Simulation of the catenary effect under wind disturbances in anchoring of small boats

Abstract

It has been conventional knowledge for as long as ships have existed that the catenary effect of an anchor line augments the efficiency of an anchoring system. This is achieved by making the anchor line as heavy as possible thus lowering the effective angle of pull on the anchor. This notion has, however, come under criticism in recent times. Many small boat owners have shifted to lighter tauter lines for anchoring. The argument in favor of this new method is the cost savings associated with lighter anchoring and the tension relief that comes with using lighter and more elastic anchor lines. The purpose of this study is to therefore compare the performance of long slack lines that form catenary shapes with that of shorter taut lines. An analysis is presented that describes the surge motion of a small anchored boat exposed to an input forcing function and various retarding forces and effects. The anchoring system used in the analytical model results in a non-linear but symmetrical restoring force, which resists the force-induced motion of the boat. Two main types of anchor lines are considered: uniform-material and two-material anchor lines. Each anchor line is evaluated both in catenary configuration and taut configuration in terms of its ability to minimize the motions of the boat and tension force in the anchor line due to wind disturbances.