Boating Through Ice: A Guide
Handling icebergs, sea ice, and sub-zero waters.
Ryder Bay, at the southern end of Adelaide Island, is littered with obstacles. In most parts of the world, such a landscape would be considered impassable by boat. But in Antarctica, the obstruction—ice—is a given. To avoid it entirely would mean abandoning boating altogether, and with it, a significant part of Britain’s marine polar science. Boat handling in and around ice is different to open water: our main focus is on maintaining safe distances from large ice structures, and protecting our propellers.
Types of Ice
Ice comes in two primary forms, decided by origin. Freshwater ice, mainly glacial, forms through shearing glaciers and calving ice cliffs. This type of ice is perennial and can drift with the wind across the boating area throughout both the austral summer and winter. Sea ice, by contrast, forms directly from seawater under calm but extremely cold winter conditions. While glacial ice is present all the time, it is often passable, with our boats able to push through. Sea ice forms for a much shorter period at our latitude, but makes sheets which are thick enough to fully stop our boating.
Icebergs
The largest freshwater ice features are icebergs. These can be easily identified by size—anything larger than a van qualifies. But that’s the smaller end of the scale: some icebergs span several miles and rise hundreds of feet above sea level. Once an iceberg reaches a size of twelve miles or more, it is named and tracked, although these are typically confined to deeper, open waters and don’t enter our boating area. For example, iceberg A23a, which is the size of Greater London, recently drifted to South Georgia, where two other BAS stations are located.
Icebergs are inherently unstable. Their buoyant, irregular bases mean they lack a natural righting mechanism, as a boat would have. Consequently, once they begin rocking, they often gain momentum and roll. Occasionally, a large chunk will calve off, destabilising the whole structure and causing a dramatic roll. When this occurs, the submerged ‘foot’ of the iceberg can lift with enough force to displace a small boat from the water.
Avoiding icebergs altogether is unrealistic, but the risk can be managed by keeping a safe distance. Our standard rule is to remain at least twice the iceberg’s height away from its foot. This submerged base appears light blue in the water and is usually easy to identify. Grounded icebergs (those in shallow water that aren’t floating) pose less of a risk and may be approached more closely if necessary. Conversely, tall, uneven, or visibly fractured icebergs are treated with extra caution and given greater clearance.
Rapid breakup is another risk. Heavily cracked icebergs, often appearing flaky and deep blue, tend to be older and structurally weaker. Fault lines widen from prolonged direct sunlight, particularly on warm days. When expansive enough, these fractures can cause the iceberg to break apart suddenly, sending chunks of various sizes drifting outwards. Some of these are large enough to damage a boat, others, heavy and travelling with momentum, pose a risk to divers and instruments in the water. There’s no fixed rule for managing this risk, other than judging where fragments might disperse if the iceberg breaks up.
Smaller Ice: Bergy Bits and Growlers
Following breakup, icebergs often leave behind smaller fragments known as bergy bits. Roughly the size of a car, these retain many of the same hazards as full-sized icebergs. Their lower profile means that they usually disintegrate with less force. Low-lying bergy bits can actually be moved by our boats. Pushing the front of the boat against a flat section of the ice, then powering forward enables us to clear them from areas we need to access.
Growlers are smaller still (no larger than the size of a grit bin) and typically much older. They are composed of glacial ice that has lost nearly all its air, leaving clear, dense ice. This clarity makes them hard to spot; their density makes them dangerous. Growlers are extremely hard and their low buoyancy reduces their profile in the water. Striking one at speed risks damaging the hull or propellers. The only realistic mitigation is to maintain a sharp lookout and reduce speed in areas where they might be present.
Sea Ice
Sea ice forms on the ocean’s surface during winter. In some parts of Antarctica, it becomes perennial and can reach depths of hundreds of metres. For instance, flights to BAS’s Fossil Bluff field station overfly sea ice around 300 metres deep. At the US’ McMurdo Station, sea ice persists year-round, requiring an icebreaker each season to clear a path for the refuelling ship. Rothera, by contrast, only experiences seasonal sea ice formation due to its relatively low latitude and longer periods of daylight through winter.
Unlike freshwater, seawater begins freezing at -1.8°C due to its salt content. A considerable depth of water, often over 100 metres, must drop to this temperature before ice will form. The earliest visible stage is ‘grease ice’, where fine ice crystals form in the surface layer, creating a thickened, slushy texture (shown in the video below). Grease ice has little effect on boating and is easily churned through by our propellers. As it thickens, it becomes increasingly visible, but is still minimally resistant.
If calm, cold conditions persist, this slushy layer thickens into fresh sea ice. Initially, the new ice is fragile. Winds and swells easily fracture it into smaller sections, which bump against one another, developing rounded edges. These discs are known as pancake ice. Over time, the pancakes raft together, eventually forming continuous ice sheets.
Sea ice thickens from below. In extreme cases, it becomes strong enough to support vehicles such as Skidoos. Multi-year ice develops when winter conditions allow ice to thicken significantly and summer conditions are too mild to melt it completely. Around Rothera, sea ice is seasonal and typically melts out early in summer.
Our rigid inflatable boats (RIBs) can travel through newly formed sea ice while it remains slushy. Thicker, rafted ice requires more effort. We often break through by pushing the bow up onto the ice so the boat’s weight can push down and through. This technique is slow and only clears a path as wide as the boat. This leaves us open to becoming stuck in the channel we’ve created. Pressure on the sides of the boat from surrounding ice can stop forward progress entirely. In such cases, manoeuvres like rocking the boat side-to-side may help, but avoidance is preferable.
Brash Ice Accumulation
Brash ice (essentially ice debris) frequently collects across the boating area. Unlike consolidated sea ice sheets, brash ice doesn’t bond together, so boats push it aside rather than break through it. Otherwise, many issues are similar. Pressure across brash ice fields can still be significant, especially when the wind is pushing it against land. A good indicator of high pressure is when the boat’s path rapidly closes behind us—usually this is the time to turn back.
Another concern is brash ice rapidly enclosing open water. Driven by wind and currents, these accumulations can sweep into previously ice-free zones. This can happen as fast as a walking pace. Fortunately, ice movement is predictable. At Rothera, northerly winds (common in the area) tend to fill the northern coves and beaches. Southerly winds do the same to the south. Accurate weather forecasts mean that we can plan our activities, and know that we often have really tight weather windows to access some areas of our boating area.
If conditions are favourable, it’s possible to navigate through brash ice. First, speed is reduced to protect the hull and engines. Large growlers often hide within these fields, adding further risk for a boat travelling at speed. We typically run on a single engine to limit potential damage and use the inner engine while turning—leaving the outer propeller stationary acting as a buffer. In some cases, we split the throttles, powering forward with one and reversing with the other to minimise stern swing into ice. Each of our boats carries a ‘bog chisel’: an excellent piece of kit for pushing large chunks of ice out of the way when needed.
Most importantly, a clear exit route must always be maintained. Reversing through brash ice risks drawing chunks through the propellers. Relying on the entry path for exit is also risky—it may close behind due to pressure, or the ice might be too dense to permit a 180° turn. The safest approach is to identify a completely separate exit route before entering ice-filled areas.
Not necessarily harder
Although ice is a challenge in itself, it isn’t insurmountable or a huge undertaking to tackle. It can move quickly, and seriously damage boats if misjudged. However, I think the reality of boating in the UK, with other traffic, water users, and narrow channels is often more difficult than dealing with ice. It all just adds interest to time afloat!