Readiness for Ice
Experience has shown that ships that are not ice-strengthened and with a speed in open water of about 12 knots often become firmly beset in light ice conditions, whereas an adequately powered ice-strengthened ship should be able to make progress through 6/10 to 7/10-first-year ice.
The engines and steering gear of any ship intending to operate in ice must be reliable and capable of quick response to maneuvering orders. The navigational and communications equipment must be equally reliable and particular attention should be paid to maintaining radar at peak performance.
Ships operating in ice should be ballasted and trimmed so that the propeller is completely submerged and as deep as possible, but without excessive stem trim which reduces maneuverability. If the tips of the propeller are exposed above the surface or just under the surface, the risk of damage due to the propeller striking ice is greatly increased.
Ballast and fresh water tanks should be kept not more than 90 per cent full to avoid risk of damage to them from expansion if the water freezes. Good searchlights should be available for night navigation, with or without icebreaker escort.
Signs of Icebergs
Caution. There are no infallible signs of the proximity of an iceberg. Complete reliance on radar or any of the possible signs can be dangerous. The only sure way is to see it.
Unreliable signs. Changes of air or sea temperature cannot be relied upon to indicate the vicinity of an iceberg. However, the sea temperature in the North Atlantic, if carefully watched, will indicate when the cold ice-bearing current is entered.
Echoes from a steam whistle or siren are also unreliable because the shape of the iceberg may be such as to prevent any echo, and also because echoes are often obtained from fog banks.
Sonar has been used to locate icebergs, but the method is unreliable since the distribution of water temperature and salinity, particularly near the boundary of a current, may produce such excessive refraction as to prevent a sonar signal from reaching the vessel or iceberg.
Likely signs. The following signs are useful when they occur, but reliability cannot be placed on their occurrence.
In the case of large Antarctic icebergs, the absence of sea in a fresh breeze indicates the presence of ice to windward if far from the land. When a growler breaks away, or ice otherwise cracks and falls into the sea, it produces a thunderous roar, or sounds like the distant discharge of guns.
The observation of growlers or smaller pieces of detached ice is an indication that an iceberg is in the vicinity, and probably to windward; an iceberg may be detected in thick fog by this means. When proceeding at slow speed on a quiet night, the sound of breakers may be heard if an iceberg is near and should be constantly listened for.
Visibility of icebergs. Despite their size, icebergs can be very difficult to see under certain circumstances, and the Mariner should invariably navigate with caution in waters in which they may be expected. In fog with sun shining an iceberg appears as a luminous white mass, but with no sun it appears close aboard as a dark mass, and the first signs may well be the wash of the sea breaking on its base.
On a clear night with no moon icebergs may be sighted at a distance of 1 or 2 miles, appearing as black or white objects, but the ship may then be among the bergy bits and growlers often found in the vicinity of an iceberg. On a clear night, therefore, lookouts and radar operators should be particularly alert, and there should be no hesitation in reducing speed if an iceberg is sighted without warning.
On moonlit nights icebergs are more easily seen provided the moon is behind the observer, particularly if it is high and full. At night with a cloudy sky and intermittent moonlight, icebergs are more difficult to see and to keep in sight. Cumulus or cumulo-nimbus clouds at night can produce a false impression of icebergs.
Signs of Drift Ice
There are two reliable signs of drift ice.
Ice Blink whose characteristic light effects in the sky once seen, can never be mistaken, is one of these signs. On clear days, with the sky mostly blue, ice blink appears as a luminous yellow haze on the horizon in the direction of the ice. It is brighter below, and shades off upward, its height depending on the proximity of the ice field. On days with overcast sky, or low clouds, the yellow colour is almost absent, the ice blink appearing as a whitish glare on the clouds. Under certain conditions of sun and sky, both the yellowish and whitish glares may be seen simultaneously. It may sometimes be seen at night.
Ice blink is observed some time before the ice itself appears over the horizon. It is rarely, if ever, produced by icebergs, but is always distinct over consolidated and extensive pack.
