Thursday, June 30, 2022

What Size Prop For My Boat

What Problems Are You Looking To Solve

How to Find the Size of a Boat Prop | How to Identify Prop Size | Prop Diameter and Pitch

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.

Are You Under Or Over Revving The Engine

Your boat owners manual will include specs that tell you within which range your boat engine was designed to rev. Itll look something like this: 4200-5000rpm for sterndrive or 50005500rpm for an outboard.

Under revving or over-revving, your engine will damage it faster than you would like. If you find that your boat engine is either under revving or over-revving in contradiction to the specs, the simplest solution is to buy a propeller with the appropriate pitch for your specific boat, which leads us to the next point.

How To Select A Propeller

Propellers are described in numbers: the first number is the diameter, and the second number is the pitch. So, a 14 x 19 propeller has a diameter of fourteen inches, and a pitch of nineteen inches. In conversation, most people would describe this propeller as a 19-inch pitch, or simply a 19.

Choosing a Propeller

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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

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.

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

Brand new boat prop. Size 3 x 14 x 13. Also marked " 23 13 ...

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.

Pitch Points

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When Choosing The Right Propeller For Your Boat Youll Also Want To Consider The Material Used Number Of Blades And Pitch

A lower pitch allows you to build up RPM quickly which is good for big boats with a heavy load.

But there will be less forward travel with each revolution so not good for top speed.

A higher pitch is slower to build up RPM resulting in lower pulling power and acceleration but a greater top speed once the vessel winds up.

Youll also need to know how to read the prop. Meaning how to identify the size, diameter and pitch of a propeller.

Most props have a series of numbers on the side or inside the hub.

A three blade prop, 18.75 inches in diameter with a 19 inch pitch will have the numbers 3 X 18.75 X 19.

Numbers are printed on the side of your prop tell you the number of blades, diameter and pitch of your prop.

If they are not printed on the side of the prop, look for the numbers inside the hub of the propeller to determine diameter and pitch.

Should You Choose A Four

Three or four blades work well in either sterndrive or outboard applications. Three-blade designs give you all-around performance with an advantage on top end speed. Four-blade designs work well with boats that are difficult to get on plane, underpowered or used in watersports where top-end speed is not critical.

The composite cores of modular hubs are designed to break away upon significant prop strikes, helping to protect the prop body and engine drive train from damage.

Four blades in many cases will drop your rpm by 50 to 150rpm with identical pitch. Three-blade props are generally best for recreational boats with three-, four- and six-cylinder outboards and sterndrives, giving good hole shot and top-end performance.

The blades on three-blade props fill up about 50 to 55 percent of the available area inside the circle formed by the props diameter . Adding a fourth blade increases the DAR to between 60 and 65 percent, so you can expect more thrust to keep your boat planing at lower rpm, a potential boost in fuel economy, but also a reduction of 50100rpm at WOT.

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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.

Its Also Important To Note The Details Regarding Trimming Up During This Process

Do You Understand Boat Prop Size, Pitch and Diameter?

Normally you wont have the boat fully loaded when propping it out.

Its best to prop it out towards the top end of the RPM range without trimming the engine. We want to be able to add some trim to the engine and come really close to hitting the rev limiter under these conditions.

This is because whenever we load the boat up and take it out under normal conditions. There will still be room to add some trim at wide-open throttle and have the RPMs come all the way up to the top of the RPM range.

If we have to trim up the engine to get to the top of the range, then after we load the boat. We wont be able to get to the top of the range because of the additional weight that has been added to the boat.

Thats all for now folks! Now that you have a better idea of how to correctly prop your boat for the best results. Remember to share this article with a fellow boater! We also encourage you to check out one of these other helpful articles that we have written!

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How To Find Your Best Propeller

The Prop on the Boat Goes Round and Round

Propellers are the final gear in your boats driveline. Having the correct prop on your boat can make the difference between happy boating and costly floating. Because props come in a huge variety of shapes and sizes it can be a tricky task to identify the one that works best on your boat. Here are the steps that I recommend to both find your propeller and understand how it works.

Information gathering is the first step in finding your best prop. This includes the make, model and year of your boat and engine. Also note the make and size of your present prop . Then consider how you use your boat. Is it for family day trips, long cruises, or tow sports? Finally, record the maximum RPMs that you can achieve with your existing prop at wide open throttle.

What Does Prop Pitch Mean

Propeller pitch typically referred to as prop pitch, is essentially the distance a propeller would move in a single revolution through soft solid. Kind of like the distance a screw would move through wood in one rotation. So, in this case, a 21 pitch prop would move 21 inches forward in one revolution, and a 19 pitch prop would move 19 inches forward in one revolution.

