Anchoring a ship may not be that difficult a task considering the various other responsibilities of Master in respect of shiphandling and tasks other than shiphandling. Anchoring however, is still tricky in an area, considering various factors such as: probable weather (wind direction & gusting); nature of sea bed; direction of ebbying and flooding streams; & sea room available. There should be more factors than usually considered in deciding the number of shackles to use. The purpose of this article is to discuss at length, a few issues in respect of anchoring & heaving.
To understand the modern theory of shiphandling, the theory of pivot point must be understood first, which is able to answer lot of questions on ship’s behavior in various situations.
Pivot Point Pivot point is a point with a vertical axis about which the vessel will swing or turn if any couple is applied to the ship in transverse plane. Looks like, it is also the virtual position of centroid of ship in motion. Pivot point is close to or nearly at the ship’s centroid when at rest & it moves longitudinally with ahead or astern movement of ship. The force responsible for forward or aft movement of the ship and longitudinal resistance to forward or aft motion respectively created by the water around the ship, balance each other and thereby the pivot point is determined. It changes position depending on the resistance of floatation medium versus the thrust of the advancing vessel. It is seen that in the ships advancing forward at constant speed, the pivot point will be approximately 1/3 L from forward.