Tropical cyclone avoidance decision in the Arabian Sea
MV Dena (Handysize, loaded with 24,000 t of clinker, GM 2.6 m, a stiff ship given to heavy rolling) sailed from Bandar Abbas at 0600 bound for Salalah, steering 205 degrees at 12 knots. The barometer read 1007.2 hPa at 0600, 1004.3 hPa at 0900 and 1001.6 hPa at noon. The wind was from 130 degrees at force 5 at 0600 and from 145 degrees at force 7 by noon, with a heavy south-easterly swell of 3 m. The last warning available is 12 hours old and places a cyclonic storm about 200 miles south-west of the vessel, moving west-north-west at 8 knots, with forecast intensification. The radar, range 48 miles, shows no echo. The vessel has fuel for five days at economical speed and the charterers require arrival within 48 hours. The second officer suggests continuing, since the centre is behind the vessel.
1. Using Buys Ballot's law and the observed wind, determine the bearing of the centre from the vessel at noon and identify the semicircle she is in. What does the pressure trend add?
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Bearing of the centre In the Northern Hemisphere, stand with your back to the wind and the low centre bears roughly 90 degrees to your left. At noon the wind is from 145 degrees, so facing downwind is 325 degrees and the centre bears about 235 degrees, south-west, at roughly 90 degrees to the left. That corroborates the 12 hour old warning position.
Semicircle The wind has veered from 130 to 145 degrees, a clockwise shift, while the pressure has fallen. In the Northern Hemisphere a veering wind places the vessel in the right-hand semicircle of the track, the dangerous semicircle, where the storm's translation reinforces the wind and sea. Geometry agrees: the centre bears south-west and tracks west-north-west, so the vessel lies north-east of the centre, to the right of the track looking along the direction of advance.
What the pressure adds A fall of about 2.8 hPa in three hours is steep, roughly 2.7 hPa per three-hour interval. Even allowing for the storm's own deepening, that rate is not consistent with a system as far away as 200 miles; the master should assume the centre is closer than the bulletin and is still approaching or deepening. The building south-easterly swell arriving ahead of the system confirms the proximity.
Closing rate if nothing is done Steering 205 toward a centre bearing 235 closes the range at 12 cos 30 = 10.4 knots; the centre recedes at 8 cos 57.5 = 4.3 knots. Net closing is about 6 knots, so the range falls from 200 to about 128 miles in twelve hours. The second officer's idea that the centre is behind the vessel is the reverse of the truth: the ship is steering into the dangerous semicircle.
2. Recommend the avoidance manoeuvre, course and speed, and justify it in terms of the semicircle, the storm track and the vessel's behaviour in a seaway.
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Aim of the manoeuvre Three objectives: open the range from the centre, stay well clear of the forecast track rather than attempting to cross ahead of it, and keep the wind and sea from a relative direction that this stiff ship can tolerate.
Course In the Northern Hemisphere right-hand semicircle the classic manoeuvre is to bring the wind on the starboard quarter and proceed with all speed, altering as the wind hauls. With the wind from 145 degrees that means a course of about 010 to 020 degrees. Turning from 205 to about 010 achieves two things at once: it is a turn away from the centre bearing 235, and it puts the ship north-east of the track and running clear of it. Recompute the relative wind after every 20 degrees of change, because the wind direction is itself moving.
Speed Maximum practical, limited by slamming and cargo security, not by fuel. Because GM 2.6 m makes this a stiff, snappy roller, a quartering or following sea from the starboard quarter is far safer than putting the wind and swell on the beam. If rolling becomes violent, make a small alteration that brings the wind a little further aft, but never put the wind ahead of the beam, which would steer the ship into the dangerous side.
What to avoid Do not attempt to cross ahead of the storm's track to reach Salalah on schedule; a vessel that loses speed or suffers damage in the crossing becomes the one the storm runs down. Do not run before the wind with the centre directly astern in the belief that opening the range is all that matters; the track may curve.
Commercial Inform the charterers that a 24 to 48 hour delay or a diversion is likely, and consider an intermediate port or an anchorage clear of the track if the storm's forecast path turns east.
3. What information should the master obtain, and what should the bridge team and crew do over the next 24 hours?
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Information Obtain the latest bulletins from the regional specialised meteorological centre responsible for the North Indian Ocean (New Delhi) and from EGC SafetyCAST or NAVTEX if in coverage, plus any port meteorological office or weather routeing service. Extract the forecast track and its uncertainty cone, the movement and intensity trend, and the radius of 34, 50 and 64 knot winds in each quadrant; also obtain the pressure and wind trend for the Oman coast. The 48 mile radar may reveal rain bands or the eye wall even when the echo is weak, so set it up as a weather-detection aid as well as an anti-collision aid.
Ship Hourly barometer readings plotted by hand so the trend is visible, with wind direction and force, sea and swell direction, and every observation logged. Secure the deck, check hatch covers, lashings and the crane, close all watertight openings, rig lifelines, and keep the engine, steering gear and generators ready for immediate manoeuvring. No deck work once the sea builds, and never by night in the storm sector. Confirm the EPIRB and communications.
Bridge team The master should be on the bridge through the critical phase, with extra lookouts, a written night order that meets the master in any doubt, and managed rest so that the officers are not all fatigued when they are needed. Brief the crew on the plan and the alternative ports. Beware the trap of the eye: a sudden lull does not mean the storm is over, because the second half arrives from the opposite direction with equal severity.