What Goes Into Choosing Your Perfect Propeller
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.
Some Problems That May Arise
There are two problems that you want to keep an eye out for when it comes to using your new propeller: ventilation and cavitation.
This is a situation where surface air or exhaust gases are drawn into the propeller blades. This causes boat speed to reduce and engine RPMs to climb rapidly. There can be several causes for this: excessively tight cornering, over-trimming the engine, or a motor that is mounted very high on the transom. This can also be caused by prop designs that are not matched to the application, props with little to no cup, or props that have worn/damaged edges.
When small air bubbles form against a propeller blade, it is called cavitation. This happens because the water boils around the prop. Many props partially cavitate during normal operation, but excessive cavitation may result in pitting of the propeller blade surface, known as cavitation burn. Just like ventilation, there are several causes of cavitation as well: incorrect matching of prop style to application, incorrect pitch, physical damage to the blade edges, or water flow obstruction such as parts of the boats hull or running gear too close to the propeller.
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.
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What Is The Most Important Thing When Choosing A Propeller Pitch No Matter What Size Engine Or What Kind Of Boat If The Propeller Isnt The Right Pitch The Boat Isnt Going To Live Up To Its Potential
Pitch is the theoretical distance, in inches, a propeller moves forward every time it turns .
Heres the skinny: You want to be able to run the engine at, or near, the manufacturers recommended maximum RPM at full throttle, trimmed up for speed, with a typical load in the boat and if you can, your propeller is the right pitch.
Too much pitch – the engine wont reach its max RPM, is sluggish getting on plane, and has poor throttle response. Not enough pitch – the boat pops on plane and accelerates with confidence, easily exceeding the recommended full throttle RPM. Neither condition is good for engine longevity or fuel economy.
The cure? Since every inch of pitch is worth about 150-200 RPM, decreasing pitch should provide a proportionate increase in RPM drop pitch an inch, gain a couple of hundred RPM. Conversely, increasing pitch usually results in a RPM decrease go up an inch, lose a couple of hundred RPM.
After you have the pitch part of the equation figured out, then the propeller selection process can continue.
How Do I know What Pitch is Right for Me?
Consult the engine owners manual to find the recommended wide-open-throttle range for your engine. If the current propeller is at WOT RPM within the specified RPM range, select a replacement or upgrade propeller with the same pitch as the current propeller.
- Adding 1 inch of propeller pitch will reduce WOT RPM by 150 to 200.
- Subtracting 1 inch of propeller pitch will increase WOT by 150 to 200.
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.
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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.
Understanding A Boat’s Propeller Pitch
A boat propeller has two basic dimensions: diameter and pitch. These dimensions are used to describe the propeller, usually in inches, and always stated as diameter x pitch. For example, a propeller described as 14.5 x 19 has a diameter of 14.5 inches and a pitch of 19 inches. These dimensions are often stamped or cast right on the propeller. Recreational boat propellers are usually offered in two-inch pitch increments within a prop model line, but some high-performance props are offered in one-inch increments to allow for fine-tuning boat performance.
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Bass Bay & Flat Boats
V MAX SHO
An ultra-high-performance ventless design and single-inch pitches allow you to fine-tune the performance of Yamahas four-stroke V MAX SHO.
This stainless-steel series provides high performance for single-outboard boats requiring bow lift.
Stronger hole-shot than comparable 3-blade propellers. Excellent on mid-range powered flats boats or for applications requiring high engine mounting heights. Fits F70 to VF115, T50/T60 K series gearcases. Standard Shift Dampener System .
Maximizing Horsepower Minimizing Rpm
The right propeller for your boat and engine combination is based in part on the wide-open throttle operating range for your specific engine. This information is an intersection of horsepower and RPM and can be found in your owners manual.
An optimal outboard propeller will maximize your boats performance while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from over-revving and facilitate maximum horsepower with minimum RPM.
An outboard engine under load should reach the manufacturers recommended RPM at WOT, usually expressed as an RPM range. Your optimal propeller will deliver acceptable acceleration and top speed while still allowing your Yamaha outboard to reach its recommended revs at WOT.
If you get this relationship wrong, premature engine damage is too often the result.
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Learn How Propellers Are Constructed
Propellers are comprised of different materials, shapes, and sizes to suit the broad spectrum of use cases and conditions in which a propeller must perform. If you’re looking to change your propeller for one that better suits your need, it will help to learn a little about propeller construction, types of propellers, materials used in constructing propellers, as well as the number of blades on a propeller, and other blade performance factors.
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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Boat Prop Selector Guide
Whether you are buying a new propeller for your boat, simply in need of a replacement for a prop you lost/damaged, trying to address a performance issue or you simply want to gain better fuel economy, we can help you find the perfect propeller for all of your boating needs. Propeller Depot offers the best pricing in the marine industry on all inboard, outboard, ski/wake and stern drive propellers. Order your boat propeller today from Propeller Depot and save.
The Meaning Of The Numbers On A Prop
Typically, most propellers will have specific information engraved on them. This information usually consists of two to four sets of numbers and letters.
One being the propeller diameter, the second being the pitch length, third representing the rotational direction and fourth being the bore/shaft diameter.
An example of these numbers displayed resembles the following: 15.6 x 15 | RH | 1 ¼
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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.
