Question 1

In head seas at 12 knots, the vessel begins slamming badly — severe impacts every few waves, causing the whole ship to shudder. What is the risk and what is the OOW response?

Accepted Answer

SLAMMING — risk and response: WHAT IS SLAMMING: slamming occurs when the bow lifts clear of the water and then re-enters heavily, causing impact loads many times greater than normal wave loads. It occurs in head seas when the bow emerges and re-enters a wave. RISKS: (1) STRUCTURAL: repeated slamming can cause fatigue cracking in the bow structure, forecastle deck, and stem plating. High-stress areas: forecastle head, bow plating, forward hold framing; (2) PROPELLER RACING: if the stern lifts in following seas — propeller emerges, races at excessive RPM then re-enters causing cavitation and shaft stress; (3) CREW: severe physical stress on personnel; (4) CARGO: violent impacts can cause cargo shift even in well-lashed stow. OOW RESPONSE: (1) REDUCE SPEED immediately: reducing speed is the primary response — at 8-9 knots slamming often reduces significantly; (2) CALL MASTER: slamming severe enough to shudder the vessel must be reported; (3) ALTER COURSE: even 10-15 degrees off dead head-on significantly reduces slamming; (4) POST-SLAMMING INSPECTION: if slamming was severe, inspect forward spaces at first safe opportunity — check for structural cracking, water ingress in forepeak or forward hold; (5) LOG: time, sea state, speed reductions made, and any structural observations; (6) CHIEF ENGINEER: advise if propeller racing has occurred — engine rpm fluctuations should be recorded. Slamming loads: the impact pressure during a severe slam can be 100-300 kPa — far exceeding the design wave loads on the vessel's structure. Repeated slamming causes metal fatigue. Several vessel losses have been attributed to slam-induced structural failure, particularly in vessels with fine bow forms. Propeller racing: when the stern lifts in following seas, the propeller may emerge partially or fully. The sudden loss of water resistance causes the propeller to race to very high RPM. When it re-enters, the sudden load can cause shaft damage, coupling failure, or even propeller blade damage. Modern vessels have automatic RPM governing systems to limit racing. The forward inspection after heavy slamming: the forepeak tank and forward cofferdam should be sounded (checked for water ingress) after severe slamming. If water is found in spaces that should be dry, structural damage is indicated — a serious finding requiring engineering assessment and potentially a port of refuge.

Question 2

The forecast shows Force 9 developing in 12 hours. You are OOW on a general cargo vessel. What checks and actions do you take regarding cargo securing before the weather arrives?

Accepted Answer

PRE-HEAVY WEATHER CARGO SECURING CHECKS: (1) CARGO SECURING MANUAL: consult the vessel's approved Cargo Securing Manual (required under SOLAS VI/5.6 and CSS Code) — check lashing specifications for cargo type and sea conditions; (2) ROUND OF HOLDS: physically inspect all cargo lashings — check turnbuckles are tight and locked; check twist locks on containers; check blocking and bracing in tween decks; check that timber deck cargo lashings are tight; (3) HATCH COVERS: verify all hatch covers are properly closed, battened, and cleat dogs tightened. Check compression bars for integrity. Any hatch that leaked previously — doubly inspect; (4) DECK FITTINGS: secure all moveable deck equipment — liferaft cradles checked, anchors properly housed and lashed, spare parts containers secured, paint lockers locked; (5) WEATHER DECK: walk the main deck — nothing left unsecured. A 25-litre oil drum loose in heavy weather becomes a lethal projectile; (6) NOTIFY MASTER: report inspection result and any concerns; (7) LOG: record the inspection, lashings checked, any adjustments made; (8) ADDITIONAL LASHINGS: if existing lashings are borderline, add preventive lashings now — much easier before the weather than in it; (9) VESSEL SPEED/HEADING: the master should consider reducing speed and/or altering heading to mitigate rolling — synchronous rolling with natural roll period can cause cargo shift even with good lashings. CSS Code (Code of Safe Practice for Cargo Stowage and Securing): mandatory under SOLAS Chapter VI. The Cargo Securing Manual is ship-specific and approved by the flag state. It specifies lashing strengths, arrangements, and inspection requirements. Synchronous rolling: when a vessel's natural roll period matches the period of the waves, resonance occurs — small waves produce large rolls. This can cause cargo shift even in moderate conditions. The officer should report this to the master who must decide whether to alter course or speed to escape resonance. SOLAS VI/5.6: the master is responsible for ensuring cargo is properly secured, and must refuse to sail if cargo securing is inadequate. The CSS Code Amendment 2011 (MSC.269(85)): updated guidance on securing arrangements. Key point: inspecting and improving lashings BEFORE the weather is always better than trying to do it in 6m seas.

Question 3

In following seas, your container vessel begins rolling heavily even though wave height is only moderate. The roll amplitude is increasing with each wave. What is happening and what action do you take?

Accepted Answer

PARAMETRIC/SYNCHRONOUS ROLLING — recognition and action: WHAT IS HAPPENING: this is likely parametric rolling (also called parametric resonance). It occurs when: (a) The vessel is in head seas, following seas, or quartering seas; (b) The encounter frequency of waves is approximately twice the vessel's natural roll period; (c) The vessel's righting lever (GZ) changes significantly through the wave cycle — typically because the wave crest alternately ships forward and aft, altering the waterplane area and hence stability. Parametric rolling can produce extremely large roll angles rapidly (30°+ in a few cycles) in conditions that appear moderate. It is particularly dangerous on large container vessels (high GM, low freeboard amidships). SIGNS: (a) Increasing roll amplitude with each cycle; (b) Roll developing in otherwise calm-seeming conditions; (c) Rolls occurring in pairs (resonance pattern). IMMEDIATE ACTION: (1) ALTER COURSE: changing heading relative to the waves typically breaks the resonance immediately — this is the most effective action; (2) CHANGE SPEED: altering speed changes the encounter frequency and may break resonance; (3) CALL MASTER immediately; (4) REDUCE SPEED in any case to reduce dynamic forces on lashings; (5) Log the event. DO NOT ignore increasing roll — parametric rolling can capsize a vessel. Parametric rolling: a serious phenomenon that has caused major cargo losses and presented capsizing risk on large container vessels. Notable incidents: the APL China (1998) lost 400 containers; the Maersk Carolina (2008) suffered severe parametric rolling. MSC.1/Circ.1228 (2007): IMO guidance to masters on avoiding dangerous situations in adverse weather and sea conditions — specifically addresses synchronous rolling and parametric rolling. The GM trap: a vessel with very high GM (stiff vessel, quick roll period) is actually more susceptible to parametric rolling than a vessel with moderate GM. This seems counterintuitive — but high initial stability leads to rapid, violent parametric rolling when resonance occurs. SOLAS Reg. II-1/22: vessels must have sufficient stability in the intact condition — but this cannot protect against parametric rolling, which is a dynamic phenomenon. Course alteration is the primary response — even 20-30° course change can break the resonance pattern immediately.