Ramming the Ice: Icebreaker Propulsion

1.0 Introduction

This beast bares TWO nuclear reactors, with over 60 MW of propulsive power driving through anything in its path! No, it isn’t a nuclear sub or a warship. This triumph of propulsion leverages all that power against an unstoppable enemy: ice. Russia drives their biggest icebreakers with twin nuclear reactors! Icebreakers work by ramming the hull into a solid icepack. We need more than a paddle wheel to achieve that. Let’s look at the advanced propulsion of icebreakers!

2.0 Power LOTS of Power

The first thing an icebreaker requires is LOTS of power. Most ships don’t come equipped to heave the bow up onto land, which is how an icebreaker works. The wedge shaped bow rides on top of the ice, and the weight of the ship crushes down to break the ice. To achieve this, the old Mackinaw employed 7.5 MW (10,000 hp) in her six diesel engines. Enough power to supply the city of her namesake two times over! [1] And modern icebreakers go further. The icebreaker Baltika has 9.0 MW in her hull. Then Russia supersizes this, with nuclear powered icebreakers! The Project 22220 class of icebreakers come with two nuclear reactors, delivering a total of 60 MW to the propulsion shafts! [2] Icebreakers are essentially a giant floating engine set inside the hull of a tank!

3.0 Propeller Configuration

Normal ships come with a pointy bow and propellers at the back. But icebreakers break the mold. The Mackinaw came with two propellers at the stern and a bow propeller. This bow propeller added to the forward propulsion, but its main purpose was to clear away ice from the bow. The propeller created a massive stream of water, carrying the ice along the hull and clear to the stern. The propeller could also run in reverse to undercut the ice. With an icebreaker, propellers become more than mere propulsion. They transform into a tool for moving and breaking the ice.

Modern icebreakers take this one step further, using three azipod propellers. Azipods use electric motors and can rotate the propeller any direction in a 360 circle. This grants the icebreaker unparalleled maneuverability. Forward, backward, sideways, spin in a circle, the azipod icebreaker does it all. (Figure 3‑1) But now the prop wash can clear the ice in any direction. Even direct the props sideways and use their wash to widen a channel. It allows the ship to clear channels larger than its own beam. This makes the ship far more productive and reduces the chances they get stuck.

Figure 3‑1: Modern Azipod Icebreaker [4]

3.1 Oblique Icebreaking

Azipods introduced a new mode of operation: oblique icebreaking. Any icebreaker wants to clear a channel as fast as possible. Except the freighter following them may be wider than the icebreaker. This requires the breaker to take multiple passes through the same channel, widening it. A slow and repetitive process that monopolizes the icebreaker.

With oblique icebreaking, you don’t drive straight onto the ice. The azipods hold the ship at an angle, and it crabs its way into the ice. (Figure 3‑2) Since icebreakers are much longer than wide, even a slight angle drastically increases the effective width of the channel. Now the ship clears the channel in a single pass, where it needed three passes before. That’s a 3x increase in productivity! All thanks to azipods.

Figure 3‑2: Oblique Icebreaker [5]

4.0 Why Diesel Electric

The Mackinaw was built in 1944, in the infancy of modern industrial electric motors. Back then, practically every ship had the same answer: diesel driven engine directly coupled to the propeller. So why did the Mackinaw take on the risk of new electric motors? The vessel had six diesel engines, each driving DC generators. These generators then fed into three DC electric motors, which drove the propellers. (Figure 4‑1) There was no direct link between engine and propeller. But why add all this complexity?

Despite the risk of newfangled electric motors, electric propulsion offers abundant benefits for an icebreaker. The first was engine room layout. When engines directly connect to the propeller, we need a straight shaft line between the propeller and the engine. For an icebreaker, this gets especially problematic. The propeller position places engines low in the hull when connected with a shaft line. And icebreakers use especially rounded hulls, with very little space near the bottom.

“If you don’t have diesel-electric, you need a mechanical link all the way from your engines to your shaft. That means your engines have to be down low and back, fairly close to where your shaft goes through the hull. With diesel-electric you can position your engines anywhere.”

Don Witt, Senior Chief Electrician [1]

The flexibility of engine placement adds a safety feature. Notice the engines were separated into three pairs, each in their own compartment. If an engine room floods, the other two compartments remain watertight, with their engines fully operational. That makes it extremely unlikely that you completely lose power. Each engine can feed power to any of the electric motors, meaning you need to destroy all three motors to completely lose propulsion. For anything less than flooding the entire ship, chances are good that you can find some combination of engine and electric motor to control the ship.

That flexibility also helps with maintenance and efficiency. We don’t need to stop the ship for maintenance on a single engine. Just run on reduced power. Also useful for slow steaming. Sometimes the ship doesn’t need full power, but these engines get extremely inefficient it you try to run them at a low throttle. Instead, completely turn off an engine, leaving the remaining beasts roaring at full power and peak efficiency. The flexibility of diesel electric decouples engineering from ship operations.

