Tropical Cyclone Avoidance in the Bay of Bengal
At 0600 on 12 August, your vessel is in position 18°00′N 087°00′E, steering 090° at 12 knots. The barometer has fallen from 1008 hPa at 1800 yesterday to 1002 hPa now and is still falling about 1.5 hPa per hour. Wind is NE force 5, sea moderate, but a long south-easterly swell is arriving and cirrus is thickening. NAVTEX carries a tropical cyclone warning: centre 15°00′N 087°00′E, moving NW at 8 knots, maximum winds 60 knots, radius of 50-knot winds 40 nm. Your next port is 300 nm to the east. The master asks you to recommend a course of action. The vessel has a service speed of 12 knots but can make 9 knots in heavy weather. There is no other traffic. The crew are preparing for heavy weather.
1. Plot the relative position and state which semicircle the vessel is in. What signs confirm that the cyclone is approaching?
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Plot the centre at 15°N 087°E and the vessel at 18°N 087°E: the centre bears 000°T at a distance of 180 nm. The track is NW (315°). In the northern hemisphere the dangerous semicircle is the right-hand side of the track; because the vessel is north of the centre while the centre moves NW, the vessel lies on that right-hand side and is in the dangerous semicircle. The 50-knot radius is only 40 nm, so the vessel is outside the core, but the centre is moving NW and the vessel is ahead of it. The falling barometer (about 1.5 hPa per hour), the long south-easterly swell and the thickening cirrus confirm an approaching cyclone. The immediate risks are increasing wind and sea, rapid wind shifts, reduced visibility in rain, and the danger that a course change toward the west would put the vessel closer to the projected track. The vessel is not yet in the 50-knot wind radius, but the situation can deteriorate within 12 to 24 hours.
2. Recommend a course and speed, and explain the reasoning. What would you monitor and report?
Read the worked answer
Alter to the east or south-east, about 110°T, and increase to the maximum available speed, keeping the wind on the starboard side as it veers. Reason: the vessel is in the dangerous semicircle, so the aim is to increase distance from the projected track and avoid crossing ahead of the centre. The centre is moving NW at 8 knots; a course east or south-east at 12 knots opens the range, whereas holding west or turning toward the centre would reduce it. Do not turn to pass close ahead of the storm. In heavy weather reduce speed only if slamming or damage forces it. Monitor the barometer hourly, wind direction and force, swell, position every hour, NAVTEX and weather charts, radar for rain squalls, and the vessel's roll and stress. Secure deck cargo, close watertight openings, check bilges and free surface. Report to the master and company, and send a position report to the next port. If the wind shifts and the centre bears differently, re-plot and adjust, because the semicircle rule is a guide, not a fixed course.