Fixed pitch propellers (FPP), controllable pitch propellers (CPP), contra-rotating propellers (CRP) and waterjets. These are four common propeller solutions in the marine industry. But what are their advantages and disadvantages, and how do they differ from each other? In this blog article, I will describe the four different propeller solutions, give tips on how to choose the right one and outline how the propeller solutions can help improve marine businesses.
Transforms the rotational torque of the engine into tractive effort
As you probably already know, the propeller’s primary purpose is to move the vessel forward by transforming the rotational torque of the engine into propulsive force. A propeller does not operate on its own but is rather a part of a system. Included in a propulsion system is a shaft, a gearbox, sealings, support bearings, one or several propellers and sometimes even the hull. Also, steering and exhaust systems can be included, depending on the system.
Generally, the most common propeller solutions can be divided into four different main groups: Fixed pitch propellers (FPP), controllable pitch propellers (CPP), contra-rotating propellers (CRP) and waterjets.
Fixed Pitch Propellers (FPP) – a robust and common propeller solution
Fixed pitch propellers is a standard propeller variant with a fixed design. This is the most common propeller solution and it is applicable to different types of vessels with varying sizes. Since it is based on a fixed design, it is not possible to adjust the solution over time or as the running conditions change. Therefore, it is important to know what demands the propeller solution must live up to before choosing a fixed pitch propeller solution. The advantages of a fixed pitch propeller solution are that it is cost-efficient and robust. There are also many different variants of the solution. For example, it can be equipped with thick propeller blades if the vessel will operate at low speeds and in areas with floating debris. That makes the propeller stronger which, together with durable material, help protect the propeller against damages. The disadvantage of the fixed pitch propeller solution is that it has limitations in its area of use when it comes to, for example, loads, speeds and maneuverability. FPP is normally not combined into an integrated system, which demands that separate solutions for steering and exhaust are created.
Controllable Pitch Propellers (CPP) are dynamically adjustable during operation
Controllable pitch propellers (CPP) are different from fixed pitch propellers because the propeller blades can be dynamically adjusted during operation. Controllable pitch propellers have a fixed diameter of the propeller blades while the pitch of the propeller blades can be adjusted by turning the angle of attack of the propeller blades. This determines how much propulsive force is being produced, or how much water that moves when the propeller blades spin one lap. This also gives the possibility to adjust the load on the driveline when generating power. Generally, the diameter or size of the propeller dictates how much power that can be absorbed and the pitch decides the forward advance. However, sometimes vessels operate at a fixed speed with a need of added thrust, which is ideal for a propeller with an adjustable pitch. There can also be a desire to change the speed of the vessel without increasing the speed of the propeller. In that case, adjusting the pitch is also a good alternative. Some systems can even reverse the thrust while the propeller continues to spin in the same direction.
The benefit of using controllable pitch propellers is that the propellers are adjustable in order to get the most out of the engine, propulsion system and application based on the vessels specific conditions. Occasions where this could be relevant are, for example, when the vessel operates with variable high loads, in bad weather or if the vessel needs to adjust its speed and thrust. For example, this could be relevant for trawlers. The downside with using controllable pitch propellers is that the solution is very costly compared to, for example, the fixed pitch propeller solution. Also, in the same fashion as FPP, in most cases a separate solution for steering and exhaust must be designed.
Contra-Rotating Propellers (CRP) offer high efficiency and general purpose performance
Contra-rotating propellers are based on an interesting propeller technique where two propellers are positioned one after another and rotate in opposite directions. In order for the propellers to rotate in two different directions, they are placed on two concentric shafts. One of the propellers is positioned on a hollow shaft while the other propeller is positioned on a shaft inside the hollow shaft. Gear wheels drive the two axles in contra-rotating directions. The gear wheels generate power to the propellers from the engine. A contra-rotating propeller solution has a higher initial cost and is normally less mechanically robust than a fixed pitch propeller solution. However, it offers a wider area of use when it comes to, for example, efficiency, maneuverability, variations in loads and speeds and comfort. Since the second propeller uses some of the energy from the rotated water leaving the front propeller, the contra-rotating propellers are more efficient than single propellers. It is also possible to minimize the appendage drag fraction of the vessel since the torque of the engine is divided into two propeller shafts. This requires smaller gears that fit into smaller underwater housings, which provides less hydrodynamic resistance.
A reduction of the blade speed and loading provides less propeller-load generated noise and vibrations. As surely noted, since the opposing direction of the propellers require a combined gear arrangement it is natural that this type of propeller is integrated into a system where more aspects are naturally handled but also optimized. This provides pre-engineered propulsion solutions which often include steering and sometimes even exhaust. Most modern CRP systems are made for high speed applications, even though the concept’s benefits are used at all speeds.
Waterjet – a water pump suitable for high-speed vessels
A waterjet differs from the three previous propeller solutions. Since it has a propeller placed inside a tube in the vessel, the propulsion acts like a water pump. It is based on the principle that each force has a counterforce of equal size. Thus, the same force that is used to push water in one direction is also used to push the vessel in the opposite direction. When choosing a waterjet of the right size in relation to the speed and load of the boat, it can be a very effective propulsion solution. Waterjets are mainly used for high-speed vessels and one of the great advantages of the solution is that there is nothing protruding beneath the bottom of the vessel which results in zero added hydrodynamic resistance. The lack of protrusions under water also provides a great advantage when operating in shallow waters in an emergency. Waterjet systems are also normally fitted with steering capacity.
A downside with waterjets is that it is very critical to choose the right tube size (system size) in relation to the application’s needs. A mistake in size or a significant change of hull resistance can lead to very poor acceleration. It also has a high initial cost and an exhaust system must be engineered separately.
Choose the right propeller solution for your vessel, business and needs
The wrong type of propeller solution can have very negative impacts on both the vessel and a marine business operation. It can, for example, lead to low efficiency, high fuel consumption and low speeds. However, the opposite is also true. Choosing the right system and propeller for the application can provide the true competitive edge, with maximized efficiency, low fuel consumption, high operational speed, great comfort, reliability and minimized operational cost. Therefore, it is crucial that the naval architect, the shipyard and the operator have a tight dialogue and also get the right support from the provider of the propulsion solution.
At Volvo Penta, we believe there is true value in providing our customers with our support and guidance in choosing the right propeller solution for your specific vessel. It is often a long and profound process that we are happy to take part in and no matter what propulsion solution that you choose in the end, our main goal is to help our customers achieve an efficient and thriving business.
I hope you enjoyed reading my blog article about different propeller solutions and their advantages and disadvantages. Feel free to visit our website and social media channels for more information about Volvo Penta and our offer. Also, don’t forget to visit our professional power blog where you can read more interesting blog articles.