Page 144 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

132 CHAPTER 6 Thrusters
FIGURE 6.10
Fluid sealing of direct drive thruster coupling.
Drive shafts and couplings vary with the type of propeller driving mechanism. Direct drive shafts, magnetic couplings, and mechanical (i.e., geared) couplings are all used to drive the propel- ler. Technology advances are being exploited in attempts to miniaturize thrusters. In one case, a new type of thruster housing places the drive mechanism on the hub of the propeller instead of at the drive shaft, allowing better torque and more efficient propeller-tip flow management. Others are developing miniature electric ring thrusters, where the propeller, which can be hubless, is driven by an external “ring” motor built into the surrounding nozzle. Such a design eliminates the need for shafts, and sometimes seals, altogether.
There are various methods for sealing underwater thrusters. Some manufacturers use fluid-filled thruster housings to lower the difference in pressure between the seawater and the internal thruster housing pressure by simply matching the two pressures (internal and external). Still others use a lubricant bath between the air-filled spaces and the outside water (Figure 6.10). A common and highly reliable technique is the use of a magnetically coupled shaft, which allows the air-filled housing to remain sealed (Figure 6.11).