What Diameter Should You Choose
The size of the propeller is determined by two numbers: the diameter and the pitchthe number of the diameter is listed first. This is the diameter of an imaginary circle passing through the end of all the blades. It can be calculated by multiplying the distance from the center of the hub to the tip of any blade by two.
The larger the diameter, the more water the propeller will push, which will create more power. But a large diameter propeller also puts more strain on the engine. The choice of diameter depends on the space available under the anti-cavitation plate.
Props For Sailboats Trawlers And Other Non
Sailboats and other non-planing boats such as tugboats and trawlers have different propeller needs from planing type outboard and sterndrive boats. The engine on a sailboat has to push a great deal more weight and push it much slower than in normal powerboat applications. To get the full rated power out of the engine, it is necessary to go to a very low pitch propeller, sometimes in a bit larger diameter and/or with more blades.
An example is this Honda outboard with a high thrust 4 bladed prop specifically made for applications where high thrust is needed under a high load and at low speed. With a normal propeller, the engine would not be able to achieve its maximum rated RPM range, so it would be unable to produce full power and would likely be subject to side effects of overloading such as fouled spark plugs.
Inboard diesel sailboat engines are usually run with two or three bladed bronze props of fairly low pitch. The two bladed props are preferred due to lower drag, but some cruisers report that the extra thrust from a three bladed prop will push their boats faster, especially when pounding into waves.
Right Hand Or Left Hand Rotation
A 4 blade propeller will usually have a smaller diameter for the same pitch size of the 3 blade equivalent. This is one reason they spin up quickly and yield good acceleration. The blades are often a bit smaller but offer more total blade area because of the additional blade, so they have more grip on the water. When switching from a 3 blade prop to a 4 blade, youll usually need to decrease the pitch by 1 or 2 inches to keep the engine RPM in the same range.
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What Determines The Type Of Propeller Your Boat Needs
If youre new to boating, you may not have expectations around what type of performance you should get out of your engine/propeller combo. That said, there are numerous variables behind which prop is right for your boat.
Propellers can make such an impact on performance that your neighbor could have the exact same boat and engine, but if the prop is different, the two boats could yield different results.
There are so many fluctuations and options that you could literally make the same exact boat faster just by adding a different propeller.
Choices that affect this change in performance are:
Stainless is a stronger metal than aluminum, so this type of prop is tougher against damage. It also doesnt flex, so it holds its shape in the water and can increase the speed depending on boat and engine type.
Aluminum is a softer and less expensive metal, so these props are usually more budget-friendly. That said, they do tend to flex, which can hinder the boats performance and speed.
Other factors that make a difference in how your propeller performs are:
Choosing a propeller can be subjective depending on your performance goals, but different engines and hull designs do play into selecting the best prop.
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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Propeller Diameter And Pitch
Propellers are sold with a given diameter and pitch, usually expressed as Diameter X Pitch, or D X P. The diameter is usually dictated by engine and gearcase design, with few if any options offered, so changing to a propeller with a different blade pitch is the most common propeller change. A prop with a higher pitch pushes more water in each revolution than one with a lower pitch.
Given that, why wouldn’t we want a very high pitch prop? There are a few reasons. First, the engine has to work harder to get that high pitch prop spinning, meaning the boat will suffer from slower pickup. Second, a propeller with a pitch that is too high for the boat/engine combination will overload the engine, preventing it from rotating within the recommended maximum RPM range at full throttle. Third, high pitch propellers tend to pull the boat sideways during slow speed maneuvers near docks more than their low pitch counterparts.
It is important to conduct a realistic test to properly evaluate potential propellers for your boat. Do not test propellers with the boat empty. Load the boat as you would normally load it when fishing, skiing, or cruising. Don’t test it only in flat calm water. A propeller might perform fine in flat water, but be prone to ventilation when planing in chop. Going to a blade with a higher rake angle or cupped edges can reduce cavitation/ventilation when riding through large waves. More blades can also help reduce ventilation and increase blade bite in rough weather.
How Does Propeller Diameter Factor Into Your Ideal Size
The size of a propellers actual diameter will change the way your RC boat handles the water. Heres what you need to know about diameter sizing:
- The typical standard size for an RC boat propeller is between 65 to 80 mm for a gas-powered boat. If your RC boat is around the average size and weight, this diameter will work well for you.
