Why recognising ice correctly matters more than simply seeing ice
Contents
Use the links below to jump to any section:
- Introduction – Not All Ice Behaves the Same
- New Ice – The False Sense of Safety
- First-Year Ice – Where Most Structural Damage Begins
- Multi-Year Ice – Old Ice, Extreme Risk
- Brash Ice & Broken Floes – Continuous Hidden Resistance
- Pressure Ridges & Rafted Ice – The Real Threat Beneath the Surface
- Ice Concentration & Coverage – Why “10% Ice” Can Still Be Dangerous
- Ice Colour, Texture, and What the Eye Misses
- Navigational Decision-Making Based on Ice Type
- Closing Perspective
- Knowledge Check – Sea Ice Types
- Knowledge Check – Model Answers
1. Introduction – Not All Ice Behaves the Same
From the bridge, ice often looks deceptively uniform.
White, grey, or broken — it is tempting to treat it as a single navigational problem.
In reality, ice type determines risk, not ice presence.
Two areas with identical ice coverage can present radically different hazards depending on ice age, thickness, strength, and internal structure. This is why experienced ice navigators ask what kind of ice they are in — not simply how much.
Misidentifying ice is one of the most common precursors to ice damage.
2. New Ice – The False Sense of Safety
New ice forms when calm, cold conditions freeze the sea surface. It includes frazil ice, grease ice, slush, and thin nilas.
New ice is:
- thin,
- flexible,
- easily broken by most vessels.
This creates a dangerous illusion of safety.
While new ice rarely causes structural damage, it dramatically increases resistance. Speed reduces earlier than expected, propeller wash becomes inefficient, and steering response dulls. The ship may still feel “in control” while power demand climbs sharply.
New ice often masks thicker ice beyond it. Vessels that accelerate through new ice frequently encounter first-year ice at unsafe speeds.
3. First-Year Ice – Where Most Structural Damage Begins
First-year ice is ice that has survived one winter but not a full melt season. It is thicker, harder, and far more dangerous than it appears.
Key characteristics:
- thickness typically up to 2 metres,
- significant compressive strength,
- sharp edges when fractured.
First-year ice resists breaking and transfers load directly into the hull. When a ship encounters uneven first-year ice, pressure concentrates at specific hull locations, especially forward shoulders and bilge areas.
Most merchant vessel ice damage occurs in first-year ice, not multi-year ice — simply because ships encounter it more often and underestimate it.
4. Multi-Year Ice – Old Ice, Extreme Risk
Multi-year ice has survived multiple melt seasons. Over time, brine drains out, leaving ice that is:
- denser,
- stronger,
- significantly harder than first-year ice.
Multi-year ice often appears smoother and less threatening than first-year ice. This is misleading.
Contact with multi-year ice can cause:
- severe hull deformation,
- propulsion damage,
- immediate loss of watertight integrity.
For non-ice-class vessels, multi-year ice is effectively a hard obstruction, not a navigable medium.
Avoidance is the only safe strategy.
5. Brash Ice & Broken Floes – Continuous Hidden Resistance
Brash ice consists of broken ice fragments packed together by wind or current. It is common in trafficked routes, icebreaker tracks, and near shore.
While individual pieces may seem small, brash ice creates continuous resistance.
Effects include:
- constant propeller loading,
- vibration and cavitation,
- accelerated fuel consumption,
- reduced steering response.
Brash ice is exhausting for machinery and crews alike. Progress becomes slow, noisy, and mentally fatiguing — increasing the risk of decision-making errors.
6. Pressure Ridges & Rafted Ice – The Real Threat Beneath the Surface
Pressure ridges form when ice sheets collide and buckle. Rafted ice forms when one sheet overrides another.
What matters is not what is visible, but what lies below the waterline.
A ridge that appears modest above the surface may extend several metres downward. Striking submerged ridge keels has caused significant hull breaches, even at low speed.
Pressure ridges are especially dangerous because they:
- are difficult to detect visually,
- appear irregular on radar,
- often lie across otherwise navigable leads.
Ice navigation failures often occur when a vessel commits to a lead without recognising embedded ridging.
7. Ice Concentration & Coverage – Why “10% Ice” Can Still Be Dangerous
Ice charts often describe concentration as a percentage. This can be misleading.
Ten percent ice coverage does not mean ten percent risk.
If that ten percent consists of:
- thick first-year ice,
- pressure ridges,
- multi-year fragments,
then the navigational hazard is extreme.
Conversely, higher concentrations of thin new ice may be manageable.
Ice concentration must always be interpreted alongside ice type and distribution.
8. Ice Colour, Texture, and What the Eye Misses
Experienced navigators learn to read subtle visual cues.
Colour variations indicate age and strength. Bluish tones often suggest older, harder ice. Grey and white textures can mask thickness differences.
However, visual interpretation is unreliable in poor light, snowfall, or flat illumination. Ice radar, satellite imagery, and ice reports must supplement the human eye — but never replace cautious judgement.
9. Navigational Decision-Making Based on Ice Type
Ice type determines:
- safe speed limits,
- whether escort is required,
- when to abort a route,
- acceptable hull contact risk.
Professional ice navigation is conservative by design. It assumes uncertainty and plans margins accordingly.
The correct response to dangerous ice is rarely heroic ship-handling. It is usually earlier avoidance, reduced speed, or refusal to proceed.
10. Closing Perspective
Ice is not dangerous because it is cold.
It is dangerous because it is variable, deceptive, and unforgiving.
Ships do not fail in ice because they encounter ice.
They fail because they encounter the wrong ice with the wrong assumptions.
Recognising ice correctly is the first step in preserving margins — and margins are the only real safety currency in polar navigation.
11. Knowledge Check – Sea Ice Types
- Why is new ice considered deceptively dangerous?
- Which ice type causes most merchant vessel damage and why?
- Why is multi-year ice especially hazardous to non-ice-class ships?
- How does brash ice affect propulsion and steering?
- Why are pressure ridges more dangerous than they appear?
- Why is ice concentration alone an unreliable risk indicator?
- How does ice colour provide navigational clues?
- What decisions should ice type directly influence?
12. Knowledge Check – Model Answers
- Because it increases resistance while appearing easy to navigate.
- First-year ice, due to its strength and frequency of encounter.
- It is denser, harder, and behaves like a rigid obstruction.
- It creates continuous resistance, vibration, and control degradation.
- Because submerged keels extend far below the visible ridge.
- Because small amounts of hard ice can present extreme hazard.
- Colour can indicate age, density, and strength.
- Speed limits, routing, escort requirements, and abort criteria.