The ship propulsion plant denotes the whole plant that transfers to the propeller the power turned out by main engines. Marine diesel engines are generally of high rpm, rpm being the short form for revolution per minute. As far as rpm is concerned, it might be out of your expectation that if M. E. rpm

Therefore, the propulsion plant concerns not only a matter of drive but a problem of the speed reduction and clutch.

The propulsion plant in practical service nowadays mainly includes the nofollowing types:

1. Direct Drive

With simple and reliable construction, convenient operation, less energy loss during drive and high propulsion efficiency, direct drive is the most common drive type and has found a wide application.

In respect of direct drive, the shaft system is directly connected with M. E. thrust shaft and both of them obtain the same rotating direction. Comparatively speaking, direct drive is suitable for a lowspeed diesel engine.

2. Gear Drive

The merits of gear drive are as clear as daylight, for M: E. rpm will be free from the low rpm limitation of a propeller after speed reduction by means of a gear and better propulsion efficiency can thus be maintained. The only trouble with gear drive is that the size and weight of a reduction gearbox will inevitably and correspondingly be increased accompanying the steady increase of M. E. power. As a result of this, its manufacture process will run into difficulty, since the maximum diameter of a gear for a 35-50 kW reduction unit may be as large as over 5 meters.

With regard to gear drive, there may be a variety of arrangement patterns, such as single-screw and single-rudder, twinscrew and twin-rudder, twin-engine incorporation with one gearbox, etc. With relation to reduction gear, there are also single-stage reduction, multi-stage reduction and reduction with an astern clutch and so forth.

Generally, warships will adopt the twin-screw and twin-rudder arrangement due to the limited engine space and the high speed requirement. Another advantage of this arrangement is that the maneuverability and service ability of a warship will be greatly raised. For a warship is capable of obtaining a rapid turning ability with the port engine and starboard engine at ahead and astern positions respectively. Furthermore, if one main engine should break down, the rest could keep on running.

In the case of applying gear drive to the twin-engine incorporation, a flexible coupling is to be introduced so as to prevent the gear from the impact due to an uneven torsional moment. To attain this goal, a coupling of high flexibility will usually be fitted in the shafting which can absorb or reduce the torsional vibration of the main engine and shafting as well as compensate the installation errors.

3. CPP Drive

There are two models of propellers at the present time, the fixed pitch propeller and the controllable pitch propeller. As known to all, Lips is a Holland company famous for the controllable pitch propeller system, with CPP its short form.

The CPP system has acquired a relative motion, and by this very motion the pitch can be adjusted to adapt the needs of astern and variation of propeller loads. So any one knot can, theoretically and actually, be obtained ranging from the max. ahead speed to the max. astern speed. If the pitch is adjusted to nil position, a vessel can keep station even if the propeller is turning madly.

In respect of the advantages of a CPP system, we can name the nofollowing:

A: Under different working conditions, vessel resistance will correspond with M. E. rated power by pitch adjustment. Thus, the full power of main engines can be realized with success.

B: As pointed out above, any one knot may be obtained by pitch adjustment, and in this way the vessel maneuverability is raised.

C: In the event of astern, it is unnecessary for a main engine to reverse and therefore the M. E. service life will be prolonged.

D: A minus push can be produced by adjusting the pitch to a minus value, and so the sliding distance needed for harbouring will be shortened to great extent.

The power patterns for the CPP maneuvering system may be of hand, mechanical, hydraulic or electrical drive, hydraulic drive being the most common pattern.

(continued)

 

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