Page 146 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

134 CHAPTER 6 Thrusters
incidence of foreign object ingestion into the thruster propeller. Also, stators help reduce the ten- dency of rotating propellers’ swirling discharge, which tends to lower propeller efficiency and cause unwanted thruster torque acting upon the entire vehicle.
6.2 Thrusters and speed
As the speed of the vehicle ramps up, the low speed stability due to static stability is overtaken by the placement of the thrusters with regard to the center of total drag. In short, for slow-speed vehi- cles the designer can get away with improper placement of thrusters. For higher speed systems, thruster placement becomes a more important consideration in vehicle control (Figure 6.12).
Propeller efficiency and placement
Propellers come in all shapes and sizes based upon the load and usage. Again using the aircraft correlation, on fixed-pitch aircraft propellers for small aircraft there are “climb props” and “cruise props.” The climb prop is optimized for slower speeds, allowing better climb performance while sacrificing cruise speeds. Conversely, a cruise prop has better range and speed during cruise, but climb performance suffers. Likewise, a tugboat propeller would not be best suited for a high-speed passenger liner.
Propellers have an optimum operational speed. Some propellers are optimized for thrust in one direction over another. A common small ROV thruster on the market today uses such a propeller for forward and downward thrusting. The advantage to this propeller arrangement is better thrust performance in the forward direction while sacrificing turning reversal and upward thrust perfor- mance. Other propellers have equal thrusting capabilities in both directions. Both have their strengths and weaknesses. All ROVs are slow systems and should make use of propellers’ maxi- mizing power at slow speeds in order to counter the combined tether/vehicle drag. Remember, an ROV is a tugboat and not a speedboat.
Propellers also produce both cavitations and propeller-tip vortices, causing substantial amounts of drag. As the spinning propeller moves water across the blade, the vector/inertial force of the moving water (instead of moving aft to produce a forward thrust vector) throws the water toward the tips of the blade, spilling over the end of the blade in turbulent flow. Kort nozzles form a basic hub around the propeller to substantially reduce the instance of tip vortices. The kort nozzle then maintains the water volume within the thruster unit, allowing for more efficient
FIGURE 6.12
Float block drag
Frame/component drag
Turning moment
CB
CG
Bow turning moment due to asymmetrical drag as speed ramps up.