Twenty-five years ago, the newly formed Steerprop delivered its first propulsion units to the river cruise vessels River Aria. The propulsor design was also novel, with double-ended, dual propeller units that offered unique efficiency and manoeuvrability. Since then, the Contra-Rotating Propellers (CRP) has proven itself time and again on a wide range of vessels, from river cruises to heavy-duty offshore ships, high-speed coast guard vessels and icebreakers.
The benefits of the CRP configuration in both open water and through ice are now well established over a quarter of a century, along with an impressive track record of reliability and lifecycle performance. The principle of operation is backed by hydrodynamic laws, which show that a double propeller in push-pull configuration can only be more efficient than a similarly sized single propeller with similar power applied.
So now, the question remains: Why aren’t more vessels deploying a propulsion technology proven to reduce lifecycle costs and emissions?
The often-noted conservatism of the shipping industry may have a part to play. Why mess with a propulsion format that has been in widespread use for close to 150 years? But there are other factors that affect how designers, shipyards and shipowners choose and assess propulsion—and even the propulsion solutions they are offered by technology providers.
In an age where both cost and efficiency are more in focus than ever, perhaps it is time to revisit how thruster propulsion is designed, assessed and selected. With a scientific and holistic perspective on ship design and the fundamentals of hydrodynamics, Steerprop’s CRP propulsor emerges as the solution of choice for many modern vessels demanding high environmental performance and low lifecycle cost.
What is CRP?
At its core, a Contra-Rotating Propellers (CRP) unit is a high-efficiency propulsion solution that utilizes two propellers rotating in opposite directions. In Steerprop’s specific technical execution, this is realized as a dual-end azimuth propulsor.
Unlike a standard single-propeller unit, the dual-end CRP splits the propulsive load between two gear wheels, driving one pushing propeller and one pulling propeller located on opposite ends of the propulsor body. It’s this push-pull action that is the source of CRP’s unbeatable efficiency
The design is mechanically driven, typically utilizing an inverted T-gear arrangement where the power is divided between two relatively short independent shafts. This simple and robust construction allows both propeller shafts to have their own dedicated bearings and seals, which, combined with lower rotational speeds, leads to a longer calculated lifetime for critical components.
Modern iterations of the technology have further integrated advanced features like permanent magnet motors and pressure lubrication. These units are available in a wide power range, from small 500 kW thrusters to massive 16 MW units intended for the large cruise vessels.
The five facts of hydrodynamic efficiency
The superior efficiency of the CRP is not a matter of opinion, but of fundamental physics. Several key hydrodynamic principles combine to give the CRP a significant edge over single-propeller designs:
- Low propeller loading
By dividing the propulsive load between two propellers, the thrust required from each individual blade is dramatically reduced. This split load allows the CRP to achieve a total propeller disc area that is roughly 150% of a single propeller design. A further advantage is that the reduced load means even more efficient blade designs can be used.
- Rotational energy recovery
When a single propeller rotates, it imparts a swirl to the water, which represents a loss of energy. In a CRP configuration, the rear propeller recovers this rotational energy from the forward propeller’s slipstream, converting some of it back into useful thrust. Calculations indicate that this swirl recovery alone can contribute significantly to the 5-9% efficiency advantage CRP holds over single-propeller azimuth designs, and around 10-15% compared to single-screw conventional propulsion.
- Cavitation Management
Because the loading is distributed to two propellers and the propellers can rotate at a lower RPM, cavitation is cut down to virtually nothing in normal operating conditions. This not only improves efficiency but also results in exceptionally low noise and vibration levels, enhancing comfort for passengers and crew.
- High-speed efficiency
Traditional pushing azimuth propulsors often lose efficiency at speeds above 20 knots due to drag caused by the strut and housing. The dual-end CRP’s advanced hydrodynamic design overcomes this, offering a 5-20% improvement in propulsive efficiency depending on speed.
- Optimal flow conditions
The dual-end configuration places the forward propeller in a pulling position, giving it constant access to uninterrupted water flow. This interaction creates a positive wake field in front of the propulsor body that significantly increases hydrodynamical efficiency to overcome the drag typically caused by the underwater housing.
