Page 254 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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  9.3 Currents and tether management 243
  CB
CG
   FIGURE 9.28
Flow drag
“Bow” turning moment
 Minimal bow turning moment with tether on thrust line.
Natural tether lay in current
Drag
     FIGURE 9.29
 Natural tether lay behind the vehicle as the speed ramps up.
Tether pull/lay
Some vehicle manufacturers place the tether pull point atop the vehicle. The benefit to this placement is the tether will not lie as easily in the debris located on the bottom, allowing a cleaner tether channel from the vehicle to the surface. This is beneficial if the vehicle is operated in mini- mal currents with little or no horizontal offset. If either a horizontal offset or a current (or both) is encountered, the vehicle may experience difficulty through partial (if not total) loss of longitudinal and/or lateral stability.
Hydrodynamics of vehicle and tether
The most typical arrangement for an observation-class system involves a clean tether (i.e., with- out clump weight) following the vehicle to the work site. The tether naturally settles behind the vehicle and slopes in the current as it feeds toward the surface. As the vehicle speed ramps up, the flow drag on the tether correspondingly melds the tether into its wake, forming a “sail” of sorts behind the vehicle (Figure 9.29). A small reduction in the drag due to reduced angle of incidence to the oncoming water flow is more than offset with the additional form and flow drag of the exces- sive tether in the water. There is an old technique used by surface-supplied commercial divers to




















































































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