Forces acting on the ship whilst riding anchor

The chain connecting the anchor to the ship is of a heavy construction with a breaking strength with much higher than required. These chain links are made heavy so that when the anchor is dropped in sea with the attached chain and the ship is in the stand still condition, the catenary so formed is deep and almost vertical, The shape of the curve formed by the chain is called a catenary. This curve is like a parabola shaped but has a different mathematical formula when compared with the parabola. It is a physical curve caused by a hanging chain supported at the ends and the total depth of the curve from  the end supports is directly proportional to the unit length of the chain.

The Bower anchor is made up of two parts, the head with the flukes folded inwards and the shank. Both these parts are fitted together by a heavy and large pin fitted tightly on the head but loosely in the shank hole. The head has sufficient internal space to allow free movement of the shank to turn through about 150 on either side of the vertical, so that when it hits the ground it is not stable and can fall on either side of the vertical position. When the anchor is released at the time of anchoring it drops by gravity taking along with it the chain. The anchor head being curved and not flat causes the anchor to fall on either side with the shaft resting on the ground and taking a posture like a plough entering the ground and thus gets fixed in the earth or ground. The details of the forces acting on the ship when riding anchor are given in the sketch shown below.

T =  Ttotal tension acting at the hawse pipe exit and is resisted by the ship’s buoyancy (has a very small trimming effect at forward).

W = the weight of the length of chain forming the catenary (length of chain from ground at point where the chain makes contact with the ground up to the hawse pipe mouth).

T1 = The horizontal tension at ground.

This horizontal tension is the vector sum of the current and wind force. This is how the ship is moored and drift prevented. Hence the anchor resists the wind and tide. The ship is always facing the combined wind and tide in normal condition.

When a medium sized ship anchors about 5 shackles are paid out. Assuming that the ship has anchored at slack water, there may be only 2 shackles forming the catenary. A shackle is a unit of length and equal to 15 fathoms or 90 feet and is the standard length of a chain. In this condition the weight of 2 shackles is able to moor the ship. As the tide builds up the ship drifts aft picking up about two more shackles making the length of the catenary now to 4 shackles. In this condition the  weight of the anchor together with the weight of one shackle of chain is greater than the horizontal component of the weight of the 4 shackles of chain forming the catenary. This is explained mathematically as follows;

          T1  =   W Tan (ang)

Where ang is the vertical angle. For all angles below 450  W  will be greater than T1 and hence the ship has a reserve force or strength of the weight of the anchor and one chain shackle in this condition. Now if for some reason the weather deteriorates and both the current and wind force increase the remaining shackle will be lifted out of the water and further deterioration in the weather may make the catenary degenerate into a straight line. This is the limiting condition when the vertical angle exceeds 450 and the horizontal tension becomes larger than the weight of the anchor leading to the condition of anchor dragging. During anchor stations some captains ask their C/O’s “How is the anchor leading.”. the C/O’s reply is normally 5 to 10 degrees to the vertical at slack water gives confidence to the captain that there is sufficient chain in the water to take care of stronger currents.