N40475 Chapter 3 Propeller shafts and couplings Propeller shafts It is recommended that all engines are mounted on flexible mountings (1), which will reduce noise and vibration, and will prevent hull movements resulting in forces being applied to the engine. The responsibility for the design and installation of the transmission system connected to the gearbox lies with the boat designer, the boat builder, the naval architect or the engine installer. It is recommended that a Torsional Vibration Analysis (TVA) is carried out on the complete drive system. Mass Elastic Data can be provided on request from Wimborne Marine Power Centre.
Figure 1 shows a simple arrangement, where the propeller shaft is supported only by the gearbox coupling and an outboard rubber bearing at the propeller end. Entry of water into the boat is prevented by a shaft seal, which must be flexibly mounted to allow for engine movement. A flexible shaft coupling (2) is fitted to the gearbox coupling, to allow momentary angular misalignment in operation. This system is only suitable for applications where the speed, diameter, and unsupported length of the propeller shaft will not induce ‘whirling’ (i.e. the centrifugal force generated by the speed of rotation is not sufficient to bend the shaft into a bow shape).
Figure 1 also shows a cutless bearing (3), flexibly mounted shaft seals (4), reinforced rubber hose (5), graphited asbestos string (6), stuffing box (7) and the drive shaft (8). Figure 1 Where the propeller shaft length is such that it cannot be simply supported by the gearbox coupling and ‘P’ bracket, without the risk of whirling, the arrangement shown in figure 2 may be adopted. Water supply (4) for bearings (use hose from M14 x 1.5 tapping on heat exchanger end cap). End cap with tapping is optional.
In this case one or more additional bearings (3) are included in the shaft log, and flexible shaft couplings (2) (which will accept thrust) are used to permit the engine to move on the flexible mountings (1) . Warning! Use a syphon break (5) where a water lift exhaust system is specified. A variation of this is to use a thrust block (bearing) at the point where the shaft emerges from the log into the engine room, together with constant velocity joints at each end of the short shaft connected to the gearbox coupling. Figure 2 Page 7