...Damage Stability





No this is not the floodable length curves of the Edmund Fitzgerald, but the Flood curves in the Blue Sheets that you will access and reference in the problem area of this lecture presentation. They are illustrated here to point out fundamental points related to the understanding of the subject matter.



Unlike the prior lectures where conceptual understanding of the material coupled with the appropriate equations satisfied the resolution of the Coast Guard's problems; whereby, definitions were essentially subordinate, the reverse is true in the study and application of the problems as they relate to the subject area of Damage Stability. This is straight forward material. Definitions are rather important in solving the essential 4 problem areas found in the Coast Guard's bank of questions related to Damage Stability.

Note: When you finish reading over the definitions click on the link to the solution of the 4 aforementioned problems. Again you will need to back out of that link with the "back button" on your browser to access the required reference sheets in solving the problems.

Definitions :

Damage stability: The existing vessel stability after the unintentional flooding of the compartment

The ability of a vessel to survive flooding is determined by the diagram you see above, called the Floodable Length Curves.

Observe the triangles constructed from the base of each watertight compartment. You simply extend the side of the triangle which comes from below right hand side of the flooded space into the side coming from the lower left-hand side of the flooded space. If the two lines intersect above the appropriate curve, i.e. the wiggley line running fore n aft, the vessel will sink, if below the vessel will remain afloat.

Now take a look at the diagram below:



Compartments 3 and 4 are both flooded. Because the point is above 63%, the vessel will sink, but if the permeability was 50% the vessel would stay afloat. Permeabilities generally run 63% with the exception of Machinery spaces which run 80%. Permeability defined as the percentage of the volume of the compartment which can be occupied by water if flooded.

Freeboard: the distance between the waterline and the main deck at the upper edge of the deck line.

Floodable Length: the maximum length of the vessel at any point which if it were flooded the ship would sank.

Factor of Subdivision: A value used to determine the permissible length between bulkheads on a vessel.

Compartment Standard: A number indicating how many compartments can be flooded and the vessel would still remain afloat.

Margin Line: An imaginary line drawn three inches below the bulkhead deck. It provides a margin of safety to the designer's calculations.

Free Communication: A flooding condition resulting from an opening in the hall which is in free communication with the Sea.

Reserve Buoyancy: the volume of space remaining above the waterline. The vessel sinks when no reserve buoyancy remains. Vessels must have reserve buoyancy to be able to float. Reserve Buoyancy is simply the watertight hull areas above the waterline. As weight is added to a vessel it sinks in the water, and the volume of space above the waterline decreases. When this space is gone the vessel sinks immediately.

Sagging Stresses: When the bow in the stern are riding on wave crests and the midship region in the trough the ship is said to be sagging.

Hogging Stresses: When a ship advances a half a wavelength so that the crest is midship and the bow and stern are over troughs the ship is said to be hogging

The Problems
Ref. Library