Mackinaw: Designed for Icebreaking

Designed for Icebreaking

The Mackinaw has several interesting features that contribute to her icebreaking efficiency.

She is constructed with a protective ice belt, formed from steel plate 1-5/8 inches thick. During construction, a full size mold of the hull was built, and heated steel plates, white-hot, were hammered into shape against it. The plates, when cooled, were welded together to form the lower hull.

The heavy icebreaker also has a unique arrangement of propellor shafts. In addition to her two stern shafts with 14-foot diameter props, she also has a forward shaft which turns a 12-foot diameter bow prop. This bow screw serves two purposes when icebreaking. First, it draws water out from under the ice, which causes the ice to sag under its own weight and weaken. Then, as the water streams aft along the sides of the hull, it reduces friction between the ship and the ice. When underway in normal conditions, the forward propellor is left to free-wheel. To prevent ice damage to the shafts, they are encased in protective steel housings.

A third icebreaking special design is the ship's heeling system. By an arrangement of ballast tanks and pumps, the Mackinaw can shift over 100,000 gallons of ballast water from side to side in 90 seconds. The rocking motion that results will free the ship if stuck in ice.

Because of these special features and her considerable breadth (74-ft, 4-in Beam), the Mackinaw can clear a path in ice twice as wide as that of a 140-footer like the USCGC Bristol Bay.