Installation Manual

N40475 Chapter 3
Page 7
Propeller shafts and couplings
Propeller shafts
It is recommended that all engines are mounted on exible
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
exibly mounted to allow for engine movement. A exible
shaft coupling (2) is tted 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 sufcient to bend
the shaft into a bow shape).
Figure 1 also shows a cutless bearing (3), exibly mounted
shaft seals (4), reinforced rubber hose (5), graphited
asbestos string (6), stufng box (7) and the drive shaft (8).
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 gure
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 exible shaft couplings (2) (which will
accept thrust) are used to permit the engine to move on the
exible mountings (1) .
Warning! Use a syphon break (5) where a water lift exhaust
system is specied.
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 1
Figure 2