Question and Answers

The factors which affect the stability of the ship - • Free Surface effect • Suspended Weight • Structural Change • Water on deck • Listing effect • Metacentric height • Righting lever • Angle of loll • Centre of buoyancy

A Rolling period diagram is - 1. Way to find at any given time, the GM of the ship 2. Used by the ship up to 70 meters in length 3. The formula used for calculation is GM = (fB/T)^2; where f= factor for the rolling period or rolling period coefficient B=Breadth of the ship T= time taken by the ship to complete one full roll 4. The roll is started by swinging a weight of centre line or men running on the deck in unison 5. Once the rolling is induced, time is noted down for the 5 or more continuous rolls using a stopwatch 6. The average value of T is found 7. GM is then calculated

When an unstable vessel heels over towards a progressively increasing angle of heel, at a certain angle of heel, the centre of buoyancy (B) may fall vertically below the centre of gravity (G). When this occurs, the vessel goes to neutral equilibrium, and the angle of heel at which it happens is called angle of loll. Although a vessel at angle of loll does display features of stable equilibrium, this is a dangerous situation and rapid remedial action is required to prevent the vessel from capsizing. Angle off loll is often caused by the influence of a large free surface or the loss of stability due to damaged compartments.

Apparent trim is the value of the trim obtained after subtracting the VISUAL aft draft from the VISUAL forward draft. True trim is the value of the trim obtained after subtracting the CORRECTED AFT draft from the CORRECTED forward draft. The CORRECTED DRAFTS are the drafts at perpendiculars.

When the density of the water changes the underwater volume of the vessel changes. As a result of change in the underwater volume, the draft of the vessel changes.