Arm movements made in a water environment take longer to perform than in an air environment due to the drag forces experienced by the arm. Movement times for ballistic underwater movements have been accurately modelled by Hoffmann and Gan ( 1988 ). The present work models the movement time for movements that require ongoing visual control. In these movements, the 'distance-covering' phase is carried out at high speeds and will be affected by the fluid characteristics, while the 'homing-in' phase, where speeds are low, is less affected. An experiment is reported that models these effects and which indicates that a ballistic component needs to be added to the standard Fitts model in order to account for the drag forces in the distance-covering phase of the movement.
Practitioner summary: Many tasks, such as maintenance and salvage work, require work to be done underwater. Times for performing underwater tasks are generally longer than on land. This article is one step in modelling the difference in task times for land and underwater movements.