The advantage in applications
For each vessel segment, the unique design of Steerprop’s Contra-Rotating Propellers (CRP) technology provides distinct advantages that align with specific operational requirements and environmental challenges.
Arctic and icebreakers
In the harsh Arctic environment, the primary benefit of CRP is unparalleled ice-management and thrust. The unique slipstream generated by the dual-end contra-rotating propellers is exceptionally effective at destroying ice ridges, clearing rubble fields, and widening fairways.
Model tests demonstrate that CRP provides 10-17% higher net thrust than single pulling propellers at the same power level. This allows vessels to achieve 50% faster speeds in brash ice and reduces ridge penetration times by half.
Mechanically, the dual-end design enhances safety; because the propellers are located several meters apart on opposite ends of the body, a single block of ice cannot jam or damage both propellers simultaneously.
Cruise and Ferry
The passenger segment prioritises comfort and fuel efficiency at high speeds. The dual-end CRP configuration distributes the propulsive load, resulting in lower propeller loading and slower rotational speeds, which virtually eliminates cavitation and significantly reduces noise and vibration for passengers and crew.
Hydrodynamically, CRP offers a 5-9% efficiency advantage over traditional open-propeller designs. For double-ended ferries, simulations show that optimized CRP units can reduce energy consumption by up to 13% compared to standard solutions.
Offshore
For offshore vessels like PSVs and Survey vessels, the main benefits are enhanced reliability and Dynamic Positioning (DP) capability.
The high thrust of the CRP units directly improves DP station-keeping precision in demanding sea conditions, while Steerprop’s robust mechanical construction is designed for a 15-year or 100,000-hour service life without major overhauls—critical for vessels operating in remote oil fields.
The integration of Permanent Magnet (PM) motor technology and condition monitoring systems allows for proactive maintenance, reducing the risk of uncontrolled failures that can lead to expensive downtime. Additionally, the compact design requires less internal hull space for machinery.
Three objections
Despite these clear physical advantages, several systemic challenges prevent CRP from becoming the default choice for every vessel.
The shipowner´s perspective: Apples for apples
A major hurdle for shipowners is the difficulty of comparing different propulsion solutions during the bidding phase.
Historically, inquiries have focused on input power. However, power is a poor definition of performance; the better question is how much thrust a unit provides at a specific speed.
Because a CRP unit is so much more efficient, a lower-power CRP can often provide the same net thrust as a higher-power single-propeller unit. Without a holistic technical comparison, owners may inadvertently choose a less efficient system simply because the power rating looks higher on paper.
The shipyards perspective: Initial vs long-term cost
Shipyards operate in a fiercely competitive environment where the primary goal is often to minimize initial capital expenditure.
Because shipyards are often responsible for the procurement but do not pay the fuel bills, they have a natural incentive to select the cheapest component that meets the minimum contract requirements—even when the system may not be the one that minimises the cost for the shipowner.
This leads to suboptimization, where the owner is left with higher long-term operating expenses (Opex) and a higher Total Cost of Ownership (TCO).
Technology providers: Competition and complexity
While many propulsion providers have explored CRP technology, many have reverted to offering cheaper, standard single-propeller solutions.
Building a reliable dual-shaft system requires exceptional expertise in bearing arrangements and lubrication systems. Steerprop’s design utilizes independent bearings for both shafts, which is technically more complex to engineer but provides a simpler, more robust mechanical power transmission in the long run.
Breaking the barriers: Real-world examples
To move past these barriers, one must look at the real-world economic performance of CRP compared to single-propeller units across different vessel types.
The Double-Ended Ferry
A simulation-based case study of a 120 CEU double-ended ferry compared three propulsion options: a low-capex ‘standard’ solution and two high-efficiency CRP options.
- Performance: The high-efficiency CRP options consumed up to 13% less energy across all simulated routes.
- Economics: While the CRP units had a higher capex (up to 11.8% higher), the annual fuel savings were so significant that the payback time was less than a year on short, frequent routes.
- TCO: Over a 15-year period, the high-efficiency CRP option saved the owner more than 2.29 million euros in energy costs alone.
