Effectual
means,

Active, efficient, mighty; producing the desired effect.

Also, having the necessary
power or force to
produce the 
desired 
effect. 

And the prayer of faith shall save the sick,
and the Lord shall raise him up;
and if he hath committed sins,
they shall be forgiven him.
Confess your faults one to another, and pray
one for another, that ye may be healed. 
  The effectual fervent prayer  
of a righteous man  
availeth much. 

James 5   
15-16