- Heavier boats need larger propellers. Larger, heavier boats will need more prop to push them through the water. Its just physics! A larger propeller will give more force to push the boat.
- Remember to start small and test propellers out. If you arent sure how large a prop your RC boat can take, dont worry. Test out your boats motors for 20 to 30 seconds with a potential propeller, then wait 5 seconds. Gently touch it. If its too hot to touch, you need a smaller propeller because the motor cant handle it.
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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.
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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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.
To Grip The Water And Push The Boat
The result is the engine RPMs going above the specs and hitting the rev-limiter. Its exactly like what happens when the prop hub gets spun
On the opposite end of the spectrum, if were to install a prop with too big of diameter and pitch, then the engine wont have enough power to spin the propeller up to the right RPM range.
Causing the engine to never reach wide-open throttle and if ran like this for a long period of time, can eventually wear out internal components of the engine prematurely.
Its important to note that there are methods of getting propellers to turn up higher without changing the prop. This is usually done by venting the propeller some props have venting holes towards the bottom of the prop.
These holes can be either plugged or have the plugs taken out allowing water to flow freely through the prop without having to be pushed off the blades of the prop, causing force against it.
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Is A 3 Or 4 Blade Prop Better
If we had a dollar for every customer support inquiry we receive asking whether a 3 or 4 blade prop is better, wed be giving away free boats! Its important to note, we are talking specifically about the average recreational boating and fishing application here.
This prop advice is not related to racing. Although the physics dont change, there are a ton of other variables to consider for racing that is not listed here.
How To Select The Correct Propeller For Your Boat
When it comes to optimizing the performance of your boat, there are a few things that will provide immediate benefits: cleaning your boats hull, keeping your engine tuned up, and choosing the right propeller. Choosing the right propeller will allow your engine to operate in the recommended RPM range, but prevent the engine from over-revving. This is where maximum horsepower is produced, with ideal engine loading. A correctly sized prop will also help increase the fuel efficiency of your boat.
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What Is The Benefit Of New Propeller Blade Design
Blade shape has moved on in the last 15 years or so, thanks partly to computer aided design and a better understanding of what advantages a particular blade shape can offer.
The heavy sectioned blade is no longer the preserve of the plodding working boat, it can also be used to advantage on high-speed vessels where performance and economy are of equal importance.
Take a look at the original propeller on my own boat . Not much more than a three-bladed slab of bronze that did its job of pushing the boat through the water.
Three-bladed propeller: a workhorse that served Tony Davies boat well for many years
Now move on ten years and look at the sophisticated four-blade foil design that runs much more quietly, with less vibration and gives increased fuel economy at semi-displacement cruising speed.
A modern four-blade design that is more fuel efficient and more powerful
Aluminum Or Stainless Steel Prop
Another choice to make is choosing between aluminum and stainless-steel materials. Common with new package boats are aluminum props, which are inexpensive and repairable. Consider these differences when thinking of upgrading to stainless steel. Thinner blades and more advanced designs give stainless steel a performance edge over aluminum. Stainless costs more but is five times more durable than aluminum. Stainless props can be repaired, at a higher cost, to like-new condition, while repaired aluminum will suffer from metal fatigue and a loss of strength.
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Aluminum And Composite Plastic Propellers
Most outboard and stern drive engines are sold with aluminum propellers of moderate pitch, suitable for most uses. Smaller engines sometimes have composite plastic props. Aluminum and plastic propellers are lighter in weight than their stainless steel counterparts, but they are also more prone to bending under load, so the blades must be made thicker. Aluminum and plastic props are much less durable than stainless steel ones, and it is easy to damage them by running aground or hitting floating debris. Although they are less expensive, aluminum propellers must be replaced more frequently, so over time the investment in a durable stainless steel prop makes more sense.
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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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 Are The Problems You Are Looking To Solve
What are some of the issues you are facing?
- Is your boat sluggish when coming out of the hole, or are you having issues hitting the kind of speeds you want?
- Is it a fuel consumption issue?
- Do you want better all-around performance?
- Maybe your current prop is ventilating excessively?
As soon as you define the problem, you will be in a much better position to find the perfect solution.
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