Page 253 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

242 CHAPTER 9 LARS and TMS
CB
CG
“Bow-up” turning moment countered by vertical thruster
“Bow-up” turning moment
Flow drag
To the surface
uncontrolled
Flow drag CB CG
“Bow-up” turning moment exceeds thrust available to vertical thruster
Tether lengthening
“Bow-up” turning moment is much larger
FIGURE 9.27
Vehicle stability considerations.
9.3.1 Tether effects
Tether pull point
Stability testing was performed on a small ROV system at Penn State University’s Advanced Research Lab in their water tunnel. The water flow was slowly brought up while observing the vehicle’s handling characteristics as well as its computed, versus actual, zero net thrust point.
This particular vehicle had a tether pull point significantly above the line of thrust (Figure 9.27), resulting in a “bow-up” turning moment. As the speed ramped up during the tests, with little tether in the water, the vehicle was still able to maintain control about the vertical plane by counteracting the “bow-up” tendency with vertical thrust down. However, at a constant speed with the tether being lengthened, the tether drag produced an increasingly higher tether turning moment, eventually over- powering the vertical thruster and shooting the submersible to the surface in an uncontrolled fashion.
If the tether is placed in close proximity to the thruster, parasitic drag will occur due to the skin friction and form drag from the thruster discharge flow across the tether. When selecting the tether placement, it is best to design the tether pull point (Figure 9.28) as close to the center point of thrust as possible in order to balance any turning moment due to the tether pull point.