes for a wide range of both outboard and stern drive applications, andare excellent for general purpose use. 

Large diameter aluminum propellers offer enhanced mid-range performance, fueleconomy, top-end performance, exceptional holding in turns and positive reversethrust 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. Thesepropellers offer high thrust for large workboats, pontoon boats, houseboats orcruisers. 

Stainless Steel Construction
Stainless steel is often mistakenly associated with speed; however, stainlesssteel alone doesn’t make the propeller any faster. It’s the design of thepropeller and features that determines its efficiency and not its composition.Stainless steel is stronger than aluminum and makes it possible formanufacturers 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 itsresistance to corrosion. 

Remember to inspect your engine’s propeller on a regular basis, checking forobvious signs of damage or burning from excessive cavitation. Even small dingsin the blades can lead to blade failure if not dressed or repaired. Worse yet,a damaged propeller significantly reduces performance as well as fuel economyand can severely damage your engine. While a trained technician can repair somedamage, extreme damage can be more costly to repair than the cost of a newreplacement. 

Also, before installing your new propeller, it is a good idea to coat thepropeller 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 assurethat it is secure. Paddling back to the dock is not pleasant on a hot summerday, especially when every stroke of the paddle reminds you that you shouldhave 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 theblade 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 throughthe 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 propelledaft. The trailing edge extends from the tip of the blade to the hub. 

D. Cup
The cup is a small curve or lip on the trailing edge of the blade that enablesthe 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 thatfaces away from the boat.

F. Blade Back
The blade back, or negative pressure (suction) side, is the side that is facingthe 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 athrough-hub exhaust, and the exterior surface is in direct contact with thewater. The inner surface consists of the exhaust passage and ribs that attachthe outer hub to the inner hub. 

J. Ribs
Through-hub exhaust propellers use various numbers of ribs to strengthen theconnection between the inner and outer hub. While three is the most commonnumber used, there can be as many as five, depending on the size andrequirements of the motor it is designed for. 

K. Shock-Absorbing Rubber Hub
To minimize damage on impact with submerged objects, some propellers use arubber hub between the hub and propeller’s splined shaft. The rubber alsoserves to minimize the impact between the gear and clutch mechanism duringnormal gear shifting.

L. Diffuser Ring
The diffuser ring is simply a slightly outward curve of the outer hub thatreduces exhaust backpressure and helps in preventing exhaust gas from backfeeding into the propeller blades.

M. Exhaust Passage
On through-hub exhaust propellers, the hallow area between the inner hub andthe outer hub through which engine exhaust gases are discharged into the water.This passage only carries air on some stern drive installations using athrough-transom exhaust system.