Do you know the basic dimensioning principles for arctic propulsors?

Understanding how ice loads affect vessels is critical when designing and dimensioning propulsors that provide top performance and fuel efficiency, while not harming the fragile arctic environment.

How to best dimension every propulsor depends on the requirements of each ice class. So to ensure the best results when designing each propulsor, it is important to check all classification requirements for that vessel. The ice class rules for Arctic Vessels in most class societies follow the IACS Unified Requirements for Polar Ships, with PC 1 being the hardest ice and PC 7 being the weakest – old ice , in general, being harder than new.

The nature of the ice, and how much pressure is required to break through, affects the load, so it is vital to understand how ice load affects the propulsor under the water. The class rules dictate how to dimension the housing to break through different ice load magnitudes.

For example, the longitudinal load is factored differently than the transversal load, which is more critical with respect to the propulsor design. Still, both must account for the load of a consolidated ice sheet that slides down the hull and impacts the propulsor. Especially with azimuth thrusters, the transversal load can play a major role. Still, all directional ice loads and magnitudes need to be considered as they hit the propulsor from different sides while the vessel cuts through ice.

When dimensioning arctic propulsors, Steerprop analyzes the impact of ice loads on the slewing ring dimensioning – the hull attachment inside the ship’s hull, with bolts dimensioned to provide the strength of the housing that withstands the ice loads.

For most ice classes, Steerprop recommends stainless steel propellers, but bronze propellers are also available for lighter ice classes. Blade scantlings are calculated according to class rules. The entire propeller can be delivered as a monoblock or built with blades bolted on, which are easier to replace with a spare blade, even underwater at sea, rather than changing an entire propeller.

Pushing and pulling have traditionally been the preferred propulsor type, but recently Steerprop’s contra-rotating propulsion (CRP) technology has been gaining traction with Arctic expedition cruise vessels. Electric steering combined with CRP works well, in particular for ice classes, and is more energy efficient. In addition, it is silent.

Furthermore, our pollution-free sealing solution for the severe vibration conditions caused by operating in ice has proven effective through experience.

Each ice class affects not only the final propeller design and configuration but also its power transmission gear design and steering. Steerprop uses a unique gear transmission design with the proper and effective load-carrying contact area and pattern that can maintain the safety margin for any unexpected load.

Every Steerprop propulsor also comes with high-quality roller bearings for the very best performance. To maintain consistent quality, the company has worked with only two or three top-flight bearing manufacturers over the years.

The most important outcome is to be able to ensure that the vessel can handle even unexpected loads and safely make it through the ice, even if the propulsor is damaged.

That’s why Steerprop uses computerized ice simulations to take the entire line into consideration, including the torsional vibrations from the whole system. We then provide our complete analysis of the forces to the ship design house or shipyard to further calculate the hull strength needed.

Ultimately, the classification societies set the rules. But it is up to us to calculate, simulate and formulate how much tension and deformation would be allowable at each point in the propulsor for a safe voyage above and beyond the rules, in the actual arctic marine environment.

Our mission is to give modern-day vessels the ability to venture more efficiently and safely into the icy frontier.

About the author:

Olli Knihti, graduated as a B. Sc. of Mechanical Engineering, from the Porin Teknillinen Oppilaitos in 1985. 

Olli started his career at the maritime industry in 1986 and has worked at Steerprop Ltd. since the year 2001. His responsibilities have been in project, warranty and Sales. 

In recent years Olli has been involved in the development of Steerprop product line, including the demanding arctic units for the world´s harshest environment.

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