Frequently Asked Questions
What are the basic propeller parts?
In order to select a suitable propeller for any particular use, it is important to have some understanding of its main components and engineering. This illustration indicates the principal parts and features of a propeller.
1. Leading Edge
nearest the boat | extends from hub to tip | section first to cut through the water
2. Trailing Edge
farthest from the boat - extends from hub to tip | edge from which water leaves the blade
3. Blade Face
side facing away from the boat
4. Blade Back
side facing boat
5. Blade Tip
maximum distance from hub to tip | separates leading from trailing edge
lip on trailing edge | permits propeller to 'hold' water better | prevents cavitation and ventilation
7. Inner Hub
contains rubber hub
8. Outer Hub
in direct contact with the water | section to which blades are attached | inner surface in contact with exhaust gases
connection between inner and outer hub
10. Exhaust Passage
hollow area between inner and outer hub | allows exhaust gases to be expelled under surface of water
11. Diffuser Ring
reduces exhaust back pressure | prevents gas from feeding back into the blades
12. Rubber Hub
moulded to inner splined hub | relieves shock should propeller strike a solid object at low speed
What is cavitation?
Cavitation is the direct result of low water pressure which may be caused by a variety of factors. Propeller defects, such as chipping, excessive cupping and inferior blade design are principal causes of this condition.
Operating at high speed may also result in cavitation. The increasing speed of a boat, as it travels through water, causes a gradual decrease in water pressure. This decline, in conjunction with water temperature, will ultimately result in boiling, a condition occurring most often at the leading edge of the propeller.
A reduction in speed and the subsequent increase in water pressure will, however, alleviate the problem. As water pressure increases the vapor bubbles collapse and this action releases energy that chips away at the blades causing progressive erosion of the propeller material.
What is diameter
Propeller diameter is distance across the imaginary circle that a spinning propeller makes. It can be easily determined by measuring the distance from the center of the hub to the tip of one of the blades and multiplying that number by 2.
What is pitch
Propeller pitch is the theoretical forward movement of a propeller for one revolution, assuming that there is no prop slip. For example, a 21 pitch propeller theoretically moves 21 inches for every revolution. Propeller slip occurs with every propeller, but the amount of slip varies depending on propeller design. Propco's more aggressively and efficiently designed propellers will slip less.
What is thrust
As pressure draws water into the propeller from the front and accelerates 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 of water into the propeller and pushing it out in a high velocity jet stream gives the water momentum. The increased momentum caused by the acceleration of the water creates momentum and that results in a force which can be called thrust.
What is slip
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.
What is cupping?
Propeller cupping is the curved lip at the trailing edge and/or tip of the propeller. Cupping helps the propeller to get a better grip in the water for better holding at higher trim and on turns.
Cupping also increases the efficiency of a propeller and can result in higher top end speeds when properly designed.
What is rake?
Propeller rake is the degree that a propeller blade is angled in relation to the hub. Props with higher rake typically have better speeds and greater lift. The better performing propellers typically have between 20-30 degree rake angles.
Materials: Composite/Plastic vs Aluminum vs Stainless
What is the difference in having an aluminum or stainless steel prop?
The primary reason to use stainless steel is durability - a stainless steel propeller will last a lot longer and will not be damaged by minor impacts that could tear up an aluminum prop. There are also performance advantages, because stainless props can be more sophisticated than some aluminum props. Sophisticated aluminum props like the Propco Quad-R-Jet 4-blade, can and do often out preform much higher priced stainless props. So with a stainless steel prop or high performance aluminum, expect better all around performance - hole shot, speed, ability to stay on plane, better gas mileage. The downside? The cost to buy and repair stainless steel propellers can be double or triple aluminum. And if you hit something very hard with out a POSSI drive system, you can do more damage to the drive train. Stainless steel won't give like aluminum and you are very likely to cause serious damage to your drive shaft.
Material Properties: Solid material that is made when two or more substances are combined physically to create a new material that is superior to both of those original materials in a specific application.
Price: Cheapest of the three.
Upkeep: May not be repaired, some manufacturers will provide replacement blades.
Strength: Lightweight, more prone to bending under loads and blades are made thicker to compensate. There is reasonable impact resistance, blade flex is common and there may be inability accept cupping due to the relative weakness of the blades.
Durability: Almost similar to aluminum, less than stainless steel.
Ideal Usage: Emergency purposes, spares, smaller engines, low-horsepower gasoline motors or electric trolling motors.
Saltwater/Freshwater Usage: Both.
Size: Most manufacturers make up to 23 inches, 3-4 blades, round ear and low rake.
Additional Information: Composite / Plastic props are corrosion resistant.
Material Properties: Made of alloys o< other metals.
Price: Low price, slightly higher than composite /plastic propellers, lower than stainless steal.
Upkeep: The material is relatively soft and can be repaired at reasonable prices. Nicks and dings can be repaired but the coating on the propeller should be touched up to prevent corrosion.
Strength: Lightweight material, strong enough to reduce blade flex and has the ability to accept some amount of cupping.
Durability: Average durability. A well designed propeller will perform just as well as an average stainless steel propeller
Ideal Usage: Can be for low or high horsepower applications, are widely available in a range of sizes for all types of applications making it the most popular choice of material.
Saltwater/Freshwater Usage: Recommended for fresh water usage only as aluminum will corrode much quicker in salt water.
