What Goes Into Choosing Your Perfect Propeller
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You wouldnt buy the wrong size tire for your car. Having the wrong tire or the wrong tread or the inappropriate air pressure would make the car wobble or, worse, cause irreparable damage to other parts. The same method of thinking applies to choosing the right propeller for your boat. How you use your boat will matter because waterskiing and fishing require different levels of performance. As Watersports Training shows, propellers are designed in myriad patterns to address power, speed, and efficiency at the various levels of boat and application.
How To Choose The Correct Pitch And Blade Count
Before you dive into choosing a propeller, understand your boatâs intended use. Consider your average speed, load, and boating location. If you will be using the boat for multiple applications, you may need to switch props. The propeller choice directly affects the engine RPM and therefore its performance. Choose a propeller that puts the engine RPM at the midpoint or higher of the wide open throttle range with a normal load. This operating range will translate to the highest horsepower available from your outboard. Refer to your engine operating manual for the operating range.
Once you know the operating range, consult the propeller guide for your engine to pick a combination of pitch, blade number, and material. Choose a range of propellers to perform a water test. Test the props under the same conditions that you would typically use the boatâsame load, gear, and water. Set the trim angle so the boat has optimum speed on top of the water.
To test the prop, run the boat at WOT and observe the maximum engine RPM on a tachometer. If the RPM is below the recommended operating range, switch to a propeller with lower pitched blades. If the RPM is higher, switch to a propeller with higher pitched blades. Each inch of pitch size will change the RPM by 150-200 RPM. Aim for the midpoint or higher of the recommended operating range. Keep in mind that high altitudes will reduce engine power, so choose a lower pitch to achieve the same RPM as on sea level.
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 boats watersports performance for tubing, skiing or wakeboarding? Once you have defined your goals, you can move on in the selection process.
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Choosing A Three Or Four Blade Propeller
One of the perplexing questions posed at the beginning of this article is choosing between a three- or four-blade prop. Either works well for outboard motors. You get all-around performance with an advantage on top speed with three-blade designs. Boats that are troublesome to get on plane, underpowered or used in watersports where top speed is not critical perform well with four-blade props.
Expect rpms to drop by 50 to 150rpm with identical pitch in four-blade props. Three-blade props are generally best for recreational boats with three-, four- and six-cylinder outboards. You get the better of both worlds with a good holeshot and top-end performance.
How To Choose The Best Propeller For Your Boat
Barletta Pontoon Boat with Mercury Engine
Barletta Content Manager, 6+ years Manufacturer Marketing, Brand Management, Content Marketing, Customer Experience
If youre new to boating, you may have realized that there are so many things to learn before stepping foot on your new vessel. Learning everything there is to know about maintaining and operating the boat can be a chore in itself.
Did you choose the right layout for your needs? Do you have enough capacity for the whole crew? What safety gear have you bought, and does it comply with your local and state laws?
So many questions with so much to learn. It can become overwhelming, so its important to do your research ahead of getting out on the water.
Do you know what your engines break-in period is? How many times a year does your boat need to be serviced? Is it safe to power wash the exterior? The list goes on and on.
But for some reason, theres a question that is rarely thought of until youre cruising along and notice a difference in your boats performance:
How do you choose the right propeller for your boat? In most cases, the manufacturer or dealer youre working with will choose the appropriate propeller based on a few key elements. But what if its not meeting your expectations in performance?
What if the propeller that is on the boat isnt necessarily the best choice for your needs?
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Find A Replacement By Oem Part Number
If you know the OEM part number and basically just need to find the replacement, use the Prop Finder tool. Or search the pages and PDF guides below by your OEM part number to find a suitable replacement. Give us a call at 1-800-998-9508 or Intl. +1-206-780-5670 if you don’t find what you need and our boating experts will quickly help you locate a replacement.
- OEM Cross-Reference Table for Mariner, Mercury Outboard & Mercruiser Stern Drives
- OEM Cross-Reference Table for Mercury / Michigan Wheel / Quicksilver / Turbo / Turning Point
Right Hand Or Left Hand Rotation
A 4 blade propeller will usually have a smaller diameter for the same pitch size of the 3 blade equivalent. This is one reason they spin up quickly and yield good acceleration. The blades are often a bit smaller but offer more total blade area because of the additional blade, so they have more grip on the water. When switching from a 3 blade prop to a 4 blade, youll usually need to decrease the pitch by 1 or 2 inches to keep the engine RPM in the same range.