So what does all this mean to your boat?

Think of your propeller as if it were the axle on your car. The lower the ratio, the more pulling power it has from a standstill. This is the same physics that is applied to a boat propeller. A boat with a lower pitch propeller could accelerate much faster from a standstill position than one with a higher pitch propeller. Unfortunately, this lower pitch would make your engine reach maximum rpm at much slower speeds, albeit faster acceleration.

On the other hand, a higher pitch propeller will give you better top speeds but much slower acceleration. It should also be noted that simply fitting a high pitch propeller to a boat with a lower horsepower engine doesnt mean that you will make your boat faster.

While the boat will achieve higher speeds, it will do so for a short while before getting bogged down. The propellers higher pitch and diameter will eventually overwork and wear down the internal engine parts that just arent built to withstand that kind of stress.

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Pros & Cons Of A 4 Or 3 Blade Prop

For us, if we go from a three blade to a four-blade without changing the diameter or pitch of the prop, we will lose top-end speed. What we will gain, however, is our hole shot .

Meaning we will be out of the water and cruising much quicker thanks to the extra blade generating a better grip from the prop to the water.

With the extra grip also comes better control and handling of our craft. Understanding these simple concepts is the crucial base foundation for us to begin the propping procedure to get the correct prop for your boat.

At the end of the day, it is ultimately up to you and what you are looking to get from your boat. Would you rather have a better hole shot or top-end speed?

Its your call! Personally, running larger boats close to the 30-foot range or higher calls for a 4 blade as its best for the size and weight of the boat. That extra blade really makes a notable difference in getting the boat out of the water and controlling it at higher speeds!

What Are Tooth Splines

Brand new boat prop. Size 3 x 14 x 13. Also marked " 23 13 ...

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.

Like our article? Do you have some comments or suggestions? Or want to share this with a friend?!

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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.

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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Tips On Choosing The Right Propeller For Your Boat

Does your boat take a long time to get up on plane? Maybe not hitting the top speed you think you should? Are you looking for better all-around performance and fuel economy? Apart from keeping your boats hull clean and your engine tuned, selecting the right propeller is one of the easiest actions you can take to get the most out of your boat.

First of all, note that the primary criterion for selecting a propeller is to make sure it allows the engine to turn up to a speed within the range specified by the engine manufacturer usually within 500 revolutions of absolute top rpm. This ensures a long life for the engine, neither lugging it down nor letting it over-rev. Your engine probably came with a propeller that achieved that basic requirement, but you can select a variety of props that will allow the engine to turn up yet have differences in pitch and number of blades, plus possess more subtle characteristics. For example:

Propeller Size

Expressed with two numbers, diameter and pitch, with diameter always stated first. Diameter is two times the distance from the center of the hub to the tip of any blade. Smaller prop diameters generally go with smaller engines, or with fast high performing boats. Pitch is the theoretical forward distance, in inches, that a propeller travels during one revolution. Think of pitch as speed, or as the gear selection on a cars transmission.

Pitch

Material

Three or Four Blades

Prop Sizes And Gear Ratios

How to choose right prop for your boat and motor

I see a large number of discussions on bulletin boards and web sites on the relationship between prop sizes and gear ratios. Since everyone seems to have an opinion, I thought I would throw my thoughts in as well.

The weight and length of the boat, the horsepower and torque of the engine, the ratio of the drive and the size of the prop all work together to affect the boats performance. In the discussion below I am ignoring slip , the power losses by the gears themselves , and the hull shape.

A props two dimensions each affect performance in different ways. For every inch of change in prop diameter the RPM varies by about 500 RPMs and every inch of pitch changes the RPM by approximately 150 to 200 RPM. So if you have a prop that measures 14×19 and then go to one that is 15×17 instead, your RPMs will go down by approximately 100 RPMs . The effective top speed wont change much as these two props are about the same, but the increase of diameter will help the engine get up on plane quicker . With dual props however, pitch changes are measured as a function of both props. Therefore, the change is approximately 70% of single props: 280-350 RPMs or 105-140 RPMs respectively per prop.

A Volvo engine with 210 HP creates 368 foot/pound at 2400 RPM. If you had a 1:1 gear the torque at the prop shaft would be 368 foot/pounds at 2400 RPM. If instead there was a 2:1 gear , the prop shaft would turn at 1200 RPM and the torque on the shaft would be 919 foot/pounds.

Sincerely,

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