• Propeller Application of Icebreaker

Q: How do ice-breaking ships keep from breaking their propellers on the ice?

A: Sometimes, things break. Props can break. Shaft seals might be a bigger worry than props. A small subset of ice class vessels are icebreakers and for good reason.

Icebreaking is for the wise, experienced, and cautious but is no place for the timid.

A vessel operator must use the breaking modes, the procedures, the very specific ice-breaking-ballast-trim that is specified in a vessels design when icebreaking. Even then, twanging a prop happens.

While not ice breakers, the ice class ferries MV Caribou and MV Smallwood both experienced prop damage during their service life in-spite of having their propulsion relatively deep.

One method in protecting the propellers from ice is the hull design where the flow of water to the propulsors is ice reduced.

In order to push a vessel forward with power, a significant amount of water must flow over the hull and through the props. Ice bits are unavoidibly sucked in with this water. Ice propeller interaction is a major source of propulsion inefficiency in ice class vessel propulsion. It is not only common but is integral to many modern designs. However, most iceprop interaction for ice class vessels are for broken ice debris. Traversing a ridge, which maybe several ice sheet thicknesses thick, all but guarantees the props experiencing ice.

While it should be avoided for most ice class vessels, backing is a common mode of ice failure for icebreakers. The star turn maneuver is common for completing a 180 in level ice and is one of many tactics in "breaking out" vessels. Older "conventional ice breakers" make use of Icehorns, projections just aft of the rudders and props. A conventional icebreaker might have an ice strengthened shaped prop but still fabricated out of propeller brass. These shapes are less efficient but much more resilient. They are not indestructible.

Prop forward or tractor mode in Podded propulsor operation has proven it's worth in the marine world.

What is an intimidating act of trust in modern technology, many "Modern" icebreaking vessels, like the MT Mastera, fitted with azipods often use "milling" to prevent strut loading when traversing sternward to employ their "spoon stern". Milling props are often stainless steel and are very expensive.