Thickness of the hull steel is only one important consideration for an icebreaker. Steel quality is most important as it must be able to withstand not only heavy pounding but also extremes of temperature.
The Titanic was vulnerable because of the steel used in her hull construction. "A metallurgical analysis of steel taken from the hull of the Titanic's wreckage reveals that it had a high ductile-brittle transition temperature, making it unsuitable for service at low temperatures; at the time of the collision, the temperature of the sea water was -2°C. The analysis also shows, however, that the steel used was probably the best plain carbon ship plate available at the time of the ship's construction."
Steel making and metallurgy have advanced greatly since the early 1900s.
The framing that holds the hull plates also is required to be robust and to be closer spaced than on ordinary ships. One of the frames in the Risley's bow area is shown in this photograph, with my thumb as reference.
http://www.boatnerd.com/pictures/specia ... 1-1-01.jpg
Another important, and often overlooked feature, is the hull coating. Icebreakers generally have a purpose-made hull coating that allows them to slip through the ice with less friction than ordinary painted, or rusted, hulls. As this coating breaks down with use and age the icebreaker has more difficulty moving through the ice. More power is used to overcome friction than to continue moving. In this photo the black coating below the ice moustache is "Inerta", and it is applied to reduce friction.
http://www.boatnerd.com/pictures/specia ... 1-3-01.jpg
How much ice a breaker can move through at continuous speed is somewhat like miles-per-gallon calculations. Very much ideal and mostly imaginary with no reflection of reality.
The sister-ship of the Risley, CCGS Earl Grey, works in the Atlantic and Gulf of St Lawrence. In order to deal with the ocean weather conditions she is ballasted far more deeply than the Risley with the result that her performance in ice may be significantly different.
Ice conditions determine much of the capability of the breakers. Fast ice, that is water that has frozen from shore to shore and is perfectly flat, is fairly easy to break through. At a certain point in the ice growth, as it gets thicker and harder, the breaker's bow will actually ride up onto the ice and the weight and inertia will break through the ice.
If you put a little snow on the ice much of the breaker's power now goes into trying to push through the snow and progress is minimised, especially if the snow is wet or the hull coating is compromised.
Ridging or Windrows make passage through the ice much more difficult. In the case of Port Colborne the prevailing winds over the winter usually build a large pressure ridge outside the canal entrance and this will get deeper and wider and more frozen as winter progresses. It may even reach the bottom of the lake. Not only is this difficult to break through, usually you chip away at it by backing and filling, you must be concerned about the bow riding up which drives the stern down and risks damaging the props and rudders should they come into contact with the lake bottom.
In the 90s when I was Mate on the Earl Grey we were escorting the Diamond Star (Now the Dara Desgagnes) into the Miramichi River in New Brunswick. The Grey got stuck in a very heavy ridge. We could not avoid it as the navigable channel was quite narrow. Despite the best efforts of the Star's Captain the ship hit the stern of the Grey with a tremendous blow, and pushed us through the ridge. It also put a rather large dent in the stern roller, which is very heavy steel and probably absorbed most of the force thereby preventing even more damage. It threw me out of my chair and onto the deck! Once everyone had made their various checks for damage and changed their underwear we noticed that the impact had launched the Star's free-fall lifeboat and it was merrily floating around in the only bit of open water within a hundred miles.
Another consideration is where the ice is. In the St Mary's and Detroit/ St Clair rivers once the ice is broken is has nowhere to go. Further downstream is more ice and that prevents broken ice from moving along. As more and more ships pass through this ice it gets more and more chewed up until it has the consistency of very cold porridge, and also has become much deeper in the channel. No amount of icebreaking will make conditions better and escorting ships through, as shown in the St Clair Cut this winter, becomes nearly impossible.
Did I mention wind and current and tides? These greatly affect ice conditions and subsequent ice navigation.
As I write this I am holed up in a B & B in Summerside, Prince Edward Island watching a blizzard wreak havoc outside my window. Not too far from here is the Canadian Icebreaker Henry Larsen and Irving's tanker Acadian. The Larsen is a class 1200 icebreaker with 15,783 hp. It spends every summer in the Arctic. She was sent to assist the Acadian because the ice conditions in Northumberland Strait are getting worse, or better depending on your point of view, each day. This storm, with its 100+ kph (60 MPH) winds will greatly change the ice conditions and will adversely affect the escort when it resumes. Then the tide will go up and down every 6 hours or so and will, at times, make the ice cover so tight that even the Larsen will not be able to move through it.
So, to answer your question, how thick is the hull on the Risley? I forget!
Enjoy the ice.
Paul
