Pitch Prop Vs 19 Pitch Prop: Which Do You Need
by Kyle WUpdated on April 23, 2021.
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Your boat is very much like your car; without proper maintenance and the right parts, it will give you issues and might leave you stranded while out fishing for bass. The difference between your boat and your car is that the parts arent quite the same.
While it will do you well to know how to change a flat on your car, when it comes to boating, propeller diameter and pitch are some of the most important terminologies. This is because its your propeller that actually drives the boat. So understanding a bit about it could save you a whole lot of headache down the road.
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How Do I Find My Current Propeller Size
The size is usually indicated on outside of the propeller hub. It is usually written like this :
- 11 1/4 X 13
The first number is the diameter and the second number is the pitch .
Some brands use a different way to indicate the size or model.
- 11 1/4 X 13 G
- 48-832832A45 21P
- 5111-093-10
- 814629
Yamaha adds a letter to indicate the type of blade design. Mercury Mercruiser uses a number code ending with pitch size . Solas propellers use the last 4 numbers to indicate size . Volvo Penta uses a separate number system, see here for an overview table.
If you have lost your propeller in the water or it has become unreadable, please contact us for advise.
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.
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Putting It All Together
Understanding propeller pitch isnt difficult, but it is essential to choosing the perfect engine for your boat and your experience on the water. Pitch and diameter entirely change the way your boat handles. So, choose the combination that makes the most sense for the usual activities you enjoy when boating.
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.
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Brave New Blade Design
With modern propeller design and construction methods the three main criteria are no longer the only governing factors. Increasingly, shape is just as important.
If the diameter needs to be smaller to provide better acceleration then the blade area can be increased by adding more blades. For optimum performance, fuel economy, fast acceleration and smooth vibration-free running, it is the shape and cross section of the blades that make the crucial difference.
For the everyday boat owner blade shape is governed by the boats dimensions, its engine specifications and the owners choice of propeller manufacturer. High-tech computer-designed and machined propellers only become viable for yachts over 24m where the shape of the blades is tailored to the requirements of the vessel taking into account resistance as well as all the normal information required to build a propeller.
This Sunseeker propeller is a good example of what is required to propel a heavy high-performance craft. The total area is greatly increased by overlapping the leading and trailing edges of the blades. This avoids the problem of cavitation, but the blades are shaped to ensure water flows smoothly through the propeller
The shape of the older equipoise is reminiscent of the latest foil design although it has less blade area.
The round blade turbine propeller is still used on heavy displacement craft including commercial vessels. It has a large blade area and thick sections to resist damage.
Stainless Steel Boat Props
SS is identical to aluminum when it comes to styles. However, a majority of SS props do not use a hubless design. That means you are stuck with a hub that is pressed in. Or, in other words, Prop A will not work with Motor B.
The Cost of Stainless Steel Props
As mentioned, youre going to pay a pretty penny for a stainless-steel prop. The question is why pay the extra money?
The typical SS prop repair ranges from $140 $250. Yes,;stainless steel is more expensive to repair, but its also worth the effort compared to aluminum. Youll repair SS over buying a new one,;because repairs are hundreds of dollars less than buying new.
Life Expectancy of Stainless Steel Props
SS props have a much longer life expectancy. Theyre durable and stronger than aluminum props.;They also dont experience;flexing. Since aluminum is softer than SS, when its under load and rotating thousands of times a minute, it loses its;pitch. What we mean by that is the blades under a load will move and negatively, affecting your performance. SS holds a more true pitch, which gives you better performance and even better fuel economy.
Think of it this way: Aluminum is like a car with a funny transmission that sometimes slips or shifts hard. SS is like a fine-tuned performance car that shifts effortlessly and doesnt slip when put into gear.
Pros and Cons for Aluminum and Stainless Steel Props
Aluminum Pros:
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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 . Your owners manual should include this specusually 50005500rpm for an outboard or 4200-5000rpm for a sterndriveor 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.
