Choosing The Right Propeller By Crowley Marine
The right prop will get the best performance out of your boat. Matching the propeller to your application, whether cruising, water skiing or performance, maximizes the power from your engine. It also helps with handling and acceleration. Use this guide to learn how different propeller designs affect efficiency, power, and speed and then learn how to choose the correct prop for your boat and application.
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.
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.
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To Pitch Up Or Pitch Down
This is an integral question that must be answered before you buy the new propeller. As explained, by increasing your prop pitch, you will be effectively decreasing the engines rpm and vice versa.
Since the general rule of thumb is that every two-inch increase in pitch decreases the engines rpm by about 400 revolutions, if you find that your boat is under revving, then you might want to consider getting a propeller with a lower pitch and vice versa. If your boats issue is that its over-revving, then getting the 21 pitch propeller might be the best solution.
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The beauty of all this is that your boat manual will have the desired specifications, but you could also ask your mechanic or even your dealer to advise which prop pitch would be the perfect solution for your specific boat. Just remember, one of the most important considerations here is your performance goal. What is it you want the boat to do exactly?
Boat Propeller Fit Guide
If you’re buying a new propeller for your boat, you may simply need a replacement for a prop you damaged, you may be trying to address a performance issue or you may simply want to gain better fuel economy. Whichever the case, we can help you find the right propeller for your needs.
Choosing the best propeller for your boat is a key factor in performance and choosing the wrong prop can lead to poor fuel efficiency and issues with engine performance including slower speed and increased wear and tear on your engine.
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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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Propeller Specifications Explained: Propeller Theory
Line A-B x 2 = propeller diameter
All propellers can be described in basic terms using a set of two numbers: diameter x pitch. Hence a prop with an 18in diameter and 12in pitch is described as 18 x 12. The diameter of the propeller is the circle which the tip of a single blade describes in a complete rotation. It is found by measuring from the centre of the propeller boss to the tip of one blade and then doubling the result.
How to measure the pitch of a propeller
Pitch is the forward distance that a propeller would theoretically travel in a single rotation if there were no slip present imagine a screw being driven into a piece of wood. The angle at which the propeller blades are set governs the distance travelled. A fine pitch gives a lesser distance than a coarse pitch .
A propeller is usually designed with two to five blades . In the first instance, the number of blades is decided by the weight of the vessel: the greater the weight, the greater the blade area required to push it through the water with a minimum of slip and cavitation.
To specify the correct type and size of propeller, a standard set of boat, engine and gearbox measurements is required. Designers will use the boats length, beam, draught and displacement in combination with the boats underwater drag characteristics to calculate hull resistance.
Does My Boat Have The Wrong Size Prop
If the boat is over-propped the engine will not reach maximum rpm and the boat will underperform. Equally, if a boat is under-propped and the engine easily reaches or exceeds maximum rpm, the propeller needs more bite to get the boat to maximum performance level.
With outdrives, adjusting the pitch of the prop is often the only option, as the maximum diameter propeller is often already installed. Increasing the pitch will decrease the revs and vice-versa. If the engine revs seem OK with the current prop, but you want better acceleration, it is often possible to reduce the diameter and increase the pitch by roughly the same amount to give added acceleration without losing the top-end speed.
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It Has Too Much Pitch
When researching propeller sizes, youll notice two numbers associated with each one. The first is its diameter and the second is its pitch.
In boating, prop pitch refers to the distance that your propeller moves through the water in one full revolution.
For instance, say your propeller has a pitch of 18 inches. This means it should move 18 inches every time it rotates through a solid medium.
The only difference? Water is, of course, a liquid medium.
As such, expect some slippage to occur. In fact, this range of motion is necessary to move your boat along. While this adjustment is fine, it shouldnt be extreme.
If you find that your boat has way too much pitch, it might be difficult for it to come onto a plane. In laymens terms, this means its hard to accelerate.
Its akin to leaving a stoplight with your car stuck in third gear. Youre working harder at lower speeds and stressing the rest of the boat mechanics.
Ask a mechanic to adjust your propeller pitch and see if that solves the issue. Often, thats the only change necessary. Or, you could also try changing from a three-blade propeller to a four-blade one. Sometimes, that extra blade can improve acceleration, fuel efficiency, and overall performance.
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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Solas Propeller Installation Instructions
Choosing The Right Propeller For You Boat Requires Trial And Error
Often, your dealer can supply two or three props for testing when you buy a new motor for a small fee.
This is because boat design, load and the running characteristics of your vessel all play a part.
Watch our video as we chat to marine mechanic Matt Solis from Barneyss Marine Hervey Bay for some tips and considerations when choosing the right propeller for your boat.
Ryan Moody started his fishing career on the reef boats before catching bucket list marlin for the likes of champion heavy tackle angler Johnno Johnson, INXS and the King of Sweden. Branching out in the late 80’s to guided barramundi fishing, Ryan has made a name for himself as a Big Barramundi specialist and to date has put clients onto over 2000 metre plus barra. That is over 2 kilometres of metre plus barra! With attitudes changing from ‘keep all you can’ towards catch and release, Ryan has decided to share his extensive knowledge and hopefully inspire people of all ages to get out from behind the computer screen/TV and into the fishing outdoors lifestyle he has spent his life perfecting.
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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 â the engine wont reach its max RPM, is sluggish getting on plane, and has poor throttle response. Not enough pitch â 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.
What Pitch Should My Prop Be
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. Engine height also affects the performance of the propeller.
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Clear Signs Its Time To Replace Your Boat Propeller
Stranded at sea isnt the time to realize you need a new boat propeller.
As with all water equipment, preventative maintenance and routine checks are critical to making sure all your gear is in top working condition.
As soon as you notice your boat becoming sluggish, slow to start or difficult to accelerate, it could be time to give your propeller a second look.
Today, were sharing eight signs that your current model has suffered enough wear and tear and its time to look for a replacement. Vacation season is at our heels and its time to set sail with the right watersport gear.
Ready to get started? Lets go!
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.
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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
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