sailboat propeller size calculator

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Prop calculator.

Calculated values from inputs used to derive size and pitch

Power required numbers, prop pitch numbers.

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Free to use Propeller Sizing Calculators

Before you start, we would like to point out that these calculators are based on empirical data. They are not intended to perform complex cavitation or efficiency calculations. The results provide "best size" propeller dimensions based on your input data. There will always be a number of propeller size combinations that will work on your vessel, finding the best one is the trick. These calculators will help guide you to determine various combinations of power, gear ratio and propeller size for your vessel. Due to the nature of these calculators, Victoria Propeller Ltd. will not accept any liability, financial or otherwise, for the calculations or results provided.

Propeller Size Calculators:

• Planing Hull Propeller Sizing Calculator (generally applies to power boats with no keel)
• Displacement and Semi-Displacement Hull Propeller Sizing Calculator (generally applies to boats with keels)
• Minimum Propeller Diameter Calculator (good for checking the propeller diameter against the hull size)

Conversion Calculators:

• Liquid Volume to Weight Converter (helpful for calculating fuel and water weights in your tanks)
• Speed Converter (enter speed in knots, mph, or kph and see the equivalent in knots, mph or kph)

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Boat Propeller Calculator

You can use this boat propeller calculator to determine one of five variables: boat speed, propeller slip, propeller pitch, engine gear ratio, or engine revs.

To use the calculator, input four of the five variables. The calculator will automatically compute the 5 th variable.

Boat Speed mph kts kph

Engine Revs RPM RPS

Gear Ratio : 1

Propeller Pitch inch cm mm

Propeller Slip %

Propeller slip provides an indication of the efficiency with which a boat is traveling through the water. However, it is not to be confused with propeller efficiency.

If there was an absence of slip as the propeller circulates through the water, the boat will, theoretically, move forward at a distance that matches the propeller pitch.

A propeller pitch can be understood by considering the action of a screw. The pitch indicates the distance that each rotation contributes to the extent to which the boat moves forward on each rotation. For instance, if the propeller moves forward 15 inches every time it completes a full turn, the nominal propeller pitch is 15 inches.

The propeller revolution rate is determined by dividing the engine rpm by the gear ratio. The propeller rpm multiplied by the propeller pitch indicates the distance forward a boat will move forward every minute.

However, as a result of propeller slip, there is a difference between actual and theoretical speed. Specifically, the actual speed is typically 10-20% slower than the propeller than the theoretical speed when the boat is traveling at its top speed.

The slip can be much higher at lower speeds, often above 50%. Typically, the slip reduces as the speed increases, generally diminishing to under 10%.

'Gear ratio' defines the number of drive shaft revolutions per propeller revolution. As such, if an engine has a gear ratio of 2:1, the engine drive shaft will turn twice for every propeller revolution. It is common for 200-HP engines to have a ratio of 1.86:1. Engines that have a smaller horsepower will have a higher ratio in the region of 2.33:1.

Calculator Equations

The following equations are used within this calculator:

V = [RPM × PP × (1 − (PS/100))] / [GR × C]

RPM = [V × GR × C] / [PP × (1 − (PS/100))]

GR = [RPM × PP × (1 − (PS/100))] / [V × C]

PP = [V × GR × C] / [RPM × (1 − (PS/100))]

PS = [1 − (V × GR × C) / (RPM × PP)] × 100

V is the speed at which the boat is traveling,

RPM is the crankcase speed (rpm),

GR is the number of revolutions the crankshaft needs to produce one revolution of the prop shaft,

PP is the blade pitch of propeller (inches),

PS is the index of propeller performance (as a percentage),

C is the constant to convert inches-per-minute of revolution to boat speed V ;

V (in mph), C = 1056 ;

V (in knots), C = 1215.2 ;

V (in kph), C = 656 .

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Prop-It-Now

Learn about our Inboard Propeller brands below and use the form to the right to get help finding the right inboard propeller for your boat.

• Prop It Now Calculator

Calculate Inboard Propeller Size

Engine Horsepower: *

Engine RPM: *

Gear Reduction: *

Vessel Speed: *

No. Blades: *

Blade Area (EAR):

Desired Diameter:

Output Data

Optimum Diameter:

Efficiency:

Suggested Cup:

Contact Information

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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 owner’s manual and can also be found online.

• 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.  Most boats have between 5% and 25% prop slip and some slip is even necessary to thrust your boat forward.  An easy way to estimate your prop slip is with my prop slip calculator .

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 (WOT) 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 (until the maximum RPM rating is reached).

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 engine’s rev-limiter (to protect the engine).   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.

Of course, most boaters will want something that is in between – good hole shot with acceptable top end speed.  Just be sure that whatever size prop you use your engine operates within its recommended RPM range.

