An estimation of the speed the vehicle can move through the water or the amount of current in which the vehicle can hold its position is made by equating the available thrust in a particular direction to the drag force:

Fd = p Cd Ad Vc2/2

p = mass density of water

Cd = drag coefficient that is a function of the Reynolds number and may vary from 0.2 to 2.0

Ad = the projected area normal to the flow direction

Vc = the current velocity or speed of the vehicle

The available thrust can be theoretically determined by computing the change in momentum of the water that is accelerated through the thruster using

FT =  p AT VT2

FT - thruster force

p = mass density of water

AT = the cross-sectional area of the thruster's stream of water being accelerated from zero to the average velocity (VT) through the thruster.

The velocity of the water through the thruster is determined from

VT = NPE/60

N = revolutions per minute (RPM)

E = the propeller efficiency

P = propeller pitch