controlling the winches for the cargo runner. Variations on this rig are often
adopted; for example the ‘butterfly’ rig (Figure 24.2(c)) which is used where
cargo is discharged from a hold to both sides of the ship.
Where a weight exceeding the safe working load of the derricks is to be
lifted the ‘yo-yo’ rig is sometimes adopted (Figure 24.2(d)). The heads of
the derricks are brought together, and a travelling block may be arranged on
the cargo runner or a wire connecting the two cargo purchases as illustrated.