If you liked our prop calculator, here are some other tools you might find useful:

• Prop Slip Calculator
• Fuel Consumption Calculator 
• Total Cost of Boat Ownership Calculator
• New Boat Price Estimator
• Boat Loan Calculator

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Propeller Shaft Size Calculator

Start » Propeller Shaft Size Calculator

This free propeller shaft size calculator helps you determine the proper propeller shaft diameter for your boat. The calculator determines a safety factor (design coefficient) based on shaft diameter , max engine RPM , shaft horsepower , gear ratio , and torsion strength of the shaft material used. The calculator utilizes the following industry accepted formula endorsed by shaft manufacturers, classification societies, and industry governing bodies:

SF = Safety Factor (Design Coefficient)

D = Shaft Diameter (this unit is squared [^3] in formula

St = Shaft Strength, Torsional Yield Shear (psi) Note : see note below

N = Max Shaft RPM

321,000 = Formula Constant

HP = Engine Horsepower

Interpreting Shaft Safety Factor

This calculator computes an objective safety factor (design coefficient) based on user provided data. So what is a propeller shaft safety factor?

Think of a propeller shaft safety factor as a protection against risk. The higher the safety factor, the more protected your boat is from a catastrophic propeller shaft failure.

In perfect conditions with limited use, a safety factor of 1 can feasibly transfer power from the engine, through the reduction gear, to the propeller. However, boat’s are not designed for perfect conditions and rarely are they intended for limited use. Commercial vessels, offshore fishing boats, racing boats, and vessels operating in extreme environments should have an appropriate safety factor based on the projected wear and tear on the boat’s running gear.

As is the case, the American Boat & Yacht Council recommends a safety factor “ approaching five ” for inboard diesel powered pleasure boats. For “heavy commercial and racing boats”, Dave Gerr (author of Propeller Handbook ) recommends a safety factor between 5 and 8.

For more information regarding propeller shaft safety factors or to order a propeller shaft give us a call at (207) 422-6532 or send us an email through our contact page .

What is Torsional Yield Strength?

While most of the data required for the propeller shaft size calculator are readily available, the one variable least understood is torsional yield strength . For the purposes of this calculator, torsional yield strength refers to the amount of force in pounds per square inch (PSI) that it takes to “twist” the shaft by 0.2%. This 0.2% yield strength is a common measurement when discussing material strength and it has been adopted by the marine industry.

At R.E. Thomas Marine Hardware, we use Western Branch Metal Aqualoy 22 and Aqualoy 22 HS (high strength) exclusively. The minimum torsional yield of these values are published on Aqualoy’s website and are defined as per ASTM A-276, ASTM A-479, and AMS 5764 specifications.

The default value in the calculator is 70,000 psi which is the standard minimum yield strength of Aqualoy 22 shaft diameters between 1 1/4″ to 2″. For diameters over 2″ to 2 1/2″ the standard minimum yield strength is 63,300 psi . Diameters over 2 1/2″ to 3″ have a standard minimum yield strength of 50,000 psi . For Aqualoy diameters over 3″ the minimum torsional yield strength is 36,600 psi .

No matter what alloy you’re using (Aqualoy 22, Aqualoy 19, Aqualoy 17, Aquamet 22, Aquamet 19, Aquamet 17, 316 stainless, or bronze), be sure to familiarize yourself with the supplier’s reported 0.2% torsional yield strength to help ensure the accuracy of your calculations.

Gear Ratio Effect on Shaft Safety Factor:

Another important variable in our propeller shaft safety factor calculator is shaft RPM. This calculator factors shaft RPM by considering the engine’s max RPM and dividing it by the gear ratio. For example, if the gear ratio was 1.5 to 1 (often presented as 1.5:1) the engine needs to turn 1.5 revolutions for the propeller shaft to make one turn. In other words, if an engine had an max RPM of 3000, the propeller shaft would theoretically have a max RPM of 2000 (3000/1.5).

This is where things get tricky and counter-intuitive for some people; the higher the gear reduction ratio, the lower the propeller shaft RPMs will be, and the LOWER the shaft safety factor will be. One way to think about it is pulling a vehicle out of a ditch. If you hook a chain from your vehicle to the vehicle in the ditch and try to pull it out in 3rd gear very little force is applied to the chain. However, if you put your vehicle in 4-low (reducing the engine gearing down), there is a significant amount more force pulling on the chain.

If the boat’s gear ratio is 1.5 to 1, just put “1.5” in the gear reduction ratio in the calculator. If the ratio is 2.07 to 1, use “2.07”, and so on…

How to Order a New Propeller Shaft

R.E. Thomas Marine Hardware is proud to be an OEM supplier of Aqualoy 22 propeller shafts to many of the world’s finest boat builders. Call us today for a quote or for more information about our complete line of Marine Hardware. While you’re there, check out our Piranha Dual Line Cutters and our heavy duty Self-aligning shaft seals .

Disclaimer : The information on this page (including the calculator) is provided for general information only. For more specific information regarding shaft diameters, shaft safety factors, shaft alloys, and other related inquiries, please contact us directly.