5.0 Power On Your Terms

I can’t ignore one of the best benefits for diesel electric propulsion: power decoupling. Diesel engines only develop full power when spinning at full speed. If you want more power, the rotation speed goes up. With a direct mechanical connection, full power requires the propellers to spin at full speed.

When breaking ice, we may not want full speed. Ice chunks suck into the blade, like a giant blender. For heavy ice, we may want the propellers rotating slower to prevent damage. But it takes a lot of power to break ice. And with diesel electric, we don’t need to chose between power and speed. The engines rotate at full speed to generate the power, and the electric motor decides how to use that power. The motor controls the rotation speed and torque on the propeller, independent of engine speed. You no longer need to choose between rotation speed or power. Diesel electric allows you to pick both on your terms.

This decoupling provides another safety feature. In the worst case scenario, a chunk of ice may wedge in the propeller and completely stop it. With a direct connection, that also forces the engine to suddenly stop, which does not end well. But with diesel electric, that stop scenario only results in tripped circuit breakers. Very big circuit breakers, but they work on the same principles as your house breakers, protecting all the electrical circuits. They automatically respond in milliseconds and can quickly reset to regain propulsion control. Plus, with the electrical system, we can easily measure the electrical circuit and monitor to know if we approach that cutoff point. Diesel electric grants safety and knowledge, always good things on an icebreaker.

6.0 Engine Cooling

Hard to imagine, but engine cooling becomes difficult in ice conditions. Marine engines work different from your car; the engines in the boat use seawater for cooling. We suck this water in through a special inlet, called a seachest, which is located on the bottom of the ship. What else do we find near the ship hull on an icebreaker? Yes, giant chunks of ice that get sucked in and clog the seachest. Without a steady stream of cold water, the engines overheat fairly quick. What to do?

If only we had a steady stream of hot water to melt the ice and keep the seachest clear. Like the cooling water that the engine just heated up! In the icebreaker, some of that now hot water gets redirected back into the seachest. The heat melts any ice, keeping the seachest operational. That’s a very efficient use for the waste heat generated by the engines.

7.0 Conclusion

A giant engine, set in the hull of a tank. That is an icebreaker. Except, this tank comes with a few surprises. Azipods allow the behemoth hulk to pirouette and dance through the ice field. We can even break at an oblique angle, drastically cutting the time to clear a channel. And the underlying electrical power grid grants redundancy and safety. Very important and when your ship goes around intentionally trying to collide with things.

With icebreakers, propulsion is more than just a massive power plant. It’s smart power.

8.0 References

[1]	S. L. Planisek, Icebreaker Mackinaw, 2nd. Edition, Mackinaw City, MI: Great Lakes Lighthouse Keepers Association, 2008.
[2]	Wikipedia Authors, “Project 22220 icebreaker,” Wikipedia, [Online]. Available: https://en.wikipedia.org/wiki/Project_22220_icebreaker. [Accessed 17 Feb 2023].
[3]	Naval News, “Russian Project 22220 icebreaker Arktika starts trials after repairs,” Naval News November 2021 Navy Forces Maritime Defense Industry, 26 Nov 2021. [Online]. Available: https://www.navyrecognition.com/index.php/naval-news/naval-news-archive/2021/november/11061-russian-project-22220-icebreaker-arktika-starts-trials-after-repairs.html. [Accessed 17 Feb 2023].
[4]	ABB Marine, “How icebreaking is enabled with ABB’s Azipod® propulsion,” YouTube, 13 Oct 2022. [Online]. Available: https://www.youtube.com/watch?v=PCgEETYmWRE. [Accessed 17 Feb 2023].
[5]	Aker Arctic, “Oblique Icebreaker Baltika – Ice trials on 19 March – 10 April 2015,” YouTube, 5 May 2015. [Online]. Available: https://www.youtube.com/watch?v=zwe0MHRaqhA. [Accessed 17 Feb 2023].
[6]	Wikipedia Authors, “Torque-Speed Curve for a typical AC motor,” Wikimedia Commons, 31 Aug 2013. [Online]. Available: https://commons.wikimedia.org/wiki/File:Torque-Speed_Curve_for_a_typical_AC_motor.jpg. [Accessed 21 Feb 2023].
[7]	A. Growcott, E. Georglades and D. Kluza, “Literature Review: In-Water Systems to Remove or Treat Biofoulding in Vessel Sea Chests and Internal Pipework,” Ministry for Primary Industries: , Vols. 2016-16, 2016.
[8]	Wikipedia Authors, “Mackinaw WLBB-30 Azipod thruster,” Wikimedia Commons, 6 Jan 2007. [Online]. Available: https://commons.wikimedia.org/wiki/File:Mackinaw_WLBB-30_Azipod_thruster.jpg. [Accessed 10 Aug 2023].

Ramming the Ice: Icebreaker Propulsion

1.0 Introduction

2.0 Power LOTS of Power

3.0 Propeller Configuration

3.1 Oblique Icebreaking

4.0 Why Diesel Electric

5.0 Power On Your Terms

6.0 Engine Cooling

7.0 Conclusion

8.0 References

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