Question 1

You are loading a timber deck cargo on a vessel assigned timber freeboard. As Chief Mate, describe the special conditions for the timber freeboard and the stability considerations.

Accepted Answer

TIMBER DECK CARGO AND TIMBER LOAD LINE: TIMBER LOAD LINE (TLL): the International Load Line Convention 1966, Annex I, Part IV (Timber Load Lines) provides for a REDUCED FREEBOARD for vessels carrying qualifying timber deck cargoes. The TLL mark is the "L" mark (Lumber) — e.g., LW (Lumber Winter), LS (Lumber Summer), LT (Lumber Tropical). Vessels can load deeper (less freeboard) when carrying qualifying timber because the deck cargo itself provides additional freeboard by preventing deck-level flooding. CONDITIONS FOR TIMBER FREEBOARD: the International Code of Practice for the Safe Carriage of Timber Deck Cargoes (TDC Code — adopted by Resolution MSC.287(87)) specifies: (1) TIMBER MUST COVER AT LEAST THE FREEBOARD DECK: the deck cargo provides the effective additional freeboard — if it doesn't cover the full freeboard deck continuously, the standard freeboard applies; (2) TIMBER MUST BE PROPERLY SECURED: lashings per the TDC Code (special timber lashing requirements); (3) SUPERSTRUCTURE PROTECTION: uprights of adequate strength and height; (4) WEATHERTIGHT CLOSURES: all access points to the deck below must be weathertight; STABILITY CONSIDERATIONS: (a) WINDAGE: timber deck cargo has large windage area — heeling moment from wind on the timber face must be included in stability calculation; (b) WATER ABSORPTION: green timber can absorb significant water, increasing topweight. Allow a water absorption allowance in the stability calculation; (c) SHIFTING: timber in heavy weather can shift. Lashings must accommodate some movement without failure; (d) GM: the IS Code criteria must be met after allowing for windage and water absorption effects. Initial GM for timber cargo is typically higher than the IS Code minimum. Water absorption effect: green (unseasoned) timber can absorb a significant percentage of its own weight in water. A 1,000-tonne timber deck cargo that absorbs 10% of its weight adds 100 tonnes of topweight during the voyage. This can substantially reduce GM. The TDC Code and IS Code require that a water absorption allowance be included in the stability calculation. Timber windage: a full timber deck load can have a windage area of 500-1,000m² for a typical cargo vessel. The heeling moment from wind on this area must be calculated and the resultant stability assessed against the minimum weather criterion (IS Code). Timber lashings — TDC Code: the TDC Code specifies the number, strength, and positioning of lashings for timber deck cargo. Key requirements: (a) lashings at each end and at intervals not exceeding 3m; (b) minimum SWL for each lashing; (c) use of uprights (stanchions) of specified height and spacing; (d) deckload restrictions. Timber on deck and stability (option D): this is a dangerous misconception. Timber on deck increases G (centre of gravity). Whether this helps or harms stability depends on the starting condition: (a) If the vessel is very stiff (high GM), adding topweight reduces excessive GM and improves sea-keeping; (b) If the vessel is at marginal stability, adding topweight reduces GM dangerously. Timber on deck NEVER improves stability if it was already at the minimum IS Code criteria. The timber "improves freeboard" (for load line purposes) — it does not improve stability.

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.