Polar Low and Icing on a Northbound Coastal Passage
M/V Fjordhavn, a 5,000 GT general cargo ship, is northbound off Helgeland at 0300 in February. The 0600 synoptic chart shows a 985 hPa low near 67°N 008°E deepening 8 hPa in three hours. Air temperature is -2°C, sea temperature 4°C, wind NE 35 kn gusting 50 kn. Heavy spray is breaking over the forecastle, and ice is accumulating on the forward mast and hatch covers. The vessel has a 2° starboard list from deck cargo and a calculated GM of 0.45 m. Next port Bodø is 90 nm ahead; shelter at Rørvik is 60 nm to starboard. Visibility is 3 nm and falling. Fuel is sufficient for 48 hours at service speed. The forecast mentions a possible polar low. The master must decide whether to press on or divert.
1. Interpret the meteorological and icing risk, including the effect on stability.
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The 8 hPa deepening in three hours, cold air at -2°C over a sea at 4°C, and NE 35 kn gusting 50 kn indicate a rapidly developing polar low, not a normal gale. Convective snow squalls and rapid wind shifts are likely. Icing is the critical hazard: air below freezing, a relatively warm sea, strong wind, and heavy spray produce heavy spray icing on the windward side and forward. Ice on the mast and hatch covers adds weight high above the centre of gravity, raises VCG, and reduces effective GM. The existing 2° starboard list will worsen if ice accumulates asymmetrically. GM 0.45 m is positive, but it is not generous for a 5,000 GT ship with deck cargo and topside ice. Visibility falling to 3 nm in snow further reduces the margin.
2. What route and speed decision would you make, and what stability measures would you take?
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Divert to Rørvik. It is 60 nm away against 90 nm to Bodø and gives shelter from the NE gale, reducing spray and icing. The approach in a polar low carries navigational risk, so keep well off the lee shore, use radar, reduce speed, and have the anchor ready. Alter course to put wind and sea on the quarter to reduce spray, and reduce speed to 8–10 kn to reduce shipping water. Secure all crew from exposed decks. De-ice the forward mast and hatch covers only with safe access and harnesses.
Monitor list, GM, and freeboard. Use the stability computer before any ballast transfer; ballast can lower VCG and correct list, but it also adds weight. Check lashings and freeing ports. If icing continues and list increases, heave to or enter the nearest shelter immediately. Avoid beam seas, which combine with ice weight and list.
3. What communications and onboard precautions should follow?
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Report the diversion and weather conditions to coast radio or VTS, and send a weather report if the polar low is not well captured by the forecast. Brief the crew on no unnecessary deck work, use of harnesses, and the icing monitor. Keep a close watch on the barometer and wind shifts. Prepare for a rapid wind shift by keeping the engine ready and the anchor available. Log the decision, the reasons, and the times. Inform the office and the next port of the revised ETA.