Mercury Mariner 4, 5, and 6HP 4-Stroke Factory Service Manual, Page 41Get this manual
OUTBOARD MOTOR INSTALLATION
Fastening Security Line
WARNING
If the length of security line being used is long enough to allow the outboard to disengage off the boat transom but is too short to not allow the outboard to submerge behind the boat and stop running, the outboard could continue running and propel itself back into the boat with the propeller rotating under powerThis exposes the occupants to serious injury or deathThe primary purpose for installing security line is to prevent loss of the outboard if the outboard becomes detached from the boat transomAn effective security line should be of working strength of at least five times the weight of the outboardThe security line should be attached between the boat and outboard following one of these stepsStep 1The length of the security line should be short enough and affixed in manner to prevent the outboard from rising up and disengaging off the boat transomStep 2The length of the security line should be long enough and affixed in manner to permit detached outboard to submerge completely behind the boat and stop running but not too short that could allow the outboard to continue running and propel itself back into the boat