5 thoughts on “Propeller Shaft Size Calculator”

Does length of shaft make a difference? I have a Yamaha 30 sailboat with a 22mm (slightly bigger than 7/8″ ) with a length of approximately 11′ bronze shaft ! The 12Hp diesel engine is in the V-Berth. Want to upgrade the engine to either 16hp or 25hp and wonder if I can stay with 22mm shaft again.

the engine is 12hp, reduction is 2:1 (prop turning approx half the speed of the engine; max engine speed is 3500; shaft is 22mm; shaft lenght is 11 feet.

We are Planning to buy propellar , but we dont have dimensions , we have only the tensile strength of 45 kgf/sqmm, how can i calculate ?

We have a crew boat with these specifications

Vessel Data

Hull Type – semi-planning/plan hull shape – Steel Builders

Boat Weight 60 ton

Waterline L 17 meters

Waterline beam 4.8 meters

Depth 1.8 Meters

Engine Data

Engine Power MTU 12V 2000 – 2X 916 KW @ 2200 rpm

ZF2500 V Gear Ratio 2.029

Propeller Data

Prop Diameter 920 mm

Pitch 1459 mm

Speed 9 Kts

We have doubts about the engine being overloaded due to the propeller was changed a long time before with the wrong specification.

According to our calculation, the propeller should be as follows:

Prop Diameter 1040 mm/ 41 inch

Pitch 869 mm/ 35.1

Estimated speed 26 Kts

Please, your kind cooperation is required to send to us the original propulsion system for the given engines and gearbox to confirm about the current propeller dimensions.

Greetings! Unfortunately, this calculator is only for calculating the “safety factor” of the propeller shaft itself. It is not designed to calculate the whether or not the engine itself is under/over propped. Thank you and good luck with your research. – RE Thomas Team

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Sailing Boat Propeller Calculator

Sailing boat propellers vary in type and size, with common options including fixed pitch, feathering, folding, 2-blade, 3-blade, 4-blade, and 5-blade props. Materials like aluminum, stainless steel, and bronze are used. Choosing the right propeller depends on your sailboat’s design, engine, and intended use, with considerations for thrust, maneuverability, and sailing performance.

However, I can provide you with a simplified table that outlines some common types of sailboat propellers based on their characteristics:

How do you size a propeller for a sailboat? Sizing a sailboat propeller involves considering the boat’s hull shape, engine specifications, and intended use. A rough estimation would be based on engine horsepower, boat displacement, and vessel type.

Is a 4 blade prop better than 3? A 4-blade prop can provide better grip and maneuverability, especially at lower speeds, but it may have slightly more drag at high speeds. The difference in speed may not be significant.

How do I know what size propeller to buy? Consult your boat’s manufacturer or a propeller specialist who can consider your boat’s specifications and intended use to recommend the right size.

Is a 19 or 21 pitch prop faster? A 21-pitch propeller is typically designed for higher speeds, so it may offer more top-end speed compared to a 19-pitch prop.

Will a bigger diameter prop make my boat faster? Not necessarily. A larger diameter propeller may provide more thrust but could increase drag, potentially affecting speed. Balancing diameter and pitch is crucial.

How do I know what prop will fit my boat? Your boat’s specifications, including engine type and horsepower, will determine the compatible propeller size. Consult a propeller expert for precise recommendations.

What is the disadvantage of a 4 blade prop? 4-blade props may have slightly more drag at high speeds, potentially reducing top-end speed. They can also be more expensive than 3-blade props.

How much speed do you lose with a 4 blade prop? The speed loss with a 4-blade prop compared to a 3-blade prop is typically minimal, often within a few knots. The exact loss would depend on various factors.

Should I go down a pitch for a 4 blade prop? Sometimes, yes. To maintain similar performance, you may need to decrease pitch when switching from a 3-blade to a 4-blade prop.

What happens if a prop is too small? A prop that’s too small may not provide enough thrust, leading to poor acceleration and reduced top speed. It can also overload the engine.

What is the best propeller for speed on a boat? The best propeller for speed depends on your boat’s characteristics and intended use. Consult with a propeller expert to find the optimal prop for your specific situation.

What prop makes a boat go faster? A propeller with the right combination of pitch, diameter, and blade count, suited to your boat and engine, can help maximize speed.

What happens if you put the wrong prop on a boat? Using the wrong propeller can lead to reduced performance, poor fuel efficiency, and potential engine damage due to overloading or overheating.

What is the most efficient propeller pitch? The most efficient pitch depends on the boat’s design and purpose. It’s often a compromise between acceleration and top speed.

What propeller pitch is best for takeoff? A lower pitch is generally better for takeoff and acceleration because it allows the engine to reach its ideal RPM quickly.

What is the most efficient propeller shape? Propeller design is complex, and efficiency depends on various factors. Modern propellers are designed for specific applications, and efficiency varies accordingly.