Size: Most manufacturers make up to 23 inches, 3-4 blades, round ear, moderate pitch and low rake.
Additional Information: Most popular type of propeller.
Material Properties: Steel alloy with a minimum of 10.5-11%chromium content by mass.
Price: Much more expensive than the aluminum or composite/plastic.
Upkeep: Can be easily repaired, at a higher cost.
Strength: Very strong and efficient as it has stiffer, thinner blades that reduces resistance in water and almost eliminates flex. The blades are strong enough to accept significant cupping.
Durability: Longest lasting. These blades can withstand small rocks, sand and various loose objects in water.
Ideal Usage: Very versatile, but ideal for heavy or high speed applications, for improving acceleration and top speeds, increase trimming or holding ability.
Saltwater/Freshwater Usage: Both.
Size: Most manufacturers make around 29 inches and up, 4-5 blades, usually a round ear, adequate rake and large cup size.
Additional Information: There is minimal give, which means that the lower unit of a vessel can become damaged if an object hits it hard enough rather than damaging the blades. The heaviness of the blades can damage gears if the motor set too high.
General Blade Types: Conventional - Cleaver - Weedless
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.
My propeller just has a little ding. Why should I get it repaired?
Sometimes a little ding is all that shows, but often one or all of the blades may have been ever so slightly knocked out of line. You may think that a small 'ding' or band will not affect the performance of your boat. In reality, a propeller affects a whole series of things on your boat. If you have an improper balanced or damaged propeller, it can change your RPM's, speed, hole shot, gas mileage and in a lot of cases, cause damage to your lower unit and even destroy the bearing and or seals. Hard strikes can often bend the shaft, especially if the propeller does not have a rubber cushion or PSSI drive type cushioning system. Many propellers today have hard plastic inserts in the hubs and are prone to causing damage to the shaft even with what may seem to be an insignificant hit.
What is a shaft?
The shaft is the rod that you mount the propeller to in order to transfer the power from the engine. It is tapered at the end with a threaded section where the propeller is attached with a prop nut and a tab washer or cotter pin to hold it in place. It is routed in different ways, depending on whether it is on an outboard, I/O, or inboard, but all go through a gear and enclosure systems with bearings and seals to keep out water and contain lubricants. The propeller shaft must be free of any bends and cracks or it will cause vibration and can erode the bearing and seals. Once that starts, system failure is inevitable over time as the lubricant leaks out and things begin to heat up due to the metal to metal high speed friction.
Why change propellers?
Often the stock outboard with which most outboards are equipped is a standard prop that will work for many different applications but not work well for you in a specific use since they go on all different kinds of boats. I/0's and inboards come with a standard prop that may or may not match the way you load or use your boat. Since it has fixed diameter and pitch, it is limited in its use and may not provide satisfactory performance for every combination load that will be encountered. One important fact to note is that the propeller moves the boat through the water at a specific engine RPM, and horsepower (HP) is directly related to that RPM. The engine cover is marked with a certain HP rating, but in most instances the full benefit of the possible HP is not realized. Along with the HP rating, equal emphasis should be placed on the RPM at which the related HP is developed. This is where the propeller comes into the picture. Outboard engines are designed to run at peak RPM for full efficiency. Excessive RPM with its increased friction and wear is harmful. It is equally harmful to run the engine overloaded to the point that it cannot achieve its rated RPM. This results in excessive carbon build up with subsequent problems of poor fuel economy, pre-ignition, frequent spark plug failure, scoring of the cylinder walls and even burned pistons.
What prop should I use with my boat and motor?
You must first, determine how the boat will be used or what the normal load will be. If this boat normally operates with one specific passenger load, propeller size is relatively easy. If it has multiple uses ranging from light to heavy loads, the selection of one or two propellers may be necessary.
The best propeller size for your boat and engine combination is based on the recommended operating range at wide open throttle (w.o.t.) for your engine, which you will find in your operator's manual. This will be expressed in terms of a certain horsepower at a certain RPM (revolutions per minute).
The goal in prop selection is to determine what propeller style and size will maximize performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum RPM where maximum horsepower is produced.
Run the boat/motor at w.o.t. under normal operating load to determinate maximum RPM you are able to obtain. A tachometer is necessary for this test. Adjust the motor trim angle for the optimum performance. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded.
If your test results in being able to over-rev the engine, you need to increase the pitch of the propeller. Increasing the pitch increment by 1" will result in approximately 200 RPM drop. If your testing shows, however, that you are only able to obtain a RPM somewhat lower than the maximum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing pitch would increase your RPM.
- Operating Range = 5000-5600 RPM
- Top End of Operating Range = 5600 RPM
- Tachometer Reading = 4800 RPM
- Difference = 800 RPM
Most modern propellers are cupped for high performance. Switching from an uncupped to a cupped propeller will also reduce your RPM. The cupped propeller of the same pitch and diameter will typically reduce your RPM by approximately 200.
Once your wide open throttle RPM falls within the recommended range of the engine manufacturer, you have a propeller that is suited correctly for your boat with respect to RPM. If you use your boat for fishing, cruising and skiing, one prop probably won't do all three things equally well. It is best in circumstances like this, if you want to have the very best performance for multiple activities and are willing to change a prop from time to time, to have two propellers. One to accommodate one set of circumstances and the other to perform bast under the different load or use. It is imperative that the wide open throttle RPM fall within the range specified by your engine manufacturer.