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Do you need a new prop?
Your boat’s propeller converts the power of the engine into underwater thrust, moving you where you want to go in forward and reverse, whether you have an inboard, sterndrive or outboard, a powerboat or a sailboat. Perhaps you have managed to damage your prop running in shallow water, have hit a submerged object or backed your trailerable boat into a tree. Or perhaps you have a brand new boat or have purchased a new engine. In these types of situations you need a new prop—right now.
How is your boat performing?
If you are pleased with your boat’s performance, your choice of a replacement prop may be simple, and you can select another prop similar to your last one. The basic rule for prop selection—your engine should be running within the designed rpm range at Wide Open Throttle (WOT). Your owner’s manual should include this spec—usually between 5000 and 5500 rpm, depending on the engine type—or your mechanic or dealer may know. If your engine is not able to reach its designed range it may be operating in an extremely overworked state, and mechanics indicate that running a highly loaded, lugging engine is a prime cause of premature failure. Allowing your engine to over-rev is nearly as bad, and can also cause severe damage.
Dollar for dollar, changing the characteristics of your boat’s prop provides more bang for your buck, leading to more improvement in performance, than any other modification you can make. Don’t be afraid to spend what it takes to get the correct prop the best size, configuration and material for your boat and how you use it.
What information do you need to buy a prop?
We stock propellers to fit all outboards, sterndrives, and some inboard engines. Before you walk into a store or call our Technical Sales department at 1-800-BOATING you will need to gather as much of the following information as possible to help us select a replacement prop.
Propeller data: Essential information -1) present prop diameter 2) present prop pitch 3) prop rotation (clockwise is right hand) right or left hand rotation 4) number of blades 5) material. Other very useful information - 1) manufacturer’s part number 2) shaft diameter and number of splines or keyway type.
Engine data: Essential information – 1) number of engines 2) rated horsepower 3) gear ratio-direct reduction or XX:1 4) rpm operating range 5) manufacturer 6) model and year. Other very useful information – 1) displacement in cubic inches or centimeters 2) power trim or trim tabs
Boat data: Essential information – 1) length overall 2) hull material. Other very useful information – 1) manufacturer 2) model 3) hull shape 4) present/desired top speed
Propeller Selection Basics
Propeller Size is expressed with two numbers, diameter and pitch, with diameter always stated first. Diameter is two times the distance from the center of the hub to the tip of any blade. Smaller prop diameters generally go with smaller engines, or with fast high performing boats. Diameter is a critical dimension for converting your engine’s power into thrust, and the right diameter is an engineering question, usually left to an expert. Pitch is the theoretical forward distance, in inches, that a propeller travels during one revolution. There is always some “slip” between the propeller and the water (generally 10 to 15%) so the actual distance traveled is somewhat less than the theoretical value. Think of pitch as speed, or as the gear selection on a car’s transmission.
Rake is the degree that the blades slant forward or backward in relation to the hub. Rake can affect how water flows through the propeller, which can make a difference regarding boat performance. Aft rake helps to lift the boat’s bow, decreasing the hull’s wetted surface area and improving top end planning speed. Negative rake, with blades slanted forward, pushes the boat’s bow down. It is only common in props for heavy displacement workboats.
Cupping of the trailing edge of the propeller blade is common on many propellers. A downward curve of the lip of the blade (like a plane’s wing with the “flaps” down) allows a better hole shot, less slippage and ventilation, and helps the propeller get a better bite on the water. A cupped prop may allow the engine to be trimmed with the prop closer to the surface, and will also decrease rpm by 150 to 300.
Material used in today’s props includes aluminum, stainless steel, bronze (often an alloy called NiBrAl) or nonmetallic composite reinforced plastics.
Ventilation is a problem that occurs when air from the surface or exhaust gas from the engine gets drawn into the prop’s blades. The boat’s speed drops, the engine over-revs and screams, and the prop sucks air. Ventilation results from excessively tight turns, a motor that is mounted too high on the transom, or an engine that is overtrimmed. Props having blades with aft rake often work better where ventilation is a problem.
Cavitation (often confused with ventilation), results from water vaporizing or "boiling" due to the extreme lack of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation may result in “cavitation burn”, metal erosion or pitting of the prop's blade surface. Causes of cavitation include dings or sharp corners in the leading edge, poor polishing, too much cup or crummy blade design.
