Whilst it is possible to use a pulley or fairlead which may be already available on the survey vessel, it is normally better to use a purpose-designed pulley.The advantage of using a C-MAX Counting Pulley is that the amount of tow cable paid out is continuously displayed to the sonar operator. This allows the operator to make estimates of layback (the horizontal distance of the fish behind the navigation reference point), to assess the vulnerability of the towfish to changes in ship speed, and to monitor the operation of the winch if it is being controlled by a deck crew