You are anchored 0.5 miles ahead of another anchored vessel. you found anchor not holding , started to drift aft(current from fw

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FOLLOW

You do not have space for lowering more shackles and second anchor too cannot be dropped ....well you can say all the precautions of dragging anchor but in the end do not forget to say:
     You will use the rudder and keep it hard toward the side opposite to that in which the anchor is dropped.
Refer
Book : The Theory and Practice of Seamanship
Edition : 11
Author : Graham Danton
Pg No. : 23