Page 97 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 3.23
Tether length
3.5 Design theory 85
Linear tether drag at constant speed with varying diameter.
Tether speed through water
FIGURE 3.24
Linear tether drag at varying speed with constant diameter.
Accordingly, the propulsion power used is proportional to the velocity cubed. To increase the for-
3 ward velocity by 50%, for example, from 2 knots to 3 knots, the power increases by (3/2) , or
33
(1.5) , which is 3.4 times more power. To double the speed, the power increases by (2) or eight
times. Increased speed requirements have a severe impact on vehicle design.
Table 3.5 lists some observation-class systems tested during US Coast Guard procedure trials (without specific names and using figures within each vehicle manufacturer’s sales literature) with
their accompanying dimensions.
At a given current velocity (i.e., 1 knot), the drag can be varied (by increasing the tether length)
until the maximum thrust is equal to the total system drag. That point is the maximum tether length for that speed that the vehicle will remain on station in the current. Any more tether in the water (i.e., more drag) will result in the vehicle losing way against the current. Eventually (when the end
Total drag
Total drag
5 units diameter
4 units diameter 3 units diameter
2 units diameter 1 unit diameter