Read more about the study here: https://bit.ly/4cIno0e
The Arctic Icebreaker
In ice-going applications, the CRP’s ability to generate high thrust at low speeds is critical.
- Efficiency: A comparative study of an icebreaker project showed that three CRP units provided equal ice-going capability to three pulling units while using 10% less power (17.5 MW vs. 19 MW).
- Thrust: Net thrust measurements in bollard conditions revealed that CRP produces 10-17% higher thrust than a single pulling propeller with the same power.
- Opex Advantage: Higher thrust translates directly into shorter ridge penetration times and increased speed in brash ice, significantly reducing operational time and fuel consumption in harsh conditions.
The expertise behind Steerprop CRP
The Steerprop CRP system is a complex azimuth propulsor comprising several integrated subsystems. These components work in synergy to provide superior efficiency, robust mechanical reliability, and a low total cost of ownership across diverse maritime sectors.
Key components and subsystems
- Dual-end propeller configuration: The system utilizes two propellers mounted on the same axis—one pulling propeller at the front and one pushing propeller at the rear—rotating in opposite directions.
- Hydrodynamically optimized housing: The propulsor features an elongated torpedo-shaped housing (strut and body) made of cast steel, which is specifically designed using Computational Fluid Dynamics (CFD) to optimize water flow, resulting in either symmetrical or asymmetric designs depending on the application profile.
- Mechanical power transmission: Power is typically distributed through an inverted T-gear arrangement, where two gearwheels divide the torque between two relatively short, independent shafts.
- Integrated permanent magnet motor: Modern units are designed to integrate a high-efficiency PM motor directly into the propulsor assembly inside the vessel's hull.
- Advanced lubrication systems: While traditionally smaller units were immersion-lubricated, Steerprop’s latest generation of CRP propulsors – even at smaller sizes - use sophisticated pressure lubrication and temperature management systems that have been used to reduce energy consumption on bigger units.
- Next-generation shaft seals: The system includes advanced propeller shaft seals, such as zero-discharge air-type seals, intended to eliminate oil leakage into the environment.
- Steerprop Care: Using sensors and data analytics to track the health of mechanical components in real-time. Steerprop Care is included as a fully integrated part of every propulsor delivery, allowing crew members to follow machinery trends and detect any deviation that could affect the propulsion system in advance.
Performance and durability
The contra-rotating design allows the rear propeller to recover rotational energy (swirl recovery) from the forward propeller’s slipstream, contributing to a 5-9% efficiency advantage over single-propeller designs. The dual-end configuration also results in less hydrodynamic drag than would be suffered with a single propulsor units, improving efficiency further. Integrated PM motors enhance this by maintaining high efficiency even at low loads.
When it comes to durability, the mechanical transmission is designed for a service life of 15 years or 100,000 hours without major overhauls. Because the propulsive load is split, each shaft rotates at a lower RPM, which reduces stress and extends the calculated lifetime of bearings and seals. In Arctic applications, the physical distance between the forward and rear propellers prevents a single block of ice from jamming or damaging both power trains simultaneously.
Lifecycle cost and adaptability
The high hydrodynamic efficiency translates directly into significant fuel savings over the vessel's life. The condition monitoring subsystem shifts maintenance from a reactive to a proactive strategy, preventing catastrophic failures and minimizing unplanned downtime. Additionally, the use of same-handed components for the underwater propulsors simplifies spare parts inventory and service logistics.
When it comes to designing units that fit the application, Steerprop utilizes a parametric design process, allowing units to be tailor-made to a vessel's specific space and operational profile rather than being limited to standard sizes. With a power range spanning from 500 kW to 16 MW, CRP technology is adaptable for everything from shallow-water river cruise ships to the most demanding Arctic icebreakers and high-speed ferries.
When trust means thrust
The hydrodynamic facts are clear: for many vessel applications, CRP is the most efficient thruster configuration available today. In an era where reducing emissions and operational costs is paramount, the industry must shift from outdated propulsion assessment methods and a focus on short-term costs, to evaluating solutions based on Total Cost of Ownership.
By involving propulsion experts early in the design phase and focusing on thrust rather than just power, shipowners can unlock millions in lifetime savings and lead the way toward a more efficient maritime future.