How do I choose a propeller pitch? Choose pitch based on your boat’s weight, engine power, and desired performance. Lower pitch for better acceleration, higher pitch for top speed.

Can a prop be too big? Yes, a prop can be too big for a boat’s engine, causing excessive load, poor performance, and potential damage.

What is the best shape for a boat propeller? Modern propellers come in various shapes optimized for specific applications. Consult with a propeller specialist to determine the best shape for your needs.

What is the best angle for a boat propeller? The blade angle is designed into the propeller and optimized for its intended use. There’s no universal “best” angle, as it depends on the prop’s design and purpose.

How far should a boat prop be in the water? The prop should be fully submerged, typically below the boat’s hull, to operate efficiently.

Which is better 3 or 4 blade prop for speed? Both 3 and 4-blade props can be suitable for speed, but it depends on your boat and engine. A 3-blade may be slightly faster, but a 4-blade can offer better low-speed control.

Are aluminum props better than steel? Aluminum props are lighter and less expensive, but steel props can be more durable. The choice depends on your budget and priorities.

How many blades on a propeller is best? The number of blades depends on your boat’s design and intended use. 3 and 4-blade props are common choices, with 4-blades offering better low-speed control.

What are 4 blade props good for? 4-blade props are good for boats that require better low-speed maneuverability and control, such as larger vessels or those with heavy loads.

Does a smaller prop make you go faster? Not necessarily. A smaller prop may increase engine RPM, but it could also reduce efficiency and speed if not properly matched to the boat and engine.

Does a 4 blade prop help with hole shot? Yes, 4-blade props can improve hole shot performance by providing more initial thrust and better low-speed control.

Does a shallow pitch propeller perform better? Shallow pitch propellers are designed for high RPM and top speed. They may not perform well at low speeds or provide good acceleration.

Is a 4 blade prop better than 2? A 4-blade prop can offer better control and grip than a 2-blade prop, especially at low speeds, but it may have slightly more drag.

Is a 5 blade prop better than 4? The choice between a 5-blade and 4-blade prop depends on your boat’s specific needs. A 5-blade prop may provide even better low-speed control.

How do I know if my prop is worn out? Inspect the propeller for dings, cracks, or missing pieces. Measure the blade length to ensure they are uniform. If in doubt, consult a propeller specialist.

Should a prop nut be tight? The prop nut should be properly torqued to manufacturer specifications to ensure it stays securely in place.

How much slip should a prop have? Typical prop slip ranges from 10% to 30%. Lower slip indicates better propeller efficiency.

Will a lower pitch prop make my boat faster? A lower pitch prop can increase engine RPM and acceleration but may not necessarily result in higher top speed. The right balance is crucial.

Do stainless steel props make a difference? Stainless steel props are durable and can provide better performance compared to aluminum, but they are also more expensive.

How can I increase my sailboat speed? To increase sailboat speed, focus on sail trim, hull maintenance, and optimizing your boat’s weight distribution. Upgrading your sail shape and size can also help.

How do I get more speed out of my boat? To get more speed from your boat, ensure it’s well-maintained, reduce excess weight, and consider engine and propeller upgrades if appropriate.

How do I get my boat to plane faster? To plane faster, ensure your boat is properly trimmed, use trim tabs if available, and consider a propeller optimized for planing.

Is it OK to run a boat with a bent prop? Running a boat with a bent prop can lead to reduced performance, vibration, and potential damage to the prop shaft and engine. It’s best to repair or replace the prop.

How far below the hull should the prop be? The prop should be fully submerged below the boat’s hull for optimal performance.

Does a boat prop have to be balanced? Yes, a balanced propeller is essential for smooth operation and to prevent vibrations that can damage the engine or drive system.

What is feathering in propeller? Feathering refers to the ability of some propellers to change their blade pitch to reduce drag and improve efficiency when sailing or under power.

What is the best pitch for acceleration on a prop? A lower pitch is typically better for acceleration as it allows the engine to reach higher RPM quickly.

Which propeller gives both good climb and good cruise performance? A propeller designed for a compromise between climb (acceleration) and cruise (top speed) performance is ideal for versatile use.

What propeller pitch is for speed? Higher pitch propellers are designed for speed, but the specific pitch required depends on your boat’s characteristics.

What is the benefit of a large pitch propeller? A large pitch propeller can provide higher top speed, but it may sacrifice low-speed acceleration and maneuverability.

When should I change my propeller pitch? Changing the pitch may be necessary when altering your boat’s weight, engine, or intended use to maintain optimal performance.

How can I increase my propeller efficiency? To increase propeller efficiency, ensure it’s properly sized and matched to your boat and engine, and keep it in good condition.

Are larger propellers more efficient? Larger propellers can be more efficient if they are properly matched to the boat and engine, but size alone does not guarantee efficiency.