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Should You Choose A Four
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 props diameter . 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 50100rpm at WOT.
How To Calculate Propeller Pitch
Propeller diameter is simply the diameter of a circle scribed by the blade tips of the prop.
Propeller pitch is the distance the prop would move forward in one rotation if it were moving through a soft solidthink of a screw being turned into wood. The blades on a propeller are analogous to the threads on a screw. Some propellers have a constant pitch, meaning the pitch is the same at all points from the leading edge to the trailing edge of the prop blades.
Progressive pitch starts lower at the leading edge and increases to the trailing edge. The pitch number assigned to a progressive-pitch prop is the average across the entire blade. Progressive pitch improves performance in high-speed applications.
Propeller pitch determines the final gear ratio between the engine and the water. A boat should be propped to operate within its wide-open throttle , which can be found in the motor specifications or the owners manual. Ideally a motor with a WOT range of 5000-5800 RPM will reach 5400 rpm with the boat running wide open and trimmed out for optimal performance, with a full load of fuel and water and an average passenger load. That RPM may go up with a light load of fuel or passengers, and lower with a heavier loadby propping for the middle of the range there is leeway in either direction.
The best bet is compromise between these extremes, a prop size that puts the engine in the sweet spot of its RPM range for everyday use.
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Its Also Important To Note The Details Regarding Trimming Up During This Process
Normally you wont have the boat fully loaded when propping it out.
Its best to prop it out towards the top end of the RPM range without trimming the engine. We want to be able to add some trim to the engine and come really close to hitting the rev limiter under these conditions.
This is because whenever we load the boat up and take it out under normal conditions. There will still be room to add some trim at wide-open throttle and have the RPMs come all the way up to the top of the RPM range.
If we have to trim up the engine to get to the top of the range, then after we load the boat. We wont be able to get to the top of the range because of the additional weight that has been added to the boat.
Thats all for now folks! Now that you have a better idea of how to correctly prop your boat for the best results. Remember to share this article with a fellow boater! We also encourage you to check out one of these other helpful articles that we have written!
What Does Prop Pitch Mean
Propeller pitch typically referred to as prop pitch, is essentially the distance a propeller would move in a single revolution through soft solid. Kind of like the distance a screw would move through wood in one rotation. So, in this case, a 21 pitch prop would move 21 inches forward in one revolution, and a 19 pitch prop would move 19 inches forward in one revolution.
So what does all this mean to your boat?
Think of your propeller as if it were the axle on your car. The lower the ratio, the more pulling power it has from a standstill. This is the same physics that is applied to a boat propeller. A boat with a lower pitch propeller could accelerate much faster from a standstill position than one with a higher pitch propeller. Unfortunately, this lower pitch would make your engine reach maximum rpm at much slower speeds, albeit faster acceleration.
On the other hand, a higher pitch propeller will give you better top speeds but much slower acceleration. It should also be noted that simply fitting a high pitch propeller to a boat with a lower horsepower engine doesnt mean that you will make your boat faster.
While the boat will achieve higher speeds, it will do so for a short while before getting bogged down. The propellers higher pitch and diameter will eventually overwork and wear down the internal engine parts that just arent built to withstand that kind of stress.
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Other Factors To Consider When Purchasing A Boat Propeller
When you browse the available propellers to buy, youll notice there are two definitive numbers. One reflects the diameter , and the other describes the pitch . The diameter should be matched to the size of your boat. The pitch will help keep your engine within its ideal RPM operating range, according to Crowley Marine.
Youll also want to consider the cupping or the trailing edge of each blade on the propeller. The cupping can be gradual or angled and describes the bite on the water or the propellers ability to grab and push water, Boat U.S. explains. Cupping matters if you need a quick acceleration or want to operate in shallow areas. Increased cupping is what speed enthusiasts talk about, and there are even more complicated variations based on twin-engine designs. But every boat can achieve its best performance with the right degree of all three of these measurements: diameter, pitch, and cupping.
How Does A Boat Propeller Work
Lets first breakdown how a boat propeller functions to better understand why its important to choose the right one for your boat.
Although propeller theory is straightforward, there are a few variables behind what makes it spin. A propeller displaces water and creates thrust by rotating its blades.
The angle at which those blades are placed relative to the hub is called pitch. The pitch of the propeller will dictate how far it moves with each thrust.