Propeller Selection Criteria: Choosing the right pitch
Changing the pitch will inversely effect engine rpm, so matching the right prop having the correct pitch with your engine will let you achieve the recommended WOT operating range. A two-inch increase in pitch results in a reduction of 300 to 400 rpm. A two-inch decrease in pitch results in an increase of 300 to 400 rpm. Therefore, if your engine operates below the proper rpm, consider a propeller with less pitch. If your engine revs beyond maximum recommended rpm, consider a propeller with more pitch. Many small boats are used at vastly different elevations, trailering to high mountain lakes and to bays at sea-level. Reduced concentrations of oxygen at high altitude cause engines to produce less power (about 20 percent less at 7,000’). You can partially compensate for this decrease in performance by carrying a second prop with reduced pitch, making it easier for the engine to achieve the correct rpm and WOT.
Your boat may have a lot of heavy gear inside, or you may sometimes use it to tow skiers or wakeboarders, in which case the original-equipment prop may have too much pitch, causing your engine’s rpm to be on the low side. If you change configurations, sometimes running light and other times loaded with camping equipment, two props with different pitches make sense. There are three ways you can tailor your prop’s pitch to match a variety of conditions; carry two complete props, two modular props or one prop, like the ProPulse composite prop, with adjustable pitch.
Modular props
like the Quicksilver Flo-Torq II, use separate modular hubs and easily swappable outer propellers. These systems offer three advantages over the traditional all-in-one props.
Shock-absorbing hub systems dissipate stress from shifting gears, protecting shear pins, and are designed to spin when subjected to a substantial impact, so they also protect your lower unit from damage.
Lower cost for replacement or upgrades because only the propellers, but not the hub, need to be replaced. Trying out different pitches or carrying a spare onboard is cheaper. Availability of replacement parts because the Michigan Wheel Vortex, Apollo and Quicksilver Flo-torq II props are interchangeable.
Composite ProPulse prop
The Swedish-made ProPulse prop offers the easiest ability to adjust the pitch. Loosen four allen screws and dial in 1” pitch increments (fine-tuning 150 to 200 rpm, while most other props are only available in 2” sequences). This adjustability often allows boaters to boost engine performance, because most props are slightly off of the best pitch. Smaller models offer pitch adjustments from 12”–16”, midrange sizes 16”–20”, and large models 15”–21”. Individual blades are inexpensive and can be quickly replaced in case of damage. If one of the four blades is destroyed, a replacement can be fitted, or one of the other three blades opposite the damaged side can be removed, turning an out-of-balance prop into an adequate two-blade prop for the trip home.
Should you choose a four-blade prop?
Three-blade props are generally best for recreational boats with three, four and six-cylinder outboards and sterndrives, giving good hole shot and top end performance. Four-blade props are recommended for bass boats and boats with high-performance hulls driven by high-horsepower outboards or IOs, providing improved hole shot performance with less vibration and steering torque at high speeds. More blades also help in lifting the bow, in applications with raked prop blades on high-speed planing hulls, and work better mounted close to the surface.
The blades on three-blade props fill up about 50 to 55% of the available area inside the circle formed by the prop’s diameter (referred to as the Diameter Area Ratio). Adding a fourth blade increases the DAR to between 60 and 65%, so you can expect more thrust to keep your boat planing at lower rpm, a potential boost in fuel economy, but also reduction in WOT rpm of between 50 and 100. The benefits of a fourth blade are more pronounced for larger boats (over 24’) and higher-performing boats.
What material is the best?
Most outboards and IOs are originally sold with aluminum props, which are inexpensive and repairable. Inboards use three and four bladed props of bronze, or a nickel-bronze-aluminum alloy. Replacement props for IO or outboard boats are available in aluminum, stainless steel or composite plastic. These materials compare as follows:
Aluminum: The most common, least expensive material. Suitable for most outboard and sterndrive applications.
Stainless: Performance advantage comes from thinner blades more than stiffness (as is commonly assumed). Stainless costs more but is five times more durable than aluminum. Stainless props can be repaired, at a higher cost, to like-new condition, while repaired aluminum will suffer from metal fatigue and a loss of strength.
Composite: Stronger than aluminum pound-for-pound, 40% lighter, and not subject to corrosion or damage from cavitation. ProPulse warrants their hubs for three years.
Conclusion
Many boaters choose to economize when replacing a propeller, and just purchase a replacement identical to their old prop. Replacing your boat’s prop offers an opportunity to improve performance with a more appropriate pitch, two modular props with different pitches, an adjustable pitch prop, or a four-blade prop. Don’t limit your consideration to just aluminum either, but consider stainless and composite alternatives. As discussed above, spending what is needed to get the best prop for your boat and boating application is a sensible use of your boating dollar.
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