Why are constant speed propellers better? Constant-speed propellers can automatically adjust their pitch to maintain a consistent RPM, optimizing efficiency for various conditions. They are common in aviation but less so in boating.

GEG Calculators is a comprehensive online platform that offers a wide range of calculators to cater to various needs. With over 300 calculators covering finance, health, science, mathematics, and more, GEG Calculators provides users with accurate and convenient tools for everyday calculations. The website’s user-friendly interface ensures easy navigation and accessibility, making it suitable for people from all walks of life. Whether it’s financial planning, health assessments, or educational purposes, GEG Calculators has a calculator to suit every requirement. With its reliable and up-to-date calculations, GEG Calculators has become a go-to resource for individuals, professionals, and students seeking quick and precise results for their calculations.

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Marine Propeller Calculator

• Propeller Calculator

The VEEM Propeller Calculator is the only calculator in the market that can give you a size and price estimate on the spot for your vessel. Input your vessel information below and you will be given the optimal propeller diameter for a given engine and gear ratio selection for any vessel over 26 ft.

The calculator is based on over 50 years of extensive test data and research and delivers industry-leading accuracy every time. Help parameters are given when you click in the input box.

Please note that validity of this quoted propeller size is subject to a review by a VEEM Naval Architect upon order. If you are unsure of any field please contact the VEEM Propeller team at [email protected] .

