When a vessel is anchored with both anchors leading ahead, she is said to be on open moor. Supposing a vessel is lying to a single anchor dead ahead and with a stress in her cable of tonnes. If she had two anchors leading dead ahead the stress in each would be ½ T tonnes. When the angle between the cables becomes 120 degrees. i.e. with each anchor about 5 points on the bow, the stress in each cable becomes T tonnes. This value increases to 2T and 3T as tons the angle becomes I50 degrees and 160 degrees respectively.
When the angle exceeds the safe limit of 120 degrees she is commencing ride to a tight span. Figure. 2.1 illustrates the parallelogram of forces for an angle of 120 degrees.
COMING TO OPEN MOOR
Figure. 2.2 shows the successive stages of this manoeuvre. The vessel is headed into the anchorage with the wind or current on one bow in order to assist counteraction of lee drift. The weather anchor (or upstream anchor) is let go on the run (1), and headway continued for roughly on third of the final length of cable. The second anchor is let go and the first one snubbed at the gypsy. As the vessel brings-to on her weather cable, it gradually grows taut to windward, snubbing the bows round (2).
If the engines are then worked ahead, using weather helm, so as to keep the cables taut (the second cable is checked soon after the anchor is let go) the bows will develop a rapid swing into the stream or wind. By keeping a little ahead of her anchor (3), so that both cables grow slightly aft, the manoeuvre is hastened. When heading into the wind or stream, both cables are veered (the second one only, for a short while) and the vessel brings-to in position (4).The reason for veering the second one by itself while dropping back initially is to middle the ship between her anchors. By laying out on third of the length between the anchors, each finally lies a point on the bow.
Mooring is usually taken to mean securing the ship with two anchors, one ahead and one lying astern-a cable each way, as it was once called. The upwind or upstream anchor is known as the riding anchor and cable, the other being called the sleeping or lee anchor and cable (Figure. 2.3).
The advantages of mooring are:
1. The vessel occupies little swinging room, turning almost in her own length about her stem.
2. The scopes can be pre-adjusted for the prevailing strength of wind or stream. The scope of each cable is estimated in the same way as that for a single anchor (Chapter 1).
The disadvantages are:
1. The second, or lee, anchor lies astern and is of no value to the ship if a headwind increases or if the wad begins to drag. In the latter event, if possible, it is better to drag the anchor down until the lee anchor is reached (heaving in the lee cable while dragging). The two cables can then be veered together. If cable is veered on the riding anchor initially and the vessel continues to drag, by the time the lee anchor is reached there may be so much cable out on the riding anchor that the other cable can be veered only a shackle or two.
2. There is a risk of getting a foul hawse. To avoid this, the vessel must always be swung within the same arc at each consecutive tidal change (Figure. 2.3).
3. Due to the fact that one cable leads aft, the vessel must be dropped down to it when leaving the anchorage, weigh it, and then heave herself back to the riding anchor. At 4-5 minutes per shackle, this will take a considerable time. At open moor, provided the anchor are dose together, both cables can be hove simultaneously.
4. In a beam wind the vessel will tum and lie at open hawse across the line of her anchors, creating a tight span. Both cables must be veered smartly, the vessel then riding to open moor.
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