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.
Hub
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.
Blades
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.
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.
Pitch
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.
– 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
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
Prop Walk
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
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.
Gearing
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.
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..