Question 1

After discharging ballast in preparation for loading, your vessel develops a 10° list which moves to the other side when the helm is put over. This is suspected to be an angle of loll. How does this differ from a list, and what is the correct remedy?

Accepted Answer

ANGLE OF LOLL — RECOGNITION AND REMEDY: WHAT IS AN ANGLE OF LOLL?: An angle of loll occurs when the vessel's INITIAL GM IS NEGATIVE (G is above M). A vessel with negative GM is unstable in the upright position — the righting moment at 0° is zero (or negative). The vessel seeks a position of stable equilibrium at a small angle of heel — this is the angle of loll. RECOGNITION: (1) The vessel lolls to one side but if disturbed (helm over, slight shift of weight) it moves to the SAME ANGLE on the OTHER SIDE. It is neutral/stable at the angle but not at zero; (2) Unlike a genuine LIST (caused by a transverse shift of G due to asymmetric loading): a list stays consistently to the same side regardless of helm; (3) The loll angle is typically 5-20°. HOW TO TELL THEM APART: LOLL: rolls between +10° and -10° (either side); LIST: consistently heels to one side, e.g., always 10° starboard. CORRECT REMEDY FOR ANGLE OF LOLL: (1) DO NOT BALLAST THE HIGH-SIDE DOUBLE BOTTOM TANKS FIRST: this is the critical danger. If you fill a DB tank on the HIGH side — the ship will suddenly flop violently to the loll angle on the LOW side. This violent rolling can shift any loose cargo or cause injury; (2) CORRECT APPROACH: (a) Identify the reason for negative GM. Was ballast discharged too quickly? Was high-weight cargo loaded without compensating ballast?; (b) ADD BALLAST LOW AND SYMMETRICALLY — fill double bottom tanks that are already on the low side (loll side) or fill both sides simultaneously; (c) Raise M by filling multiple low tanks; (d) Monitor the angle — as GM improves, the loll angle reduces. High-side ballasting danger — the critical trap: this is one of the most important stability knowledge tests in any Chief Mate oral examination. The instinctive response to "the ship is heeling to starboard" is to "fill the port tank." But with an angle of loll: (a) the vessel is AT STABLE EQUILIBRIUM at the loll angle; (b) adding weight to the HIGH side initially worsens the situation — as the weight first pushes the already-unstable vessel to heave; (c) when the port tank fills sufficiently to push G back below M, the vessel will suddenly and violently flop from the starboard loll angle to a port loll or even capsize in the process. The "sudden flop" can be catastrophic. Several capsize casualties have involved incorrect high-side ballasting response to angle of loll. Why lolls can move to either side: the vessel at angle of loll has a positive GZ at the loll angle but a small or negative GZ at the upright position. If disturbed (wave, helm movement), it can heel through the upright (where it is unstable) to the loll angle on the other side. This characteristic distinguishes loll from list. Discharging low-side ballast (option D): removing weight from the low side would shift G upward (if the tank is low), worsening the negative GM. It would also reduce overall displacement and could increase the loll angle. Discharging ballast when the vessel has negative GM is the wrong approach. The correct approach is to INCREASE KM (by filling low tanks) and ADD BALLAST LOW to reduce KG — improving GM from below.

Question 2

You are in the Bay of Bengal in October. Barometric pressure has fallen 8mb in 4 hours and a heavy swell is arriving from the SSW despite a cloudless sky. As Chief Mate on watch, what do you suspect and how do you respond?