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Enter any 4 Variables and it Calculates the 5th For Additional Info Read Instructions Below the Propeller Calculator Copyright 1997 polson enterprises. Propeller Handbook : The Complete Reference for Choosing, Installing, and Understanding Boat Propellers Visit our outboard propeller manufacturers and propeller industry market research page, for a list of propeller manufacturers and information on the boat propeller market., mercury has an online everything you ever need to know about propellers book that does a nice job covering propellers in general., other prop calculators on the internet, other "prop calculators" are online. give them a try also. all of us are a little different and one or the other might fit your specific needs best. obviously we are partial to ours, it is the only one that can calculate any of the 5 variables without having to set a selector switch of some type. boat performance analyzer from boatramp.com prop calculator" from continuous wave . they recognize our calculator as the inspiration for theirs near the bottom of the page. surfbaud freeware propeller calculator is an excel spreadsheet program, sizing propellers, prop slip is a measure of how efficiently the boat is going through the water. prop slip is not the same thing as propeller efficiency. if the prop did not slip at all as it screws through the water, each rev theoretically propels the boat the forward a distance equal to the prop pitch. (a 15 inch pitch prop theoretically propels the boat 15 inches in one revolution). the propeller revolution rate is the engine rpm divided by the gear ratio. this propeller rpm times the prop pitch determines the theoretical distance that the boat should have moved in one minute, which can be converted to a theoretical boat speed in miles per hour. due to prop slip the boat is actually going slower than the theoretical speed. normally on the nature of 10 to 20% slower at top speed. this ratio is found by dividing the actual boat speed by the theoretical boat speed to get a ratio. then, this ratio is subtracted from 1. the result is the propeller slip. in real life, you also have to do a number of unit conversions between hours, minutes, miles, feet, inches, etc. all of these calculations and conversions are done for you internally in the rbbi prop calculator. see our prop folder for additional information about propellers.. Shopping cart 0 You have no items in your shopping cart. Check Order Status Inboard Boat Props Outboard Boat Props Sterndrive Props PWC Impellers Ski/Wake Props Authentic Volvo Duo Props Propeller Pullers Propeller Guides Brass cutless bearings Non metallic bearings Duramax flanged bearing Stave bearings Shaft Zinc Anodes Rudder Zinc Anodes Plate Zinc Anodes Shaft Nut Zincs Stainless Prop Shafts Shaft Seals Marine Shaft Logs Shaft Packing Boxes Shaft Accessories Deck Hardware Drivetrain Hardware Marine Exhaust Fittings Hose and Clamps Steering Hardware Marine Valves Plumbing Fittings Prop Shafting Accessories Marine Strainers Miscellaneous Hardware Boat Props / Propeller Guide: Utilize our best in class boat propeller guide to find the correct propeller for your Outboard, Ski/Wake, or Stern Drive marine engine. Inboard Sizing: Choosing the right propeller for your boat requires several pieces of information to allow us to give you the best suggested propeller size for your boat and engine combination. By providing us with this information, we can complete a propeller analysis for you.  Deep Blue Yacht Supply has completes thousands of free inboard boat propeller sizings each year. Submit your inboard boat prop sizing request today. Propeller Cases Cutlass Bearings Shafts & Seals Boat Hardware 1-800-454-6960 1-321-978-0253 1-954-444-6259 [email protected] DeepBlueYacht Contact Us With Questions Shipping and Return Policy Privacy Policy Conditions of Use Company Overview Skip to primary navigation Skip to main content Skip to primary sidebar Skip to footer Yacht Cruising Lifestyle Everything fun you can do from your yacht How to Choose the Right Sailboat Propeller November 10, 2021 by Martin Parker 1 Comment Introduction Choosing the right sailboat propeller for your needs is a complex process. Make the wrong decision, and you could end up with an underpowered boat or a constantly over-revving engine. Get the right propeller, though, and your engine will run more efficiently and with less vibration, ultimately allowing for more comfortable cruising. By understanding propeller basics and seeking the advice of professionals, you can easily find the right propeller for your sailboat. Below, we explain the basics so that you know what to look for when buying your next sailboat propeller. We’ll also take a deep dive into the choices of propellers available for purchase. Propeller Basics Before we jump into the more technical details of propellers, it’s essential that you know the basic parts of a sailboat propeller and what they do. Understanding the basics will help you choose the right propeller and give you some knowledge when speaking to the experts. The hub serves two purposes. First, it connects the propeller to the driveshaft, allowing the engine to turn the propeller and drive the boat forward. Second, the blades are attached to the hub, either as a one-piece casting or using some mechanical means. There are three terms relating to the blades. Root: The point where the blade attaches to the hub. Leading-Edge: When rotating, the blade edge cutting through the water is the leading edge, while the opposite side of the blade is the trailing edge. Tip: The furthest end of the blade from the hub. Basic Dimensions Two measurements define the basic propeller dimensions, the diameter and the pitch, and propellers are described in terms of their pitch and diameter. The size of the circle made by a rotating propeller. Imagine attaching a marker to the end of a blade and then spinning the propeller one full rotation, with the marker drawing a circle on a piece of card. Measure the distance across the circle, and that is the propeller’s diameter. The distance the propeller would move forward during a single rotation. Think of a nut and bolt. The pitch of the bolt’s thread is equivalent to that of the propeller. When you rotate the nut through a single rotation, it moves forward a set distance. The distance moved is the pitch, measured in inches. Sailboat Propeller Sizing Both diameter and pitch are measured in inches, and the values describe the prop size. A 14/22 prop has a diameter of 14 inches and a pitch of 22 inches. While these dimensions will give you a reasonable propeller size for your boat, other factors also affect the correct values for your propeller. Your Propeller Choices When you are considering a new propeller, you have a significant number of choices to make. Some may be forced on you by budget constraints. Others you may deem essential regardless of cost. Constant or Variable Pitch The pitch is the blade’s angle of attack through the water, and blades either have constant or variable pitch. A constant pitch blade has the same pitch from root to tip, whereas the pitch of a variable pitch blade reduces towards the tip. When you look at it, a variable pitch blade twists from the hub to the tip. If your budget is tight, you may opt for a constant pitch prop, as they are cheaper to manufacture. However, a variable pitch prop offers better efficiency and is generally the better choice. To understand why variable pitch is more efficient, we need to look at propeller speed. As your propeller spins at a constant engine rpm, the hub turns much more slowly than the tip of the blade. Increasing propeller speed equals increased thrust, so to maintain constant thrust along the blade, the pitch reduces towards the tip. Fixed or Controllable Pitch Fixed propellers all have twisted blades giving a variable pitch along the blade, but the pitch cannot be adjusted as the name suggests. This makes them cheaper to produce than variable pitch props. A variable pitch prop allows the blades to be rotated at the root, allowing you to set the correct pitch for varying conditions. Because of the increased cost and complexity, variable pitch blades are rarely seen on cruising yachts and are primarily used on superyachts. There are several advantages that you may consider worth the extra cost; – Your engine rpm can be set at the most economical speed, with thrust controlled by varying the pitch. – You may be able to reverse the pitch of your propeller, which is very useful when maneuvering. This eliminates the need to stop the propeller from spinning before changing its direction. – A variable-pitch propeller can be fine-tuned to get the best efficiency, power, and speed from your propulsion system, something you cannot do with a fixed pitch prop. – Variable pitch propellers can often be feathered. The reduced drag and therefore reduced fuel consumption offered by a feathered prop may go some way to offsetting the initial cost. Feathering Propellers A feathering propeller is a direct replacement for a fixed propeller but with the advantage of less drag when you’re sailing. Once under sail, the prop automatically aligns the blades with the direction of travel, removing the drag caused by a fixed prop. The only disadvantage is that the