What Is Hull Resistance
Resistance is the hulls natural tendency to drag through the water. Bare hull resistance is produced by skin friction of the hulls surface, waves created by the hull running through the water and the shape of the hull below the water. To the bare hull, the drag of appendages must be added such as rudders, shaft support brackets, shafts, depth sounder transducers, stabiliser fins, etc. However, the design of the lift surfaces of the hull also have a very significant effect. Designers work to ensure that the lifting characteristics give the hull the least resistance at normal operating speed.
Once this resistance is known, engine and gearbox specifications can be combined with maximum propeller diameter to determine the props minimum requirements. The vessels duty cycle, which is simply a judgement as to whether a boat is to be used for heavy or light commercial or pleasure purposes, then also influences a designers final specification.
Propeller diameter is first decided by the type of boat. A large-diameter slow-revving propeller is the most efficient type but this wont provide the necessary acceleration to get a sports cruiser onto the plane. Acceleration is achieved by increasing the rotation speed, which usually means a smaller diameter propeller. The fact that many modern sports cruisers have limited propeller clearance space is another reason for fitting smaller diameter propellers.
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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.
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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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.
What Are Tooth Splines
A new propeller will only fit if the number of ´tooth splines´ on your propeller shaft is the same as the number of tooth splines on the inside of your propeller hub. When selecting your engine in our shop, it will automatically select the propellers with the right number of tooth splines. To verify if your new propeller will fit, you can check the number of tooth splines as mentioned on the product page.
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How Can I Make My Boat Come On Plane Faster
There are many factors that can cause a boat to have difficulty coming onto plane. Consider for example, your total boat weight, weight distribution, engine trim, hydrofoil, and transom tabs before investing in a new propeller. To assist in the correction of on plane problems, you may also need a different propeller. Your first choice should be a propeller with more blade area. Keep the pitch the same, but provide more blade area either through a slightly larger diameter or an extra blade. In cases where this would not be enough, you may have to trade some top speed for a better bottom end through reducing the pitch a couple of inches
How To Choose The Right Propeller
Let me start by saying, Ive been an avid boater all my life.; Ive driven numerous different types of boats and have always been pleased with the propeller that was included from the factory every time.;
If youre new to boating or dont have a strong opinion on the type of speed and performance youre getting out of the ride, then you might have the dealer recommend a prop or roll with what the boat manufacturer provides.;;
For instance, here at Barletta Boats, we try to find a middle ground prop that allows the motor to spin close to its max RPM.; This way, youre getting a propeller that is made for your boat and will achieve near-peak performance without breaking the bank.
The traditional method of selecting a propeller takes the horsepower, boat weight, and hull type into account.; Then the pitch and diameter are selected that allow the engine to run at the top end of the RPM range.
Once the boat is loaded with people and gear, this method typically provides good all-around performance.; Some boaters may have different performance goals such as better hole shot or faster top-end speed, which will require a change in the pitch and diameter.
You generally want to find the comfort zone for props on your specific boat. If youre hitting the rev limiter, youve got too low of a pitch.; Think of this as a 10-speed bike and youre going downhill on the highest gear.;
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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.
Choosing The Right Pitch
If you are currently satisfied with the performance of your engine, we recommend to choose the same pitch. However, if you wish to change the performance you can consider a different pitch. For example;if your engine currently makes too much or too few RPM , or;if you want to use your boat for a specific purpose .
A good indicator for selecting the optimal pitch is the RPM your engine makes at ´wide open throttle´ . Each engine has a prescribed ´optimal´ RPM that it should be making at wide open throttle. This differs per engine but often it is between 4500 and 6000. You can check your manual, or check this RPM Range Chart for the most common engines.
If currently your RPM is too high, you may consider a larger pitch that will improve your top speed. If the RPM is too low, the engine is not reaching its full potential because of a too large pitch. In this case choosing a smaller pitch can result in better acceleration and higher top speed .
If youre changing pitch on a recreational boat, remember that each inch of pitch translates to a difference of approximately 200 to 300 rpm. Lowering the pitch will increase rpm and choosing a larger pitch will reduce the rpm. For example, going from a 23 pitch to a 21 pitch will increase engine rpm by about 400 to 600. For smaller horsepower engines this difference is usually bigger: one pitch can already make such a difference.
NEW: for direct advice on the optimum pitch!
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