Whether you, as a boat owner, should choose a 2-blade, a 3-blade or a 4-blade folding propeller is a question that we have been asked a lot here at Flexofold. And regardless of how many times we are asked, and no matter by who, and no matter how insistent the question is, our answer will always be the same.
Basically, it is about your boat’s motor, its gearbox, your boat type, and, last but not least, your personal preferences.
What are the differences between a 2-, a 3- and a 4-blade folding propeller?
The way the various propellers are constructed and how they work is the same. But, in spite of this, there are some significant differences. That is why it is important that you know both your boat engine, its gearbox and the model, when you want to find out which folding propeller is the right one for you (and your boat). It is, in fact, the combination of these things that is determining your choice.
Here at Flexofold, it is a given that we help you with this. You just need to give us the precise information, and then we can make the necessary calculations. In order to find these pieces of information, you might want to find the papers relating to your boat. Some pieces of information, like which gearbox your boat has, can be found on the information plate on the boat itself.
A 2-blade folding propeller provides for the least drag resistance because of the minimal profile that is in contact with the water.
In spite of this, the 3-blade propellers are the most common.
In rough winds as well as strong currents, it is an advantage, as it ensures that the boat maintains its speed, and you will also experience a smoother sailing, as the vibrations are reduced if you have a sailboat with more horsepower.
A 4-blade propeller is a good choice for heavier boats with a lot of horsepower. In this case, more power is required before the boat gains momentum – that applies to rough winds and seas as well. That is because the bigger blade area makes for an efficient pitch.
However, it is important to mention that there is not, per definition, one specific folding propeller with one specific number of blades that is better than the others. Instead, it is the conditions that the propeller is to function under as well as motor type, gearbox and boat type that are determining factors.
Why is it possible to choose between a different number of blades?
In relation to the number of blades, it is especially motor size and gearbox that determine what you should choose.
The bigger your boat’s engine is, the bigger the propeller you theoretically need, as a bigger blade area ensures the necessary propulsion. That means that when you have more boat power which needs to perform well in water, the bigger the blade surface needs to be, as the blades push the water and thereby optimise everything from the centrifugal force to water currents. Maybe it sounds simple, but it is also important to take the gearbox into consideration, as this in particular is determining for the propeller’s diameter and pitch, which need to match the motor. Therefore, you should be aware of the fact that the same motor type and model can have different gearboxes and maybe also differences when it comes to propulsion and moving backward. If you for example have a gearbox of 1:2,36, it is recommended that you use a propeller with a smaller blade profile instead of a gearbox of 1:3,16, which requires a bigger blade profile.
If you look at a boat propeller from the back, you can see that the blade on, for example a 3-blade propeller covers about 50-55 % of the circle that the blades rotate in. You can increase this percentage by adding a fourth blade. When you look at a 4-blade propeller, you will likewise see that a bigger area of the rotational circle is covered. And that also means that the propeller has much more contact with the water.
And to just stick with 2 blades, that are scaled up in size, is not a possibility due to, among other things, the tipping speed.
What should you choose for your sailboat?
In particular, there are 3 factors that are crucial, when it comes to finding the right number of blades on your folding propeller:
Your boat’s motor size and gearbox
Your boat type
Your personal preferences
The number of blades depends on your boat’s motor size and gearbox
The two most important parameters when it comes to finding the right folding propeller are determined by your boat’s motor and gearbox.
If you for example have a sailboat with 150 horsepower, then a 2-blade propeller is not the best solution. In this case you need something bigger and more powerful instead.
Conversely, a boat with less horsepower should not have a folding propeller with 4 blades, as the motor in that case would have a hard time keeping up with the propeller, and that could cause damage to your boat.
Your boat type is important when it comes to the number of blades
Following the above, your boat type is also important.
However, this should always be seen in correlation to both the motor and the gearbox, as many different combinations exist. And the same gearbox can easily be mounted on different boats.
That is why we always recommend that you contact us when you need to get the best assessment.
Your personal preferences can also be a determining factor
Last but not last, your personal preferences are also important, of course.
Even after thorough calculations when it comes to motor size, gearbox and boat type, it will in some cases be relevant with, for example, both a 2-blade folding propeller and a 3-blade folding propeller, and in that case, it will be up to you to decide what you want to achieve with your time spent at sea.
If you want the absolute smallest tensile pull as possible, then you should of course choose the model with the fewest blades, i.e. a 2-blade folding propeller. And the same thing applies if it is a matter of money. The fewer the blades, the lower the price will be, more often than not.
If, on the other hand, you want maximum effect even in the roughest weather, then a 3-blade folding propeller is a more optimal choice.
At Flexofold we help you make the right decision
We know that it can be difficult to choose a folding propeller with the right number of blades. Therefore, we offer to help you in the process – no matter which type of sailor you are.
All you need to do is send us the exact details for your boat, including your boat’s motor size, gearbox and boat type, then we do the math for you. From there, we can give you the best and most well-founded assessment of how many blades the folding propeller for exactly your boat needs to have.