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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What Determines The Type Of Propeller Your Boat Needs
If youre new to boating, you may not have expectations around what type of performance you should get out of your engine/propeller combo. That said, there are numerous variables behind which prop is right for your boat.
Propellers can make such an impact on performance that your neighbor could have the exact same boat and engine, but if the prop is different, the two boats could yield different results.
There are so many fluctuations and options that you could literally make the same exact boat faster just by adding a different propeller.
Choices that affect this change in performance are:
Stainless is a stronger metal than aluminum, so this type of prop is tougher against damage. It also doesnt flex, so it holds its shape in the water and can increase the speed depending on boat and engine type.
Aluminum is a softer and less expensive metal, so these props are usually more budget-friendly. That said, they do tend to flex, which can hinder the boats performance and speed.
Other factors that make a difference in how your propeller performs are:
Choosing a propeller can be subjective depending on your performance goals, but different engines and hull designs do play into selecting the best prop.
Right Amount Of Blades
How many blades should a prop have? Theoretically, one blade is best it has the least amount of drag and no other blades disturbing the water flow. But making one blade balanced is like trying to walk with one leg it cant be done. Two blade props need oversized blades to create enough blade area for effective thrust, and that creates excessive drag and or vibration. So, most common props used for todays power plants have three blades, which offers the best compromise between balance, efficiency, blade area, and vibration.
Three-blade props generally can propel a boat faster. But if the engine has enough torque, four-blade props can harness that power more efficiently because of the greater blade surface area.
Four-blade props are popular on boats which often encounter ventilation issues, such as tunnel hulls and powercats. They usually get a better bite on the water while three-blade props may slip too much. The extra blade can also improve hole shot and reduce vibration. Top-end, however, is usually cut by a couple of mph, as that extra blade also adds drag. Props with fewer than three blades are reserved for applications, such as sailboat auxiliary motors and electric trolling motors. Five or more blades are only commonly used on props for large vessels or special applications.
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Other Reasons To Change
Why else might you want to change props? If your motor over-revs from ventilating, going from a three to a four-bladed prop will often solve the problem. If you want to increase both top-end and cruising speed immediately, changing an aluminum prop for a stainless one will do the trick. And if you want to reduce vibrations going from a three to a four-blade prop will make a noticeable difference.
A dual-use boat can benefit from a propeller change, depending on the type of activity involved.
Another reason you might want to consider changing a properly sized prop is if you run a dual-use boat. If, for example, the kids want to go water skiing from your center console which is propped for the best cruising speed, but as a result is slow to get on plane you may want to swap for a prop that will provide a better hole shot, when you plan to spend the day water skiing.
In any case, whenever you buy a new boat you should plan on trying several different props to find the one that best fits your boat and your needs. If youve never played the prop-swapping game, you might discover that you can give your old boat some new life.
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How To Choose A Boat Propeller
- Written by Robert Gallo on Nov 23, 2009To ensure our content is always up-to-date with current information, best practices, and professional advice, articles are routinely reviewed by industry experts with years of hands-on experience.Reviewed by H.R. Helm on Feb 25, 2020
Knowing how to choose a boat propeller really comes down to knowing whether or not you want a lot of speed, or more power to transport a boatload of people over the lake comfortably and easily. This can only be accomplished through trial and error, research and visiting several boating outlets. It may even entail asking for an individual demonstration. Keep in mind that you might have to purchase more than a single propeller because there are different varieties that will be able to give you either top speed or good handling, but you wont be able to get both of these with just one propeller.
Sizes of Propellers
Why Size Matters
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What Is Propeller Diameter And Why Does It Matter
Any time you want to buy a new propeller for your boat, you need to understand what the specs are and what all those terminologies mean. Propeller diameter and pitch are two of the most important specifications that directly translate into real-world performance.
Lets start by defining what they mean.
The diameter of a propeller is the distance across the full circle being made by the blade tips. Its primarily determined by the rpm at which the propeller would be turning as well as the amount of power being delivered to it. This diameter tends to increase on propellers typically used on slower boats while increasing propellers used for faster boats.
Should You Go With A Different Pitch
All marine propellers involve a tradeoff. If you prop your boat to maximize top speed, acceleration will be compromised and visa-versa. Therefore, the first consideration is whether to optimize top speed, acceleration or some combination of the two.
To increase acceleration, consider reducing your pitch. This makes particular sense if you are NOT hitting the max RPM level established by the engine manufacturer when running at Wide Open Throttle with your current prop. To see the potential impact on top speed, enter your current values for Max RPMs, Gear Ratio, Pitch and Prop Slip in the Boat Prop Calculator tool. Then, decrease the pitch by an inch or two. However, as you do this, you should increase your RPMs by approximately 200 for each 1 reduction in pitch .
Conversely, to increase top speed, consider increasing your pitch. This is especially relevant if you ARE hitting the max RPM level established by the engine manufacturer with your current prop. However, it is hard to tell whether your RPM level is the best your engine can do or if it is being capped by the engines rev-limiter . If it is the later, then you likely have room to improve your top speed. If it is the former, then it might not make much difference after you account for the fact that each 1 increase in pitch will result in approximately a 200 decrease in RPMs.
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