Pitch is calculated by the theory that if a 12 pitch propeller is moving through a solid structure, one revolutionor turnof the propeller will move it forward 12 inches. This is what happens when a screw moves through a solid such as wood.
So, if the propeller pitch is 16 inches, it will move 16 inches. The more pitch you are trying to turn, the more torque is required from the engine.
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What Is The Benefit Of New Propeller Blade Design
Blade shape has moved on in the last 15 years or so, thanks partly to computer aided design and a better understanding of what advantages a particular blade shape can offer.
The heavy sectioned blade is no longer the preserve of the plodding working boat, it can also be used to advantage on high-speed vessels where performance and economy are of equal importance.
Take a look at the original propeller on my own boat . Not much more than a three-bladed slab of bronze that did its job of pushing the boat through the water.
Three-bladed propeller: a workhorse that served Tony Davies boat well for many years
Now move on ten years and look at the sophisticated four-blade foil design that runs much more quietly, with less vibration and gives increased fuel economy at semi-displacement cruising speed.
A modern four-blade design that is more fuel efficient and more powerful
How Do I Find The Right Prop For My Boat
When choosing a propeller, choose a pitch that will keep the engine RPM in its recommended operating range. Going under the range will cause the boat to lug, while going over can cause engine strain. Blades have other important features built into their design. Rake is the angle between the blade and the hub.
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Stainless Steel Propellers Vs Aluminum Or Plastic
Most boat propellers are going to be one of these three materials. Aluminum props tend to be the most common. They are lighter and cheaper and can do a great job. However, stainless is definitely stronger. Stainless is also able to have thinner blades. If you want a top quality boat propeller, stainless steel is probably what you want. Most boats come with aluminum propellers. Large boats like yachts and small fishing boats alike.
Plastic props are more common on small boats. They require less power and the prop will do less work. The propeller blade on a plastic prop is going to be flimsy, as you might expect. Heavier boats cant use these. Theyll have more drag than the prop can handle.
For optimum performance, stainless steel props are best. The boats performance will reflect this. However, you may see increased fuel efficiency with a lighter aluminum prop. Much of it depends on the type of boat and engine you have.
Basically, a stainless steel prop is going to last longer than the others. Up to five times longer, in fact. It can stand up to more abuse. Youll be less likely to need to replace it as soon as another type.
To Grip The Water And Push The Boat
The result is the engine RPMs going above the specs and hitting the rev-limiter. Its exactly like what happens when the prop hub gets spun
On the opposite end of the spectrum, if were to install a prop with too big of diameter and pitch, then the engine wont have enough power to spin the propeller up to the right RPM range.
Causing the engine to never reach wide-open throttle and if ran like this for a long period of time, can eventually wear out internal components of the engine prematurely.
Its important to note that there are methods of getting propellers to turn up higher without changing the prop. This is usually done by venting the propeller some props have venting holes towards the bottom of the prop.
These holes can be either plugged or have the plugs taken out allowing water to flow freely through the prop without having to be pushed off the blades of the prop, causing force against it.
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How To Use The Boat Prop Calculator Tool
Knowing the theoretical speed for your boat can help you select the proper pitch. To use our boat prop calculator, you need four inputs:
- RPMs: Revolutions Per Minute. Enter the high end of the operating range established by the engine manufacturer or the maximum RPMs you can attain with your current propeller. The recommended operating range should be listed in your owners manual and can also be found online.
- Pitch: This is the theoretical distance the propeller would travel through the water with each revolution . Think of a screw going through wood. It is often stamped on the outside of the propeller. In this example, the propeller has a 24 pitch.
- Gear Ratio: This is the number of drive shaft revolutions for one revolution of the propeller. You can find this in your owners manual or by searching online.
- Prop Slip: This is the relationship between the actual vs. theoretical distance your boat travels with each revolution of the propeller and is impacted by the hull design, weight of your boat, propeller design and other factors. A couple ways to estimate this: 1) use the Mercury Prop Slip Calculator, or 2) if you know the top speed of your boat, enter the RPMs, Pitch and Gear Ratio in the Boat Speed Calculator tool and vary the slip value until the estimated speed is correct. For example, I have a boat with a max RPM of 5,800, my propeller pitch is 24 and the gear ratio is 1.92. Based on these inputs, a Prop Slip of 13% get me to my correct maximum top speed of 60 mph.
