Selecting a Propeller
By Tom Burden and Brian Gordon, Last updated: 12/3/2018
Apart from keeping your boat’s hull clean and your outboard or engine tuned, selecting the right propeller is one of the easiest actions you can take to optimize boat performance. Depending on how you use your boat and its current performance, you might be wondering if you should “pitch up”, “pitch down”, switch from aluminum to stainless steel or move from three blades to four. Your prop might be damaged and need replacement. Or maybe it is in good shape but you're looking to improve your boat's performance. In either case, this article is for you.
What problems are you looking to solve?
Is your boat sluggish coming out of the hole and slow to get on a plane? Are you not hitting the top speed you think you should? Do you want an improvement in fuel economy? Hoping for better all-around performance? Is your current prop blowing out or ventilating excessively in turns or when you accelerate? Are you looking to improve your boat’s watersports performance for tubing, skiing or wakeboarding? Once you have defined your goals, you can move on in the selection process.
Is your engine over or under revving?
Selecting the correct prop should result in your engine running within the designed rpm range at Wide Open Throttle (WOT). Your owner’s manual should include this spec—usually 5000–5500rpm for an outboard or 4200-5000rpm for a sterndrive—or your mechanic or dealer may know. Allowing your engine to under rev or over rev at WOT can result in engine damage. Over revving or under revving can be corrected by selecting a prop with a different pitch.
Should you "pitch up" or "pitch down"?
Engine rpms and pitch are inversely related. Increasing the pitch will decrease engine rpms and decreasing the pitch will increase engine rpms. As a general rule, a two-inch increase in pitch will result in a reduction of 300 to 400 rpm. Conversely, a two-inch decrease in pitch will result in an increase of 300 to 400 rpm. Going back to the question of performance at WOT, if your engine is under revving, consider a propeller with less pitch. If your engine is over revving, consider a propeller with more pitch.
Elevation and Weight
Elevation: Many small, trailered boats are used at vastly different elevations, such as high mountain lakes and bays at sea level. Reduced concentrations of oxygen at high altitude cause engines to produce less power (about 20 percent less at 7000'). 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 at WOT.
Weight: 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 two ways you can tailor your prop’s pitch to match a variety of conditions: carry two complete props or two modular props with different pitch.
Ventilation and Cavitation
Ventilation: This problem 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 over trimmed. Ventilation can also occur from prop designs that are not matched to the application, poorly designed props, props with little or no cup, or props that are worn or have damaged edges or cup profiles.
Cavitation: Often confused with ventilation, cavitation 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 incorrect engine height (outboards), dings or sharp corners in the leading edge, poor polishing, too much cup or crummy blade design. Cavitation can also occur from thru-hulls, sensors, or other turbulence-producing protrusions under the boat forward of the prop.
We stock propellers to fit most outboards and sterndrives. Choices include propellers and hubs made by Mercury (Quicksilver) and Turning Point. Regardless of the brand you prefer, you will need to gather as much information as possible (see below) to make the best selection. After processing this information, you can make a better-informed decision. You can do this by speaking with an associate at one of our stores, calling our Technical Sales Department at 1-800-BOATING, using the Turning Point Prop Wizard. If you prefer a Mercury product, use the Quicksilver Propeller Selection Guide. NOTE: Mercury and Turning Point both offer modular hubs and propellers. Hubs are purchased separately from props. You will find an explanation of modular hub systems at the end of this article. Mercury also continues to offer one-piece, traditional props.
- Present prop diameter
- Present prop pitch
- Right or left hand prop rotation (clockwise is right hand)
- Number of blades
- Material (Usually aluminum or stainless steel)
Other useful information
- Manufacturer’s part number
- Shaft diameter and number of splines or keyway type
- Number of engines
- Rated horsepower
- Gear case size
- RPM at WOT
- Manufacturer, model and year
Other very useful information
- Displacement in cubic inches or centimeters
- Power trim or trim tabs
- Length overall
- Hull material
Other very useful information
- Manufacturer, model and year
- Hull shape
- Present/desired top speed
Propeller Nomenclature—"Prop Talk"
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. 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 percent) 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.
Turning Point Propellers use a squeeze casting process to create thinner blades. They boast stainless performance at aluminum prices.
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 planing speed. Today’s aggressively raked propellers may require that you add a high performance trim tab to your shopping list. The blade tips of these new propellers may strike the older style trim tabs on your engine.
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.
What material is 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 or stainless steel. These materials compare as follows:
Aluminum is the most common, least expensive material. Suitable for most outboard and sterndrive applications.
Stainless steel offers a performance advantage over aluminum due to stiffer, thinner blades and more advanced designs. Best choice at speeds over 50mph, or if your boat is running over oyster beds or sandbars regularly. 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.
Should you choose a four-blade prop?
Three or four blades work well in either sterndrive or outboard applications. Three-blade designs give you all-around performance with an advantage on top end speed. Four-blade designs work well with boats that are difficult to get on plane, underpowered or used in watersports where top-end speed is not critical.
The composite cores of modular hubs are designed to break away upon significant prop strikes, helping to protect the prop body and engine drive train from damage.
Four blades in many cases will drop your rpm by 50 to 150rpm with identical pitch. 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.
The blades on three-blade props fill up about 50 to 55 percent 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 percent, so you can expect more thrust to keep your boat planing at lower rpm, a potential boost in fuel economy, but also a reduction of 50–100rpm at WOT.
Modular Hub Systems
Modular hub systems consist of a hub that fits on to the splined shaft of the engine and the prop body which fits on to the hub. These systems offer two main advantages. First, in the event of a significant prop strike, the composite hub breaks away, which reduces the chance of damage to the engine, engine drive or propeller. Damaged hubs are easily user replaced and this lowers the “replacement cost” of the prop strike. Also, if the prop is bent but the hub is intact, replacing just the prop is also less expensive. Either way you stand to save $$$ if you have a problem.