blades are flat with a constant pitch. This type of propeller is a cheaper alternative to a controllable pitch prop, although with fewer features. Folding Propellers If you’re looking for more sailing efficiency, a folding propeller is a common choice. It’s a straightforward design, using the force of the water over the blades to close them flat when sailing. The centrifugal force opens the blades when the prop is turning. There are some downsides to increased sailing efficiency, though; – They are only available as fixed pitch propellers. – Older designs don’t always open with low engine rpm or may only partially open. You may need to rev the engine to open the blades. – The water pressure may not always be sufficient to push the blades back at low sailing speeds, negating any potential advantage. – Very poor reverse thrust. Number of Blades Older designs were only available with two blades, but nowadays, three and four-blade versions are available. Many of the issues listed above have been addressed with the more modern designs, such as gearing to ensure all the blades open together. Saildrives are an interesting development with propellers, providing a more compact propulsion system for sailing yachts. Many of the same choices for conventional propeller drive systems apply to saildrives, and you can use all the different propeller types mentioned above. The primary difference is that the saildrive setup moves the propeller almost directly under the engine, which means the propeller’s angle of attack is parallel to the water’s surface. Compared to a conventional engine and driveshaft system, these factors will influence the pitch, diameter, and type of propeller you choose. Sailboat Propeller Issues Most propellers rotate clockwise when moving forward. Prop walk is the sideways force generated by the spinning propeller, but it’s barely noticeable moving forward. Going astern is a different prospect, and you can usually feel a distinct movement of the stern to port. You can counter the effect with the rudder, but you can also use it to your advantage when maneuvering. Propeller design can also contribute to the amount of prop walk you experience. Cavitation happens when the prop becomes overloaded, and a vacuum forms around the blades. The effect of water rushing into the vacuum creates tiny bubbles, causing a loss of thrust, vibration, and noise. Excessive cavitation can even damage the propeller. Your propeller’s design and the shape of the boat hull can significantly impact how susceptible the propeller is to cavitation, so it is essential to get it right. Ventilation Often confused with cavitation, ventilation is the result of the propeller drawing in air. There may be a design fault when propellers sit too close to the surface, but problems can also arise when the propeller spins too fast. Factors Affecting Your Choice Of Sailboat Propeller Engine power It’s crucial to choose the right size prop for your engine. If your propeller is too large or over-propped, then the engine will struggle to reach maximum rpm. When a propeller is too small or under-propped, your engine will rev out too quickly. When looking at prop size, you must also factor in any gearing in the drivetrain. The gearbox supplies the power output from the engine, so it’s the output shaft torque and power figures that are important. Hull Resistance The wet surface of your hull produces resistance, but of course, you also need to factor in any drag caused by extra protrusions on your hull. This will include the rudder, driveshaft brackets, speed sensors, depth sounder, etc.  Choosing The Right Propeller For You. Many factors affect your choice of a propeller, and no one solution is better than the other. The first consideration is your motoring requirements, followed by your budget restraints. If your budget is tight and you only occasionally use the motor, a fixed blade prop is probably your best choice. Alternatively, a folding or feathering prop could be the best choice if you want to maximize your sailing speed, reducing drag. When maneuverability is the priority, a feathering prop might be the right choice. While the options above relate to specific uses, the main characteristics of a propeller are pitch and diameter. These define how the prop will perform and should be the starting point for any propeller choice. Once you determine these basic choices, you can consider more technical factors. FAQ’s Is A Bigger Sailboat Propeller Better Than A Small Propeller? Is bigger better? No, not always, and in the case of propellers, going bigger could cause you all kinds of problems. In terms of size, we’re talking about the diameter of your propeller. A larger diameter propeller will give you more blade area and give you more thrust – in theory. However, if your propeller is too big, the engine will struggle to turn it and never let it get to its optimum rev range. An oversize propeller will restrict your boat’s performance and increase your overall fuel consumption. Should You Choose The Highest Pitch Propeller For More Speed? This is a similar consideration to that of the size (diameter) of your propeller. Increasing the pitch of your propeller blades should, in theory, give you more thrust. As with the diameter, though, too much pitch will limit the performance. Does A Bigger Propeller Mean Your Boat Will Go Faster? No, and it may make your boat slower. As we discussed throughout this article, choosing the correct propeller involves numerous factors, and it’s not a simple question of size. What Factors Should You Consider When Choosing a Propeller? The first and most important factor is how you sail your boat and how often you use your engine. If you tend to cruise gently, using your engine only when there are light breezes, then low vibration and low cost are possible priorities. Anyone who enjoys racing will look for speed and low drag, so a folding or feathering prop will be a priority. Need more advice on buying a sailboat propeller? Get a conversation started on our forum  by leaving a question or comment! If you found this article helpful, please leave a comment below, share it on social media, and subscribe to our email list., for direct questions and comments, shoot me an email at [email protected]. Sharing is caring! Reader Interactions July 28, 2022 at 1:24 am First of all, your first prop size numbers are reversed. In almost all sailboat application propellers the diameter number is larger than the pitch number. 22D x 14P , for example is more along the ratio of diameter to pitch, for the reason of motor H.P. in relation to Tons of boat which you need to propel. The only applications, which are few, where you would run a prop with a higher pitch number than Diameter is when your transmission ratio is aprox. 2 or 2.5 -1, thus a slower turning prop shaft.. l am a prop shop operator in Cancun.. Thank You.. Leave a Reply Cancel reply Your email address will not be published. Required fields are marked * Save my name, email, and website in this browser for the next time I comment. MB #20512 PO BOX 480 Sevenoaks Kent TN13 9JY Tel: +44 56 0386 9163 Keep In Touch Thank you for reading. Join our online crew and find more about the #boatlife \n") msg.document.write("\n") makeChartBar(msg, boatName[i], boatName[j], "LOA", LOAMax, LOA[i], LOA[j]) makeChartBar(msg, boatName[i], boatName[j], "LWL", LWLMax, LWL[i], LWL[j]) makeChartBar(msg, boatName[i], boatName[j], "Beam", beamMax, beam[i], beam[j]) makeChartBar(msg, boatName[i], boatName[j], "Displacement", displacementMax, displacement[i], displacement[j]) makeChartBar(msg, boatName[i], boatName[j], "Sail Area", sailAreaMax, sailArea[i], sailArea[j]) makeChartBar(msg, boatName[i], boatName[j], "Capsize Ratio", capsizeRatioMax, Math.round(capsizeRatio[i]*100.)/100., Math.round(capsizeRatio[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Hull Speed", speedMax, Math.round(speed[i]*100.)/100., Math.round(speed[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Sail Area to Displacement", sailAreaToDisplacementMax, Math.round(sailAreaToDisplacement[i]*100.)/100., Math.round(sailAreaToDisplacement[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Displacement to LWL", displacementToLWLMax, Math.round(displacementToLWL[i]), Math.round(displacementToLWL[j])) makeChartBar(msg, boatName[i], boatName[j], "LWL to Beam", LWLToBeamMax, Math.round(LWLToBeam[i]*100.)/100., Math.round(LWLToBeam[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Motion Comfort", motionComfortMax, Math.round(motionComfort[i]*100.)/100., Math.round(motionComfort[j]*100.)/100.) makeChartBar(msg, boatName[i], boatName[j], "Pounds/Inch", PPIMax, Math.round(PPI[i]), Math.round(PPI[j])) msg.document.write(" Report any problems to . will continue to host Carl's Sail Calculator on his Web site; please direct correspondence to him. Carl's Sail Calculator v3.55 . For multihulls, try this site This page works with all standard browsers on Mac OSX, Windows 7 or later, and Linux. It does not render properly on Apple iPads and iPhones running iOS 10. This is an OS problem beyond my control. Some data were moved and recalculated from earlier versions. If you find any basic measurements that you know to be incorrect for any of the boats please send the corrections to Tom . : When you select a boat, its parameters appear below in .

