Question 1

Your vessel has a GM of 1.8m at departure. During the voyage, 200 tonnes of bulk cargo shifts 4m to starboard and 1m upward. Calculate the resulting list and GM.

Accepted Answer

CARGO SHIFT STABILITY CALCULATION: EFFECT OF A WEIGHT SHIFT: Moving a weight creates a shift of the Centre of Gravity (G) in the direction of the shift. The shift of G = (mass shifted × distance shifted) ÷ total displacement. GIVEN DATA: Mass shifted (w) = 200t; Distance shifted (d) starboard = 4m; Distance shifted upward = 1m; Original GM = 1.8m; Assume total displacement (W) = 20,000t (not given — assume for calculation). STEP 1 — HORIZONTAL SHIFT (GG1): GG1 horizontal = (w × d_horiz) ÷ W = (200 × 4) ÷ 20,000 = 800 ÷ 20,000 = 0.04m. This is the shift of G to starboard, causing a STATIC LIST to starboard. tan θ = GG1 / GM = 0.04 / 1.8 = 0.022. θ = arctan(0.022) = 1.3° list to starboard. STEP 2 — VERTICAL SHIFT (GG1): GG1 vertical = (w × d_vert) ÷ W = (200 × 1) ÷ 20,000 = 200 ÷ 20,000 = 0.01m UPWARD RISE OF G. New GM = original GM - GG1 vertical = 1.8 - 0.01 = 1.79m. RESULTS: List to starboard = approximately 1.3°; New GM = 1.79m (only marginally reduced). NOTE: this is a worked formula example — in the oral examination, the examiner tests whether the candidate understands the PRINCIPLES: (a) horizontal shift causes list; (b) upward shift reduces GM; (c) formula: tan θ = GG1 horiz / GM; (d) significance — 1.79m GM is still positive and adequate, 1.3° list is small. Cargo shift formula: the ability to calculate the effect of a cargo shift is a core STCW II/2 competency for Chief Mate certification. The formula (tan θ = GG1 / GM) is fundamental and must be understood, not just memorised. This type of calculation appears in virtually every Chief Mate oral examination and many written papers. IMSBC context: the calculation is especially relevant for bulk cargoes (grain, concentrates, fertilisers) where cargo shift is a recognised risk. The IMSBC Code (Group A and grain cargoes) establishes maximum shifting moments based on exactly these calculations. Correcting list by ballasting (option D): ballasting to the HIGH SIDE (transferring ballast water to the high side to counteract a list) is a common but potentially dangerous approach. The correct correction for a CARGO SHIFT list is to shift the cargo BACK. Filling high-side ballast tanks: (a) adds topweight if high tanks; (b) may improve the immediate list but leaves the cargo in the shifted position, which could shift further; (c) reduces GM if tanks are high. The only permanent solution is to restow the shifted cargo or, if at sea, to accept the list and monitor stability. Upward shift reducing GM: even a small upward shift of the Centre of Gravity reduces GM. In the calculation above — 0.01m GM reduction from 200t shifted 1m up on a 20,000t vessel. On a smaller vessel (5,000t) the same shift would reduce GM by 0.04m. For vessels with marginal GM, even small cargo shifts can be significant. Examiner expectation: in the oral exam, the examiner may give different displacement values and expect a worked calculation. The candidate should clearly show: (a) formula used; (b) each step; (c) interpretation of results. "Is 1.79m GM adequate?" — yes for most cargo vessels (IS Code criterion 0.15m GM minimum).

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.