Wiki claims the Polar Star can, "steam continuously through 6 feet (1.8 m) of ice at 3 knots (6 km/h)"
The Wiki on the Australian Aurora Australis says "The vessel can break level ice up to 1.23 metres (4 ft 0 in) thick at 2.5 knots (4.6 km/h; 2.9 mph)".
That is moving nonstop. The ram and back-up capabilities is 21 feet thick. Also listed in this article.
Go with wiki, they are the authority on everything