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Inboard Propeller Sizings & Prop Guide for Outboards, Sterndrives, and Ski/Wake engines.

Prop Guide: Use our revoluntionary boat propeller guide to find the perfect boat prop for your outboard, stern drive, or ski/wake marine engine.

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Boat Propeller Calculator

Boat Propeller Calculator: Looking for a free tool that calculates the boat propeller easily? Then use our free online tool that calculates the five variables easily and accurately. Just by giving inputs, you will get the output. And also we are providing steps to calculate the boat propeller manually. Read on to know the definitions, formulas.

How to Find the Boat Propeller?

Propeller slip delivers an indication of the efficiency with which a ship is traversing through the water. However, it is not to be confused with propeller efficiency.

A propeller slope can be understood by viewing the action of a screw. The pitch indicates the length that each rotation contributes to the extent to which the boat moves along on each rotation.

The gear ratio defines the number of drive shaft revolutions per propeller revolution. As such, if an engine has a gear ratio of 2:1, the machine drive shaft will turn twice for every propeller circle.

V = [RPM x PP x (1 − (PS/100))] / [GR x C]

PP = [V x GR x C] / [RPM x (1 − (PS/100))]

RPM = [V x GR x C] / [PP x (1 − (PS/100))]

PS = [1 − (V x GR x C) / (RPM x PP)] x 100

GR = [RPM x PP x (1 − (PS/100))] / [V x C]

V = speed 

RPM = crankcase speed (rpm).

GR = number of revolutions the crankshaft needs to produce one revolution of the prop shaft.

PP = blade pitch (inches).

PS = index (as a percentage).

C = constant to convert inches-per-minute of revolution to boat speed V.

V (in mph), C = 1056.

V (in knots), C = 1215.2.

V (in kph), C = 656.

Follow the guidelines that are given below to calculate the boat propeller calculator easily.

• Take the inputs from the given problem and look into the problem what to find.
• Apply the formula which you need and substitute the values in it.
• Simplify the answer and you will get the answer easily.

Arithmeticcalculator.com is the best website that offers all math calculators that gives you instant results along with the concepts easily.

FAQs on Boat Propeller Calculator

1. How to use this boat propeller calculator?

Simply give the inputs in the input fields and click on the calculate button that gives you the answer effortlessly.

2. What is the best tool to calculate boat propeller?

The boat propeller calculator is the best online tool that calculates the boat propeller.

3. What is the best website that offers a boat propeller calculator?

Arithmeticcalculator.com is the best website that offers a boat propeller calculator.