During an annual survey, the class surveyor measures the thickness of tank top plating in No. 3 hold and reports it has wasted from the original 14mm to 11mm. The allowable diminution is 20%. As chief mate, explain the significance of this reading and the action required.

A. THICKNESS MEASUREMENT AND WASTAGE ALLOWANCE: CALCULATION: Original thickness: 14mm. Allowable diminution: 20% of 14mm = 2.8mm. Maximum allowable diminution means minimum acceptable thickness = 14 - 2.8 = 11.2mm. Measured thickness: 11.0mm — this is BELOW the minimum acceptable. THE PLATING IS UNDER THE ALLOWABLE THICKNESS: the tank top plating has exceeded its allowable wastage. CLASS ACTION: when steel is found below the allowable diminution, the classification society will: (1) Issue a RECOMMENDATION or CONDITION OF CLASS requiring renewal of the plating within a specified period; (2) May suspend the vessel's class (or apply a condition) until repair is completed; (3) Issue a notation in the classification certificate. CHIEF MATE ACTION: (1) Do not dispute the reading — have it confirmed by an independent thickness measurement if uncertain; (2) Notify DPA and master immediately; (3) Arrange for a steel contractor to assess and quote for renewal at this port or the next suitable port; (4) Review adjacent areas — if one area is at minimum, adjacent areas may also be close to limits; (5) Check whether any cargo moisture management changes are indicated (corrosion is accelerated by wet cargoes); (6) The cost of renewal is significant — notify P&I Club and insurance; (7) DO NOT LOAD cargo into a hold with plating below allowable wastage until repaired unless class approves a temporary mitigation.

B. The reading is within 20% of the original — the surveyor has made an error. 11mm from 14mm is only 3mm loss which is 21%, just 1% over. Ask for a re-measurement and proceed.

C. Thickness measurement findings only become actionable at the 5-year special survey. Annual survey readings are advisory and can be deferred until the special survey for action.

D. The allowable diminution means the plating is still acceptable until it reaches zero. "Allowable diminution" is a percentage the surveyor applies as a safety margin — the plating need not be renewed until it reaches 0mm.

A. BENDING MOMENT AND SHEAR FORCE — CONCEPTS AND ACTION: BENDING MOMENT (BM): the internal moment within the ship's hull that resists the tendency of the vessel to "hog" (bend upwards at the centre when supported at ends) or "sag" (bend downwards at the centre when buoyancy exceeds weight amidships). BM is measured in tonne-metres and varies along the ship's length. SHEAR FORCE (SF): the internal vertical force within the hull at any cross-section — the tendency of the ship to "shear" (vertical displacement between adjacent sections). Caused by unequal distribution of weight and buoyancy. STILL WATER vs WAVE BM: still water BM is calculated at the loading port and is the controllable component. Wave BM is added at sea and is the classification society's additional allowance. MAXIMUM PERMISSIBLE STILL WATER BM: the value stated in the ship's loading manual is the maximum still water BM that the vessel's structure can withstand in still water, with a reserve for wave-induced BM at sea. EXCEEDING THE LIMIT: the proposed sequence results in 9,100 vs maximum 8,500 tonne-metres — this is 7% over the permissible limit. THE VESSEL MUST NOT SAIL in this condition. ACTION REQUIRED: (1) Modify the loading sequence — redistribute cargo between holds; (2) Add ballast to alter buoyancy distribution; (3) Reduce total cargo weight; (4) Recalculate until within limits; (5) Document the revised plan and recheck BM and SF on the loading computer.

B. The maximum permissible value is a conservative limit with a 10% tolerance built in. A bending moment 7% over the stated limit is within the safety margin and the vessel may sail.

C. Shear force and bending moment limits only apply in port loading operations. At sea, wave action overrides the still water values and the loading computer limits are not relevant.

D. Contact the classification society for a dispensation to sail over the limit. Classification societies routinely grant dispensations for temporary bending moment exceedances at loading ports.

A. HOGGING AND SAGGING — DEFINITIONS AND CAUSES: SAGGING: the vessel bends so that the midship section is lower than the bow and stern — the hull curves downward like a banana shape with the centre lowest. CAUSES: (a) Heavy cargo concentrated amidships with relatively empty ends — weight exceeds buoyancy amidships; (b) At sea: when the vessel is in the trough of a wave — buoyancy is reduced amidships (both bow and stern supported by wave crests). STRESS IN SAGGING: the keel and bottom structure are in TENSION; the deck structure is in COMPRESSION. HOGGING: the vessel bends so that the midship section is higher than the bow and stern. CAUSES: (a) Heavy cargo at the ends with a light or empty midships section; (b) At sea: when the vessel is at the crest of a wave — midships is supported by the wave while bow and stern overhang the troughs. STRESS IN HOGGING: the deck is in TENSION; the keel is in COMPRESSION. PRACTICAL SIGNIFICANCE: most bulk carriers are more vulnerable to hogging in heavy weather (empty holds or light cargo at midships with heavy ore at ends). Structural failures have occurred from excessive hogging loads in severe weather. LOADING TO BALANCE: a well-loaded bulk carrier distributes heavy cargo to minimise both sagging and hogging BM — check the loading computer output for the BM envelope diagram.

B. Sagging occurs when the bow is heavily loaded and the stern is light. Hogging occurs when the stern is heavily loaded and the bow is light.

C. Hogging and sagging refer to the list of the vessel, not bending. A hogged vessel lists to port, a sagged vessel lists to starboard.

D. Sagging is when the vessel is heavy overall and riding low in the water. Hogging is when the vessel is light and riding high. Both are stability terms, not structural terms.