Optimised inboard propulsion system with contra-rotating propellers

In my previous blog article, “Pros and cons with different marine propeller solutions”, I described contra-rotating propellers (CRP). An optimised placement of this propeller solution is one of the main benefits of Volvo Penta’s Inboard Propulsion System (IPS). But why did Volvo Penta choose to design the system the way they did? This blog article describes the design process of the IPS and summarises why the system helps increase efficiency and minimise fuel-consumption in the marine industry.

Multiple ways to create different types of propeller solutions

Seen through history, many different propeller variants have been used in the marine industry. A different number of propeller blades, different mounting on shafts and variants on rotations are just a few of all adjustments that can be made to create different types of propeller solutions. However, they all have one thing in common, which is to transform as much power as possible from the engine into propulsive force, enabling the vessel to operate in an effective and optimised way.

An integrated system achieves maximum efficiency and low fuel consumption

Contra-rotating propeller

The operator, in dialog with the naval architect and the shipyard, normally chooses the vessel’s propeller solution. When making this choice many different aspects need to be taken into consideration, such as operational requirements, type of vessel, legal aspects and the conditions the vessel will operate in. To make the right choice, a lot of information about the vessel needs to be available, such as load, speed, area of use, hull material, and what levels of efficiency and fuel-consumption the vessel must live up to. Also, the choice of engine must be taken into consideration in regards to, for example, power and torque performance. The whole propulsion system must work together as an integrated system to achieve maximum efficiency and benefit as well as minimised fuel consumption.  

In general, one could say that a predictable vessel operation can work really well with a standard propeller solution. However, as soon as the vessel’s operation starts to vary with a wider operational window, the more sophisticated propulsion systems become the preferred choice.

IPS – includes propulsion, steering, maneuvering and exhaust system

Volvo Penta has designed many different propulsion solutions over the years, where one of our most successful designs is our Inboard Performance System (IPS). A benefit with the IPS is that it is not just propellers generating propulsion, but a complete system that solves several technical challenges for the end user. In addition to propulsion, the IPS offers steering and maneuvering of the vessel as well as a fully integrated exhaust system. When choosing other, more simple, propeller solutions that are not integrated into a complete system you must construct your own system and choose each part separately. This process takes both time and money and opens up for the possibility to sub-optimize and choose solutions that are not ideal for the rest of the system. In worst case, this could lead to a negative effect on the vessel’s performance, behaviour and efficiency.   

High pressure above the propeller is achieved from the hull

Preferably, the propulsion system should be placed behind or underneath the vessel. It is also important to create a high pressure above the propeller in order to increase its efficiency and margin to cavitation. This is sometimes done by either mounting a plate above the propeller or, as we did with the IPS, place the propeller underneath the vessel. That way, we allowed the hull to create the desired pressure. If the propeller position is not fully controlled its design conditions may be compromised by the water flow movements and pressure distribution from the vessel. This leads to a not fully integrated solution that requires a lot of trial and error development.

Comparison of efficiency between Fixed Pitch Propellers (FPP) and Contra-Rotating Propellers (CRP)

Contra-rotating propellers that pull the vessel forward

Another benefit with the IPS system, aside from the placement of the propulsion system, is that the propeller is based on a technique called contra-rotating propellers. That means two propellers are positioned one after another and rotate in opposite directions. Thus, the propulsion system pulls the vessel forward instead of pushing it forward. This means that the propellers can operate in relatively undisturbed water. An undisturbed water flow in front of the propeller is more predictable which has allowed us to optimize efficiency and lower the fuel consumption. Also, another elegant solution enabled by using pulling propellers is that the exhaust system could be integrated in the pod housing and the gases released low and far aft under the vessel without disturbing the propellers.

In the beginning of the design process, the whole propulsion system was fixed under the vessel. However, in order to improve maneuverability, we changed that and made the system fully articulated. We chose to design electronic steering of the vessel which enabled us to further conceptualize a joystick that offers increased comfort, better drivability and enhanced safety.     

Easy to install and operates with varied load and speed

SC Falcon operating with IPS

The hull of the vessel must be designed to fit the IPS system. For example, the hull needs to be flat in the area around the IPS units, the load balance and dynamic stability must be considered, performance and system sizing must be reviewed and maneuvering requirements must be estimated. Volvo Penta’s skilled staff can support both the naval architect and the shipyard’s engineers in the design process in order to optimise the hull design for the IPS system. When the vessel’s hull is adapted to the IPS system, the installation process is very simple. Even though the IPS system is designed to fit medium and high-speed vessels, it allows you to operate with varied load and speed and yet maintain a low fuel consumption.       

The image illustrates the force-vectors acting on the vessel when mooring against a wind farm pylon.

Volvo Penta delivers the whole package

As stated, we try our best to design and pre-engineer optimized solutions for our customers. We do this by creating systems and not components. This is why Volvo Penta often tries to further the dialog from a holistic perspective, where the solution goes all the way from propeller to antenna. As such, the IPS package offers a tremendous spectrum of features that in the end will benefit our customers.

The right propulsion system is crucial in order to achieve a successful business in the marine industry. It is important for us, at Volvo Penta, to be available and offer our services to our customers in every step of the way when choosing a propulsion system. We also work on a daily basis to develop our existing propulsion systems as well as design new and innovative ones. This way, we make sure to always offer the very best propulsion systems for our customers.

I hope you enjoyed reading my blog article about the IPS system and the design process that led up to the solution. Feel free to visit our website or social media channels for more information. Also, visit our Professional power blog to read more interesting blog articles!     

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