Question 56 ONC04 - Mate of LT 500-1600 GRT

The Correct Answer is B. ### Explanation for why Option B ("1"") is correct: This question relates to the practical requirements for securing deck cargo, specifically timber, as outlined in maritime regulations (such as those from the IMO or specific flag states). The core calculation requires finding a flexible wire rope size that provides a Minimum Breaking Load (MBL) equivalent to the required MBL of a 3/4" close link chain, while applying a safety factor of 5. **Step 1: Determine the required strength (Working Load Limit, WLL) of the 3/4" chain.** * The strength of lashing gear is typically specified by its Working Load Limit (WLL) or safe working load (SWL). For lashing timber, maritime standards often mandate a minimum ultimate strength (MBL). * According to standard tables for Grade L or Grade 80 steel close-link chain used in cargo lashing: * A 3/4" (19 mm) close link chain has a typical **Minimum Breaking Load (MBL)** of approximately **25 tonnes** (or 55,000 lbs). **Step 2: Determine the required Minimum Breaking Load (MBL) for the wire rope.** * Since the wire rope must be the *equivalent* of the chain, it must also have an MBL of approximately **25 tonnes**. * The safety factor (SF) of 5 must be applied to determine the *required WLL* if the calculation started there, but here, the regulation specifies the minimum strength of the lashing material (3/4" chain MBL). Therefore, we look up the wire rope size that meets this MBL requirement directly. **Step 3: Select the appropriate wire rope size.** * We consult standard tables for flexible wire rope (e.g., 6x37 or 6x19 IWRC construction, Grade 1960 N/mm² or EIPS classification) to find the size corresponding to an MBL of 25 tonnes (55,000 lbs): * A **1" (25 mm) flexible wire rope** typically has an MBL in the range of **24.5 to 27.5 tonnes** (54,000 to 60,000 lbs). Therefore, a **1"** wire rope is the closest and most appropriate standard size that provides the required ultimate strength equivalent to a 3/4" close link chain. *** ### Explanation for why the other options are incorrect: **A) 9/16"** * A 9/16" (14 mm) wire rope has an MBL of only about 13 tonnes (28,000 lbs). This is significantly less than the required 25 tonnes and would fail to meet the regulatory strength requirement for lashing timber cargo equivalent to 3/4" chain. **C) 1 1/4"** * A 1 1/4" (32 mm) wire rope is stronger than required, having an MBL of approximately 38 tonnes (84,000 lbs). While it would certainly meet the minimum strength, it is an unnecessarily heavy and large size (over-engineering) compared to the specified 3/4" chain equivalent. The 1" size is the direct standard equivalent. **D) 1 3/8"** * A 1 3/8" (35 mm) wire rope has an MBL exceeding 45 tonnes. This is substantially stronger than required and is an incorrect selection based on providing the *equivalent* strength to the 3/4" chain.