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Home → All Courses → Engineering and Technology Courses → Naval Architecture and Ship Building & Repair → Flashcard

What specific calculation determines the maximum length of a ship's compartment that can be flooded without causing the ship to sink?



The specific calculation that determines the maximum length of a ship's compartment that can be flooded without causing the ship to sink is based on the concept of Floodable Length, which is evaluated against the Margin Line. Floodable Length refers to the maximum length of a compartment, at any given position along the ship, that can be completely filled with water without submerging the ship's Margin Line. The Margin Line is a critical safety boundary defined as a line drawn 76 millimeters (approximately 3 inches) below the upper surface of the bulkhead deck at the ship's side. If the ship's waterline after flooding reaches or goes above the Margin Line, the ship is considered to have lost sufficient reserve buoyancy and is at imminent risk of sinking.

The calculation itself is an iterative process involving hydrostatic principles and is performed for various potential compartment lengths and locations along the ship. The steps are as follows:

1. Compartment Flooding Simulation: A specific compartment of a hypothetical length and located at a particular point along the ship's length is assumed to be flooded.

2. Water Ingress Volume and Permeability: The exact volume of water that enters this compartment is calculated. This calculation accounts for the compartment's Permeability, which is the percentage of the compartment's total volume that can be occupied by water. Permeability values vary depending on the type of compartment; for example, an empty cargo hold might have 95% permeability, while an engine room, due to machinery, might have 85% permeability.

3. Hydrostatic Equilibrium Calculation: The addition of this floodwater (which can also be viewed as a loss of buoyancy in that section) causes the ship to sink deeper and potentially change its trim (its fore-aft inclination). Using the ship's comprehensive hydrostatic data – which includes information like displacement, longitudinal center of buoyancy (LCB), longitudinal center of gravity (LCG), and moment to change trim (MCT) – the ship's new equilibrium waterline after flooding is determined.

4. Margin Line Comparison: This newly calculated waterline is then compared against the Margin Line. The crucial check is to ensure that no part of the ship's new waterline, particularly at its lowest point considering the new trim, touches or rises above the Margin Line.

5. Iteration and Floodable Length Curve: If the new waterline remains below the Margin Line, that specific compartment length at that location is deemed permissible. If it crosses the Margin Line, that length is too great. This entire process is repeated for numerous compartment lengths and positions along the ship. The results are typically plotted to create a Floodable Length Curve, which graphically represents the maximum allowable single compartment length at every point along the ship's length that satisfies the Margin Line criterion, thereby preventing sinking.

This calculation fundamentally ensures that even after the flooding of a single compartment, the ship retains sufficient reserve buoyancy – the volume of the ship above the waterline – to keep the Margin Line clear of the water, preventing the ship from sinking due to an overwhelming loss of flotation.