HYDRODYNAMIC FORCES ON AN ANCHOR CABLE 



ABSTRACT 



The holding power required of an anchor for a ship anchored in shoal 

 water with a length of anchor line at least five times the water depth, i.e., 

 with a scope of five, can normally be assumed to be equal to the estimated 

 drag of the ship. Anchoring in deep water necessitates a relatively shorter 

 anchor line, which results in a considerable hydrodynamic force on the anchor 

 cable and a tension in the cable much greater than the drag of the ship. 



Curves have been computed from which the magnitude and direction of 

 the tensions in the anchor cable can be determined when the drag of the ship, 

 the velocity of the current, the depth of the water, and the type and length 

 of the anchor cable are known. Formulas are given for ship drag, current 

 parameter, breaking strength of wire-rope and chain cables, safe working loads 

 on cables, and holding power of an anchor. An illustrative example applies 

 these calculations to the determination of diameter and length of a wire-rope 

 anchor cable and of size of anchor required in a given problem. 



INTRODUCTION 



In March 1 9^5 the Bureau of Ships (1)* requested the David Taylor 

 Model Basin to calculate the forces acting on the anchor cable of a ship. An 

 anchor selected for a ship must have sufficient holding power to take the 

 greatest loads normally encountered in service. Ordinarily, however, it is 

 not practicable to carry anchors which will hold a ship under any storm con- 

 ditions and on any types of bottom. Likewise the length of anchor cable 

 carried is limited by practical considerations. The holding power of models 

 of several types of anchors in different types of sea bottom has been investi- 

 gated experimentally (2) (3). Also, tests of the holding power of full-scale 

 anchors of new designs have been conducted by the Norfolk Naval Shipyard from 

 time to time, as requested by the Bureau of Ships. 



The holding power required of an anchor for a given ship is usually 

 computed for the standard conditions of a 70-knot wind and a 4-knot tide in 

 the same direction. It is desirable where practicable to anchor with a length 

 of anchor line at least five times the water depth, i.e., with a scope of five 

 or greater. With such a scope, under the standard conditions of wind and cur- 

 rent, an anchor cable would make a small angle with the horizontal at both the 

 anchor and the ship, resulting in a tension in the cable at the anchor very 



* Numbers in parentheses indicate references on page 12 of this report. 



