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Forces Acting on the Anchor

The anchors are normally housed in hawse pipes of suitable size and form to prevent movement of anchor and chain due to wave action. The anchor arrangements provide for an easy lead of the chain cable from the windlass to the anchors and upon release of the brake, the anchor must start falling by its own weight.
While sailing the stowed anchor is held tight against the hull or the anchor pocket respectively.
It is recommended to fit anchor lashing or devil claw. Anchor lashing should be designed to resist a load at least corresponding to twice the anchor mass plus 10m if cable without exceeding 40% of the yield strength of the material. The shell plating in way of hawse pipes shall be increased in thickness and the framing reinforced as necessary to ensure a rigid fastening of the hawse pipes to the hull. Ships provided with a bulbous bow and where it is not possible to obtain ample clearance between shell plating and anchors during anchor handling, local reinforcements of the bulbous bow shall be provided as necessary.

The Bower anchor is made 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 15 degrees 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.

When the ship is anchored. The holding power depends on the angle of the chain.
While at anchor the anchor is dropped to the attached chain and the chain is paid out to a length that is 6-10 times the depth of water. The heavy chains form a catenary or a parabolic curve depending on the forces acting at the time.

Forces on the Ship Anchor:

Let T- Total tension acting st the Hawsepipe.
Let Wt- Weight of the length of the cable forming the catenary
Let the catenary make an angle to the vertical Ø
Let current be the Tidal current
Let wind be the Wind force
The Tension of the Hawse Pipe is due to weight and some other buoyant forces.
The Horizontal tension at the cable will be the resolved forces of the wind and current=T
The weight of the Cable W and the Horizontal Tension has a relationship
Which means that as long as Ø is below 45 degrees. Tension in cable will be less than the weight of cable. When the catenary angle is 5-10 degrees to the vertical there is enough reverse chain in the water. However, as the angle starts to increase the value of  TanØ becomes greater than 1 and the weight of the anchor.
This leads to drag anchor.


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