Accepted Answer

TROPICAL CYCLONE APPROACH — RECOGNITION: SIGNS: an 8mb pressure drop in 4 hours plus an anomalous swell from a different direction than the current wind — these are classic early warning signs of a tropical cyclone. October in the Bay of Bengal is the second peak of the cyclone season (post-monsoon, September-December). BAY OF BENGAL CYCLONES: rotate COUNTER-CLOCKWISE (Northern Hemisphere). A swell from the SSW suggests a system to the SSW generating that swell. LOCATING THE CYCLONE: using Buys Ballot's Law (Northern Hemisphere): STAND WITH YOUR BACK TO THE WIND — the low pressure centre is to YOUR LEFT. So if the current wind is from the NNE (facing SSW) — the cyclone centre is approximately to the LEFT (ENE). MANOEUVRING RULES: for a vessel in the DANGEROUS SEMI-CIRCLE (DSC — right-hand semi-circle in Northern Hemisphere): alter to STARBOARD and run with the wind on the starboard quarter to move away from the path. For a vessel in the NAVIGABLE SEMI-CIRCLE (NSC — left-hand semi-circle): bring the wind on the starboard bow and run at best speed. For a vessel ON THE TRACK: bring the wind on the starboard quarter and run. IMMEDIATE ACTIONS: (1) CALL THE MASTER; (2) Obtain the latest GMDSS weather broadcast/NAVTEX; (3) Plot the swell direction and barometric pressure trend on the chart; (4) BEGIN MANOEUVRING in accordance with the semi-circle rules; (5) Secure all deck equipment, hatches, and loose gear; (6) Commence ballasting if stability margin is low. Barometric pressure rate of fall: in tropical regions, the normal diurnal variation in atmospheric pressure is approximately 1-2mb. A fall of 8mb in 4 hours (2mb/hour) is far outside normal variation and indicates the approach of a significant system. This rate of pressure fall is explicitly used in seamanship as a criterion for a developing tropical cyclone. Bay of Bengal cyclones: the Bay of Bengal is one of the world's most active cyclone basins with peak activity in May-June and October-December. Bengali cyclones are noted for their rapid intensification and the shallow Bay of Bengal amplifies the storm surge to devastating heights (the 1970 Bhola cyclone killed 300,000 people partly due to surge). Swell without cloud: tropical cyclones generate long-period swells that travel far ahead of the system — typically 24-36 hours ahead of the centre. The swell arrives before the cloud cover and wind, which is why experienced mariners treat an anomalous heavy swell as a cyclone warning even in clear weather. Buys Ballot's Law: this rule for finding the low pressure centre from the wind direction works in both hemispheres (modified for Southern Hemisphere — centre is to the right when facing into the wind). Every OOW and CM should know this and be able to apply it immediately. Course into swell (option C): course selection in cyclone avoidance is NOT simply "head into the swell." The correct course depends on which semi-circle the vessel is in. Heading directly into the swell might take the vessel toward the cyclone track rather than away from it. Buys Ballot manoeuvre must override the simple "head to sea" instinct.

Question 3

As Chief Mate, you receive a NAVTEX synoptic chart showing a 968mb deep depression 400nm to your NW, moving E at 25 knots. Your position is to the S of the low track. Describe the expected weather sequence.

Accepted Answer

SYNOPTIC CHART ANALYSIS — DEPRESSION WEATHER SEQUENCE: WARM SECTOR DEPRESSION STRUCTURE: the classic mid-latitude depression has: WARM FRONT (ahead of the warm sector), WARM SECTOR, and COLD FRONT. The vessel to the SOUTH of the depression track is in the NAVIGABLE (or "safe") SEMI-CIRCLE and will experience: APPROACHING WEATHER SEQUENCE: (1) VEERING WINDS: as the warm front approaches (from WSW to W to NW) — south of the track; (2) WARM FRONT PASSAGE: (a) Clouds sequence: Cirrus → Cirrostratus → Altostratus → Nimbostratus (the "classical" cloud sequence of an approaching warm front); (b) Rain: continuous moderate to heavy rain with poor visibility; (c) Wind: south to SW, backs slightly as warm front passes; (d) Pressure: steady fall; (e) Temperature: rise; (3) WARM SECTOR: (a) Moderate SW winds; (b) Drizzle and low cloud (sea fog risk if warm, moist air moves over colder sea); (c) Pressure temporarily stabilises; (4) COLD FRONT PASSAGE: (a) Wind veers rapidly from SW to NW or N; (b) Heavy showers, squalls, possibly thunder; (c) Pressure rises sharply; (d) Temperature falls; (e) Visibility clears rapidly; (5) AFTER COLD FRONT: NW winds, showers, good visibility between showers, cumulus cloud. 968mb depression = strong system. At 400nm at 25kt movement = 16 hours to reach the vessel position. Wind force — from distance/isobar spacing on chart: calculate wind force from isobar spacing. 968mb depression = likely Force 9-10 on the south semi-circle at closest approach. 16-hour arrival time: 400nm ÷ 25 knots = 16 hours. This is a real operational planning figure — 16 hours is enough time to: (a) prepare the vessel; (b) ballast/secure; (c) brief the crew; (d) assess whether to divert to shelter or run clear of the track. A CM on watch who correctly identifies this timing gives the master critical decision information. The vessel south of the track: in a Northern Hemisphere depression moving east — the vessel south of the track is in the LEFT (navigable) semi-circle. The warm front WILL pass over the vessel south of the track. The warm sector and cold front will also pass. The CM who says "no warm front because I'm south of the track" is confusing the depression track with the location of the warm front (which extends south of the centre). Isobar spacing and wind force: on a synoptic chart, the spacing between isobars (drawn at 4mb intervals) gives a relative indication of pressure gradient and therefore wind speed. Closely spaced isobars = strong gradient = strong winds. At 968mb in a mid-latitude depression, the warm sector to the south typically shows 4-6mb spacing — equivalent to Force 8-10 (gale to strong gale). Uniform wind Force 12 (option D): incorrect. Wind in a depression varies with location — strongest winds are to the right of the track (dangerous semi-circle) and in the depression core (around the warm/cold front boundary). South of the track, the winds are typically Force 8-10 in a deep system. Force 12 is generally confined to the core of the strongest systems.