Abrupt smoothing of the sea and the gradual lessening of the ordinary ocean swell is the other reliable sign, and a sure indication of drift ice to windward.
Other likely signs include the presence of isolated fragments of ice which often point to the proximity of larger quantities.
There is frequently a thick band of fog over the edge of drift ice. In fog, white patches indicate the presence of ice at a short distance.
In the Arctic, if far from land, the appearance of walruses, seals and birds may indicate the proximity of ice.
In the Antarctic, the Antarctic Petrel and Snow Petrel are said to indicate the proximity of ice–the former being found only within 400 miles of the ice edge, and the latter considerably closer to it.
Sea surface temperatures give little or no indication of the near vicinity of ice. When, however, the surface temperature falls to +1° C, and the ship is not within one of the main cold currents, the ice edge should for safety be considered as not more than 150 miles distant, or 100 miles if there is a persistent wind blowing off the ice, since this will cause the ice temporarily to extend and become more open. A surface temperature of -0.5°C should generally be assumed to indicate that the nearest ice is not more than 50 miles away.
Detection of Ice by Radar
Though an invaluable aid, the limitations of radar in detecting ice must always be borne in mind. Absence of an indication of ice on the radar screen does not necessarily mean that there is no dangerous ice near the ship. The strength of the echo received from an iceberg depends as much on the inclination of its reflecting surfaces as on its size and range. When approaching the ice edge a continuous visual lookout is essential. Operators must be aware of the limitations given below and that less than full operating efficiency will greatly reduce the chance of detecting ice.
The following conclusions have been reached from recent experience, but abnormal weather conditions may substantially reduce detection ranges.
In a calm sea, ice formations of all sorts should be detected; from large icebergs at ranges of from 15 to 20 miles down to small growlers at a range of possibly 2 miles. However, bergy bits weighing several tons, and protruding up to 3 m out of the water, are unlikely to be detected at a range of more than 3 miles. As warning of ice may therefore be short, radar should be operated continuously in low visibility where ice is expected.
In rough weather, it is unsafe to rely on radar when sea clutter extends beyond 1 mile, as insufficient warning will be given of the presence of growlers large enough to damage the ship, or of drift ice.
Fields of concentrated hummocked ice should be detected in all sea conditions at a range of at least 3 miles.
Ridges show clearly, but shadow areas behind ridges are liable to be mistaken for leads or the closed tracks of ships, and the large area of weak echoes given by a flat floe may be mistaken for a polynya. It is difficult to distinguish between 10/10 hummocked or rafted ice and 3/10 small floe and ice cakes.
Large floes in the midst of brash ice will usually show on radar.
A lead through static ice will not show on radar unless the lead is at least a mile wide and completely free from brash ice.
Areas of open water and smooth floes appear very similar, but in an ice field the edge of a smooth floe is prominent, while the edge of open water is not.
Snow, sleet and rain squalls can sometimes be detected. Lookouts can then be increased, or speed or course altered to avoid the squalls.
Signs of Open Water
Water sky, distinguished by dark streaks on the underside of low clouds, indicates the direction of leads or patches of open water. A dark band on the cloud at a high altitude indicates the existence along this line of small patches of open water which may connect with a larger distant area of open water. If low on the horizon, water sky may possibly indicate the presence of open water up to about 40 miles beyond the visible horizon.
Dark spots in fog give a similar indication, but are only visible at shorter distances than reflections on clouds.
The sound of a surge in the ice indicates the presence of large expanses of open water in the close vicinity.
Effects of Abnormal Refraction
Ice or open water in the distance may often be detected by super-refraction raising the horizon. The image of the ice or areas of open water, or a mixture of the two, may be seen as an erect or inverted image. Alternatively, both images may be seen at once, one above the other and usually in contact, in which case the erect image is the higher of the two. Allowance must be made for the fact that the refraction causing the mirage will increase the apparent dimensions of small ice, sometimes so greatly as to make small pieces appear like icebergs. The areas of open water are dark relative to the ice.