Chapter 2 TPD1317e 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. In this case one or more additional bearings are included in the shaft log, and flexible shaft couplings (which will accept thrust) are used to permit the engine to move on the flexible mountings. A variation of this system 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. Warning!
Use a syphon break where a water lift exhaust system is specified. See section 3.4 Water supply for bearings (use hose from 1/4” BSP tapping on heat exchanger end cap). End cap with tapping is optional. Flexible mountings Flexible shaft couplings Shaft bearings Figure 2 - Alternative shaft arrangement 2.2 Gearbox Output Couplings Flanged propeller shaft couplings, to suit the gearbox output couplings offered by the many gearbox manufacturers, come in a number of different shapes and sizes, and a variety of methods can be employed to grip the propeller shaft.
A traditional and sound method is to provide the same taper at both ends of the propeller shaft, so that the gearbox coupling can be machined at the same time as the propeller. This system is still commonly employed, particularly where high thrusts and heavy boats are involved. If the shaft becomes worn it can be turned end for end, and re-used at minimum cost. As an alternative to the above procedure the method shown in figure 3 may be adopted. Flanged couplings supplied with Twin Disc gearboxes are intended for this system.
The shaft is gripped by means of a split coupling, with a key to provide positive drive, and a dowel pin to eliminate the possibility of the shaft being drawn out of the coupling when going astern. For lighter duty applications the system shown on figure 4 may be employed, utilising the flanged couplings supplied with Hurth gearboxes. In this case ahead thrust is taken directly by the gearbox output shaft, as the propeller shaft is butted against it. The shaft is secured by means of two half dog setscrews which also prevent loss of the shaft when going astern. Positive drive is provided by a key. Page